Q. What is React.js?
React is a JavaScript library created for building fast and interactive user interfaces for web and mobile applications. It is an open-
source, component-based, front-end library responsible only for the application view layer.
The main objective of ReactJS is to develop User Interfaces (UI) that improves the speed of the apps. It uses virtual DOM (JavaScript
object), which improves the performance of the app. The JavaScript virtual DOM is faster than the regular DOM. We can use ReactJS
on the client and server-side as well as with other frameworks. It uses component and data patterns that improve readability and
helps to maintain larger apps.
Reference:
↥
Q. How React works?
React implements a virtual DOM that is basically a DOM tree representation in Javascript. So when it needs to read or write to the
DOM, it will use the virtual representation of it. Then the virtual DOM will to find the most efficient way to update the browsers
DOM.
Unlike browser DOM elements, React elements are plain objects and are cheap to create. React DOM takes care of updating the
DOM to match the React elements. The reason for this is that JavaScript is very fast and it is worth keeping a DOM tree in it to
speedup its manipulation.
↥
Q. List some of the major advantages and limitations
of React?
Advantages:
It relies on a virtual-dom to know what is really changing in UI and will re-render only what has really changed, hence
better performance wise
JSX makes components/blocks code readable. It displays how components are plugged or combined with.
React data binding establishes conditions for creation dynamic applications.
Prompt rendering. Using comprises methods to minimise number of DOM operations helps to optimise updating process
and accelerate it. Testable. React native tools are offered for testing, debugging code.
SEO-friendly. React presents the first-load experience by server side rendering and connecting event-handlers on the side
of the user:
oReact.renderComponentToString is called on the server.
oReact.renderComponent() is called on the client side.
oReact preserves markup rendered on the server side, attaches event handlers.
Limitations:
Learning curve. Being not full-featured framework it is requered in-depth knowledge for integration user interface free
library into MVC framework.
View-orientedness is one of the cons of ReactJS. It should be found 'Model' and 'Controller' to resolve 'View' problem.
Not using isomorphic approach to exploit application leads to search engines indexing problems.
↥
Q. Why does React emphasize on unidirectional data
flow?
It is also known as one-way data flow, which means the data has one, and only one way to be transferred to other parts of the
application. In essence, this means child components are not able to update the data that is coming from the parent component. In
React, data coming from a parent is calledprops.
In React this means that:
state is passed to the view and to child components
actions are triggered by the view
actions can update the state
the state change is passed to the view and to child components
The view is a result of the application state. State can only change when actions happen. When actions happen, the state is updated.
One-way data binding provides us with some key advantages
Easier to debug, as we know what data is coming from where.
Less prone to errors, as we have more control over our data.
More efficient, as the library knows what the boundaries are of each part of the system.
In React, a state is always owned by one component. Any changes made by this state can only affect the components below it, i.e
its children. Changing state on a component will never affect its parent or its siblings, only the children will be affected. This is the
main reason that the state is often moved up in the component tree so that it can be shared between the components that need to
access it.
↥
Q. How to declare constant in react?
// Constants.js
exportconstPOSTURL = "http://localhost:3000/api/v1/patterns";
exportconst DELETEURL = "http://localhost:3000/api/v1/patterns/";
exportconst DeleteButton = require("./images/delete-icon.png");
exportconst LoadingWheel = require("./images/loading-wheel.gif");
// App.js
import * as Constants from "./Constants";
constemployee = {
emp_id: 10,
name: "Nakul Agate",
email: "[email protected]"
};
classApp extends React.Component {
render() {
return (
<div>
<div>Employee Details :{JSON.stringify(employee)}</div>
<div><img src={Constants.LoadingWheel} alt="Loading..." /></div>
</div>
);
}
}
⚝
↥
Q. What is Destructuring in React?
Destructuring is a convenient way of accessing multiple properties stored in objects and arrays. It was introduced to JavaScript by
ES6 and has provided developers with an increased amount of utility when accessing data properties in Objects or Arrays.
When used, destructuring does not modify an object or array but rather copies the desired items from those data structures into
variables. These new variables can be accessed later on in a React component.
Example:
/**
* Destructuring in React
*/
importReact from "react";
exportdefault function App() {
// Destructuring
const [counter, setcounter] = React.useState(0);
return (
<>
<button onClick={() => setcounter(counter + 1)}> Increment </button>
<button onClick={() => setcounter(counter > 0 ? counter - 1 : 0)}>
Decrement
</button>
<h2>Result: {counter}</h2>
</>
);
}
⚝
↥
Q. Why is it necessary to start component names with
a capital letter?
In JSX, lower-case tag names are considered to be HTML tags. However, lower-case tag names with a dot (property accessor) aren't.
When an element type starts with a lowercase letter, it refers to a built-in component like or and results in a
string <div>or<span> passed to React.createElement. Types that start with a capital letter like compile
toReact.createElement(Foo) and correspond to a component defined or imported in your JavaScript file.
<component /> compiles to React.createElement('component') (html tag)
<Component /> compiles to React.createElement(Component)
<obj.component /> compiles to React.createElement(obj.component)
↥
Q. What are fragments?
Fragments allows to group a list of children without adding extra nodes to the DOM.
Example:
classApp extends React.Component {
render() {
return (
<React.Fragment>
<ChildA />
<ChildB />
<ChildC />
</React.Fragment>
)
}
}
Benefits:
It's a tiny bit faster and has less memory usage (no need to create an extra DOM node). This only has a real benefit on very
large and/or deep trees, but application performance often suffers from death by a thousand cuts. This is one cut less.
Some CSS mechanisms like Flexbox and CSS Grid have a special parent-child relationship, and adding divs in the middle
makes it hard to keep the desired layout while extracting logical components.
The DOM inspector is less cluttered.
↥
Q. What is Virtual DOM?
In React, for every DOM object, there is a corresponding "virtual DOM object". A virtual DOM object is a representation of a DOM
object, like a lightweight copy. A virtual DOM object has the same properties as a real DOM object, but it lacks the real thing's power
to directly change what's on the screen.
Manipulating DOM is slow, but manipulating Virtual DOM is fast as nothing gets drawn on the screen. So each time there is a
change in the state of our application, virtual DOM gets updated first instead of the real DOM.
⚝Virtual DOM Example
↥
Q. What is the difference between ShadowDOM and
VirtualDOM?
1. Document Object Model:
It a way of representing a structured document via objects. It is cross-platform and language-independent convention for
representing and interacting with data in HTML, XML, and others. Web browsers handle the DOM implementation details, so we can
interact with it using JavaScript and CSS.
2. Virtual DOM:
Virtual DOM is any kind of representation of a real DOM. Virtual DOM is about avoiding unnecessary changes to the DOM, which are
expensive performance-wise, because changes to the DOM usually cause re-rendering of the page. It allows to collect several
changes to be applied at once, so not every single change causes a re-render, but instead re-rendering only happens once after a set
of changes was applied to the DOM.
3. Shadow DOM:
Shadow DOM is mostly about encapsulation of the implementation. A single custom element can implement more-or-less complex
logic combined with more-or-less complex DOM. Shadow DOM refers to the ability of the browser to include a subtree of DOM
elements into the rendering of a document, but not into the main document DOM tree.
Difference:
The virtual DOM creates an additional DOM. The shadow DOM simply hides implementation details and provides isolated scope for
web components.
↥
# 2. REACT SETUP
Q. How to set up a react project with create react app?
Create React App is an officially supported way to create single-page React applications. It offers a modern build setup with no
configuration. This tool is wrapping all of the required dependencies like Webpack, Babel for React project itself.
Requirements:
The Create React App is maintained by Facebook and can works on any platform, for example, macOS, Windows, Linux, etc. To
create a React Project using create-react-app, you need to have installed the following things in your system.
Node version >= 14
Visual Studio Code Editor
Installation:
npxcreate-react-app my-app
cd my-app
npmstart
Output:
Running any of these commands will create a directory called my-app inside the current folder. Inside that directory, it will generate
the initial project structure and install the transitive dependencies:
my-app
├──README.md
├──node_modules
├──package.json
├── .gitignore
├──public
│├──favicon.ico
│├──index.html
│├──logo192.png
│├──logo512.png
│├──manifest.json
│└──robots.txt
└──src
├──App.css
├──App.js
├──App.test.js
├──index.css
├──index.js
├──logo.svg
├──serviceWorker.js
└──setupTests.js
Reference:
↥
Q. What are the features of create react app?
Create React App is a command-line program that lets us create a new React project easily and build the project into artifacts that
we can deploy. It is created by the React team and creates a scaffold to the app.
Below are the list of some of the features provided by create react app.
React, JSX, ES6, Typescript and Flow syntax support.
Autoprefixed CSS
CSS Reset/Normalize
Live-editing CSS and JS in local development server.
A fast interactive unit test runner with built-in support for coverage reporting
A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps
An offline-first service worker and a web app manifest, meeting all the Progressive Web App criteria.
↥
Q. What does eject do in create react app?
The create-react-app commands generate React App with an excellent configuration and helps you build your React app with the
best practices in mind to optimize it. However, running the eject script will remove the single build dependency from your project.
That means it will copy the configuration files and the transitive dependencies (e.g. Webpack, Babel, etc.) as dependencies in
the package.json file. If you do that, you'll have to ensure that the dependencies are installed before building your project.
After running the eject, commands like npm start and npm run build will still work, but they will point to the copied scripts so you
can tweak them. It won't be possible to run it again since all scripts will be available except the eject one.
↥
Q. How to put React in production mode?
Create a simple hello-world-app using create-react-app.
npxcreate-react-app hello-world-app
Modify the App.jsfile as shown below.
import'./App.css';
function App() {
return (
<div className="App">
<header className="App-header">
<h1>Hello world app</h1>
</header>
</div>
);
}
exportdefault App;
Run the app local server by running the following command
npmstart
On the local server (http://localhost:3000) you can see a simple React app displaying a "hello world" message. The next step is to
make this app production-ready for deployment. Inside the root directory run the following command:
npm runbuild
This creates a build directory inside the root directory, which bundles your React app and minifies it into simple HTML, CSS, and
JavaScript files. This build folder serves your app via a simple en point, index.html, where your entire React app resides. Running
your app via a remote server means running this index.html file on the server.
↥
Q. What are the common folder structures for React?
React doesn't have opinions on how you put files into folders. That said there are a few common approaches popular in the
ecosystem you may want to consider.
1. Grouping by features or routes:
One common way to structure projects is to locate CSS, JS, and tests together inside folders grouped by feature or route.
common/
Avatar.js
Avatar.css
APIUtils.js
APIUtils.test.js
feed/
index.js
Feed.js
Feed.css
FeedStory.js
FeedStory.test.js
FeedAPI.js
profile/
index.js
Profile.js
ProfileHeader.js
ProfileHeader.css
ProfileAPI.js
2. Grouping by file type:
Another popular way to structure projects is to group similar files together, for example:
api/
APIUtils.js
APIUtils.test.js
ProfileAPI.js
UserAPI.js
components/
Avatar.js
Avatar.css
Feed.js
Feed.css
FeedStory.js
FeedStory.test.js
Profile.js
ProfileHeader.js
ProfileHeader.css
↥
Q. What are the popular React-specific linter?
1. ESLint:
ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React
is an npm package called eslint-plugin-react.
npminstall -g eslint-plugin-react
This will install the plugin we need, and in our ESLint config file, we just need a few extra lines.
"extends": [
"eslint:recommended",
"plugin:react/recommended"
]
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test--env=jsdom",
"eject": "react-scripts eject",
"lint": "eslint src/**/*.js src/**/*.jsx"
}
2. eslint-plugin-jsx-a11y:
It will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt text
and tabindex, for example, will not be picked up by regular plugins.
↥
Q. What is the browser support for react applications?
By default, Create React App generated project supports all modern browsers. Support for Internet Explorer 9, 10, and 11 requires
polyfills. For a set of polyfills to support older browsers, use react-app-polyfill.
The browserslist configuration controls the outputted JavaScript so that the emitted code will be compatible with the browsers
specified.
Example:
// package.json
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
}
↥
Q. Explain the use of Webpack and Babel in React?
1. Babel:
Babel is a JS transpiler that converts new JS code into old ones. It is a very flexible tool in terms of transpiling. One can easily add
presets such as es2015, es2016, es2017, or env; so that Babel compiles them to ES5. Babel allows us to have a clean, maintainable code
using the latest JS specifications without needing to worry about browser support.
2. Webpack:
Webpack is a modular build tool that has two sets of functionality — Loaders and Plugins. Loaders transform the source code of a
module. For example, style-loader adds CSS to DOM using style tags. sass-loader compiles SASS files to CSS. babel-loader transpiles
JS code given the presets. Plugins are the core of Webpack. They can do things that loaders can't. For example, there is a plugin
called UglifyJS that minifies and uglifies the output of webpack.
3. create-react-app:
create-react-app, a popular tool that lets you set up a React app with just one command. You don't need to get your hands dirty
with Webpack or Babel because everything is preconfigured and hidden away from you.
Example: Quick Start
npx create-react-app my-app
cd my-app
npm start
↥
Q. What is the difference between ReactDOM and
React?
The ReactDOM module exposes DOM-specific methods, while React has the core tools intended to be shared by React on different
platforms (e.g. React Native).
React package contains: React.createElement(), React.createClass(), React.Component(), React.PropTypes(),React.Children()
ReactDOM package contains: ReactDOM.render(), ReactDOM.unmountComponentAtNode(), ReactDOM.findDOMNode(), and react-
dom/server that including: ReactDOMServer.renderToString() and ReactDOMServer.renderToStaticMarkup().
Example:
/**
* React vs ReactDOM
*/
import { createRoot } from "react-dom/client";
exportdefault function App() {
return <h1>Hello React</h1>;
}
constrootElement = document.getElementById("root");
constroot = createRoot(rootElement);
root.render(<App />);
⚝
↥
Q. What is ReactDOM?
ReactDOM is a package that provides DOM specific methods that can be used at the top level of a web app to enable an efficient
way of managing DOM elements of the web page.
ReactDOM provides the developers with an API containing the following methods
render()
findDOMNode()
unmountComponentAtNode()
hydrate()
createPortal()
1. render():
ReactDOM.render(element, container, callback)
Render a React element into the DOM in the supplied container and return a reference to the component (or returns null for
stateless components). If the React element was previously rendered into container, this will perform an update on it and only
mutate the DOM as necessary to reflect the latest React element. If the optional callback is provided, it will be executed after the
component is rendered or updated.
⚝
2. hydrate():
ReactDOM.hydrate(element, container, callback)
This method is equivalent to the render() method but is implemented while using server-side rendering. This function attempts to
attach event listeners to the existing markup and returns a reference to the component or null if a stateless component was
rendered.
⚝
3. unmountComponentAtNode():
ReactDOM.unmountComponentAtNode(container)
This function is used to unmount or remove the React Component that was rendered to a particular container. It returns true if a
component was unmounted and false if there was no component to unmount.
⚝
4. findDOMNode():
ReactDOM.findDOMNode(component)
If this component has been mounted into the DOM, this returns the corresponding native browser DOM element. This method is
useful for reading values out of the DOM, such as form field values and performing DOM measurements.
⚝
5. createPortal():
ReactDOM.createPortal(child, container)
createPortal allow us to render a component into a DOM node that resides outside the current DOM hierarchy of the parent
component.
⚝
↥
# 3. REACT JSX
Q. What is JSX?
JSX ( JavaScript Expression ) allows us to write HTML elements in JavaScript and place them in the DOM without
any createElement()orappendChild() methods. JSX converts HTML tags into react elements. React uses JSX for templating instead
of regular JavaScript. It is not necessary to use it, however, following are some pros that come with it.
It is faster because it performs optimization while compiling code to JavaScript.
It is also type-safe and most of the errors can be caught during compilation.
It makes it easier and faster to write templates.
When JSX compiled, they actually become regular JavaScript objects. For instance, the code below:
consthello = <h1 className = "greet"> Hello World </h1>
will be compiled to
consthello = React.createElement {
type: "h1",
props: {
className: "greet",
children: "Hello World"
}
}
Example:
exportdefault function App() {
return (
<div className="App">
<h1>Hello World!</h1>
</div>
);
}
⚝
↥
Q. How JSX prevents Injection Attacks?
React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that's
not explicitly written in your application. Everything is converted to a string before being rendered.
For example, you can embed user input as below,
exportdefaultclassJSXInjectionExample extends React.Component {
constructor(props) {
super(props);
this.state = {
userContent: `JSX prevents Injection Attacks Example
<script src="http://example.com/malicious-script.js></script>`
};
}
render() {
return (
<div>User content: {this.state.userContent}</div>
);
}
}
// Output
User content: JSX prevents Injection Attacks Example
<scriptsrc="http://example.com/malicious-script.js></script>
⚝
↥
Q. What are the benefits of new JSX transform?
The React 17 release provides support for a new version of the JSX transform. There are three major benefits of new JSX transform,
It enables you to use JSX without having to import React.
The compiled output relatively improves the bundle size.
The future improvements provides the flexibility to reduce the number of concepts to learn React.
↥
Q. Is it possible to use React without rendering HTML?
It is possible with latest version (>=16.2). Below are the possible options:
render() {
return false
}
render() {
return null
}
render() {
return []
}
render() {
return <React.Fragment></React.Fragment>
}
render() {
return <></>
}
Note that React can also run on the server side so, it will be possible to use it in such a way that it doesn't involve any DOM
modifications (but maybe only the virtual DOM computation).
↥
Q. How to write comments in React and JSX?
Writing comments in React components can be done just like comment in regular JavaScript classes and functions.
React comments:
function App() {
// Single line Comment
/*
* multi
* line
* comment
**/
return (
<h1>My Application</h1>
);
}
JSX comments:
exportdefault function App() {
return (
<div>
{/* A JSX comment */}
<h1>My Application</h1>
</div>
);
}
↥
Q. How to add custom DOM attributes in JSX?
Custom attributes are supported natively in React 16. This means that adding a custom attribute to an element is now as simple as
adding it to a render function, like so:
Example:
// 1. Custom DOM Attribute
render() {
return (
<div custom-attribute="some-value"/>
);
}
// 2. Data Attribute ( starts with "data-" )
render() {
return (
<div data-id="10" />
);
}
// 3. ARIA Attribute ( starts with "aria-" )
render() {
return (
<button aria-label="Close"onClick={onClose} />
);
}
⚝
↥
Q. How many outermost elements can be there in a
JSX expression?
A JSX expression must have only one outer element. For Example:
constheadings = (
<div id = "outermost-element">
<h1>I am a heading </h1>
<h2>I am also a heading</h2>
</div>
)
↥
Q. How to loop inside JSX?
You can simply use Array.prototype.map with ES6 arrow function syntax.
Example:
/**
* Loop inside JSX
*/
constanimals = [
{ id: 1, animal: "Dog" },
{ id: 2, animal: "Bird" },
{ id: 3, animal: "Cat" },
{ id: 4, animal: "Mouse" },
{ id: 5, animal: "Horse" }
];
exportdefault function App() {
return (
<ul>
{animals.map((item) => (
<li key={item.id}>{item.animal}</li>
))}
</ul>
);
}
⚝
↥
Q. How do you print false values in JSX?
In React, boolean values (true and false),null, and undefined are valid children, but these values will not be rendered in UI if you
put them directly inside {} in JSX.
For example, all these JSX expressions will result in the same empty div:
<div/>
<div></div>
<div>{false}</div>
<div>{null}</div>
<div>{undefined}</div>
<div>{true}</div>
If you want a value like false, true, null, or undefined to show in the output, you have to convert it to a string first.
<div>{String(true)}</div>
<div>{String(false)}</div>
<div>{String(undefined)}</div>
<div>{String(null)}</div>
In the output, this will render true, false, undefined, and null respectively.
⚝
↥
Q. How to use React label element?
If you to render a <label> element bound to a text input using the standard for attribute, then it produces HTML missing that
attribute and prints a warning to the console.
<labelfor={'user'}>{'User'}</label>
<inputtype={'text'} id={'user'} />
Since for is a reserved keyword in JavaScript, use htmlFor instead.
<labelhtmlFor={'user'}>{'User'}</label>
<inputtype={'text'} id={'user'} />
↥
Q. How to use InnerHtml in React?
The innerHTML is risky because it is easy to expose users to a cross-site scripting (XSS) attack. React
providesdangerouslySetInnerHTML as a replacement for innerHTML. It allows to set HTML directly from React by
using dangerouslySetInnerHTML and passing an object with a __html key that holds HTML.
Example:
function App() {
return (
<div
dangerouslySetInnerHTML={{
__html: "<h2>This text is set using dangerouslySetInnerHTML</h2>"
}}
></div>
);
}
⚝
↥
Q. How to show and hide elements in React
1. Returning Null:
constAddToCart = ({ available }) => {
if (!available) return null
return (
<div className="full tr">
<button className="product--cart-button">Add to Cart</button>
</div>
)
}
2. Ternary Display:
When you need to control whether one element vs. another is displayed, or even one element vs. nothing at all (null), you can use
the ternary operator embedded inside of a larger portion of JSX.
<divclassName="half">
<p>{description}</p>
{remaining === 0 ? (
<spanclassName="product-sold-out">Sold Out</span>
) : (
<spanclassName="product-remaining">{remaining} remaining</span>
)}
</div>
In this case, if there are no products remaining, we will display "Sold Out"; otherwise we will display the number of products
remaining.
3. Shortcut Display:
It involves using a conditional inside of your JSX that looks like checkIfTrue && <span>display if true</span>. Because if
statements that use && operands stop as soon as they find the first value that evaluates to false, it won't reach the right side (the JSX)
if the left side of the equation evaluates to false.
<h2>
<spanclassName="product--title__large">{nameFirst}</span>
{nameRest.length > 0 && (
<spanclassName="product--title__small">{nameRest.join(" ")}</span>
)}
</h2>
4. Using Style Property:
<divstyle={{ display: showInfo ? "block" : "none"}}>info</div>
↥
# 4. REACT COMPONENTS
Q. What are React components?
Components are the building blocks of any React app and a typical React app will have many of these. Simply put, a component is a
JavaScript class or function that optionally accepts inputs i.e. properties(props) and returns a React element that describes how a
section of the UI (User Interface) should appear.
In React, a Stateful Component is a component that holds some state. A Stateless component, by contrast, has no state. Note that
both types of components can use props.
1. Stateless Component:
importReact from 'react'
constExampleComponent = (props) => {
return <h1>Stateless Component - {props.message}</h1>;
};
constApp = () => {
constmessage = 'React Interview Questions'
return (
<div>
<ExampleComponent message={message} />
</div>
);
};
exportdefault App;
The above example shows a stateless component named ExampleComponent which is inserted in the <App/> component.
The ExampleComponent just comprises of a <h1> element. Although the Stateless component has no state, it still receives data via
props from a parent component.
2. Stateful Component:
importReact, { useState } from'react'
constExampleComponent = (props) => {
const [email, setEmail] = useState(props.defaultEmail)
constchangeEmailHandler = (e) => {
setEmail(e.target.value)
}
return (
<input type="text" value={email} onChange={changeEmailHandler} />
);
}
constApp = () => {
constdefaultEmail = "[email protected]om"
return (
<div>
<ExampleComponent defaultEmail={defaultEmail} />
</div>
);
};
exportdefault App;
The above example shows a stateful component named ExampleComponent which is inserted in the <App/> component.
The ExampleComponent contains a <input>. First of all, in the ExampleComponent, we need to assign defaultEmail by props to a
local stateby auseState() hook in ExampleComponent.
Next, we have to pass emailtovalue property of a input tag and pass a function changeEmailHandler to an onChange() event for a
purpose keeping track of the current value of the input.
↥
Q. What is the difference between Component and
Container in React?
The presentational components are concerned with the look, container components are concerned with making things work.
For example, this is a presentational component. It gets data from its props, and just focuses on showing an element
/**
* Presentational Component
*
**/
constUsers = props=> (
<ul>
{props.users.map(user=> (
<li>{user}</li>
))}
</ul>
)
On the other hand this is a container component. It manages and stores its own data, and uses the presentational component to
display it.
/**
* Container Component
*
**/
classUsersContainerextends React.Component {
constructor() {
this.state = {
users: []
}
}
componentDidMount() {
axios.get('/users').then(users =>
this.setState({ users: users }))
)
}
render() {
return <Users users={this.state.users} />
}
}
↥
Q. How to import and export components using
React.js?
// Importing combination
importReact, { Component } from 'react';
importReactDOMfrom 'react-dom';
// Wrapping components with braces if no default exports
import { Button } from './Button';
// Default exports ( recommended )
import Button from './Button';
classDangerButton extends Component {
render()
{
return <Button color="red" />;
}
}
exportdefault DangerButton;
// or export DangerButton;
By using default you express that's going to be member in that module which would be imported if no specific member name is
provided. You could also express you want to import the specific member called DangerButton by doing so: import { DangerButton
} from './comp/danger-button'; in this case, no default is needed
↥
Q. What is difference between declarative and
imperative in React.js?
1. Imperative programming:
It is a programming paradigm that uses statements that change a program's state.
conststring = "Hi there , I'm a web developer";
letremoveSpace = "";
for (let i = 0; i < i.string.length; i++) {
if (string[i] ===" ") removeSpace+= "-";
else removeSpace+=string[i];
}
console.log(removeSpace);
In this example, we loop through every character in the string, replacing spaces as they occur. Just looking at the code, it doesn't say
much. Imperative requires lots of comments in order to understand code. Whereas in the declarative program, the syntax itself
describes what should happen and the details of how things happen are abstracted way.
2. Declarative programming:
It is a programming paradigm that expresses the logic of a computation without describing its control flow.
Example:
const { render } = ReactDOM
constWelcome = () => (
<div id="App">
//your HTML code
//your react components
</div>
)
render(
<App />,
document.getElementById('root')
)
React is declarative. Here, the Welcome component describes the DOM that should be rendered. The render function uses the
instructions declared in the component to build the DOM, abstracting away the details of how the DOM is to be rendered. We can
clearly see that we want to render our Welcome component into the element with the ID of 'target'.
↥
Q. What is the difference between Element and
Component?
1. React Element:
It is a simple object that describes a DOM node and its attributes or properties. It is an immutable description object and you can
not apply any methods on it.
constelement = <h1>React Element Example!</h1>;
ReactDOM.render(element, document.getElementById('app'));
2. React Component:
It is a function or class that accepts an input and returns a React element. It has to keep references to its DOM nodes and to the
instances of the child components.
function Message() {
return <h2>React Component Example!</h2>;
}
ReactDOM.render(<Message />, document.getElementById('app'));
↥
Q. How to conditionally render components in react?
Conditional rendering is a term to describe the ability to render different user interface (UI) markup if a condition is true or false. In
React, it allows us to render different elements or components based on a condition.
1. Element Variables:
You can use variables to store elements. This can help you conditionally render a part of the component while the rest of the output
doesn't change.
functionLogInComponent(props) {
constisLoggedIn = props.isLoggedIn;
if (isLoggedIn) {
return <UserComponent/>;
}
return <GuestComponent />;
}
ReactDOM.render(
<LogInComponent isLoggedIn={false} />,
document.getElementById('root')
);
2. Inline If-Else with Conditional Operator:
render() {
constisLoggedIn = this.state.isLoggedIn;
return (
<div>
{isLoggedIn
? <LogoutButton onClick={this.handleLogoutClick} />
: <LoginButtononClick={this.handleLoginClick} />
}
</div>
);
}
⚝
↥
Q. How to conditionally add attributes to React
components?
Inline conditionals in attribute props
/**
* Conditionally add attributes
*/
importReact from "react";
exportdefault function App() {
const [mood] = React.useState("happy");
constgreet = () => alert("Hi there! :)");
return (
<buttononClick={greet} disabled={"happy" === mood? false : true}>
Say Hi
</button>
);
}
⚝
↥
Q. How would you prevent a component from
rendering?
React shouldComponentUpdate() is a performance optimization method, and it tells React to avoid re-rendering a component,
even if state or prop values may have changed. This method only used when a component will stay static or pure.
The React shouldComponentUpdate() method return true if it needs to re-render or false to avoid being re-render.
Syntax:
shouldComponentUpdate(nextProps, nextState){ }
Example:
/**
* Prevent a component from rendering
*/
exportdefaultclass App extends React.Component {
constructor() {
super();
this.state = {
countOfClicks: 0
};
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
this.setState({
countOfClicks: this.state.countOfClicks + 1
});
}
shouldComponentUpdate(nextProps, nextState) {
console.log("this.state.countOfClicks", this.state.countOfClicks);
console.log("nextState.countOfClicks",nextState.countOfClicks);
return true;
}
render() {
return (
<div>
<h2>shouldComponentUpdate Example</h2>
<p>Count of clicks: <b>{this.state.countOfClicks}</b></p>
<button onClick={this.handleClick}>CLICK ME</button>
</div>
);
}
}
⚝
↥
Q. When would you use StrictMode component in
React?
The StrictModeis a tool for highlighting potential problems in an application. Like Fragment, StrictMode does not render any visible
UI. It activates additional checks and warnings for its descendants.
Strict mode checks are run in development mode only; they do not impact the production build.
Example:
/**
* StrictMode
*/
import { StrictMode } from "react";
importMyComponent from "./MyComponent";
exportdefault function App() {
return (
<StrictMode>
<MyComponent/>
</StrictMode>
);
}
React StrictMode, in order to be efficient and avoid potential problems by any side-effects, needs to trigger some methods and
lifecycle hooks twice. These are:
Class component constructor() method
The render() method
setState() updater functions (the first argument)
The static getDerivedStateFromProps() lifecycle
React.useState() function
Benefits of StrictMode:
Identifying components with unsafe lifecycles
Warning about legacy string ref API usage
Warning about deprecated findDOMNode usage
Detecting unexpected side effects
Detecting legacy context API
↥
Q. Why to avoid using setState() after a component
has been unmounted?
Calling setState() after a component has unmounted will emit a warning. The "setState warning" exists to help you catch bugs,
because callingsetState() on an unmounted component is an indication that your app/component has somehow failed to clean up
properly.
Specifically, callingsetState() in an unmounted component means that your app is still holding a reference to the component after
the component has been unmounted - which often indicates a memory leak.
Example:
/**
* setState() in unmounted component
*/
importReact, { Component } from "react";
importaxios from "axios";
exportdefaultclass App extends Component {
_isMounted = false; // flag to check Mounted
constructor(props) {
super(props);
this.state = {
news: []
};
}
componentDidMount() {
this._isMounted = true;
axios
.get("https://hn.algolia.com/api/v1/search?query=react")
.then((result) => {
if (this._isMounted) {
this.setState({
news: result.data.hits
});
}
});
}
componentWillUnmount() {
this._isMounted = false;
}
render() {
return (
<ul>
{this.state.news.map((topic) => (
<li key={topic.objectID}>{topic.title}</li>
))}
</ul>
);
}
}
Here, even though the component got unmounted and the request resolves eventually, the flag in component will prevent to set the
state of the React component after it got unmounted.
⚝
↥
Q. What is Lifting State Up in ReactJS?
The common approach to share state between two components is to move the state to common parent of the two components.
This approach is called as lifting state up in React.js. With the shared state, changes in state reflect in relevant components
simultaneously.
Example:
The App component containing PlayerContent and PlayerDetails component. PlayerContent shows the player name buttons.
PlayerDetails shows the details of the in one line.
The app component contains the state for both the component. The selected player is shown once we click on the one of the player
button.
/**
* Lifting State Up
*/
importReact from "react";
importPlayerContent from "./PlayerContent";
importPlayerDetails from "./PlayerDetails";
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.state = { selectedPlayer: [0, 0], playerName: "" };
this.updateSelectedPlayer = this.updateSelectedPlayer.bind(this);
}
updateSelectedPlayer(id, name) {
const arr = [0, 0, 0, 0];
arr[id] = 1;
this.setState({
playerName: name,
selectedPlayer: arr
});
}
render() {
return (
<div>
<PlayerContent
active={this.state.selectedPlayer[0]}
clickHandler={this.updateSelectedPlayer}
id={0}
name="Player 1"
/>
<PlayerContent
active={this.state.selectedPlayer[1]}
clickHandler={this.updateSelectedPlayer}
id={1}
name="Player 2"
/>
<PlayerDetailsname={this.state.playerName} />
</div>
);
}
}
/**
* PlayerContent
*/
importReact, { Component } from "react";
exportdefaultclassPlayerContent extends Component {
render() {
return (
<button
onClick={()=> {
this.props.clickHandler(this.props.id, this.props.name);
}}
style={{ color: this.props.active ? "red" : "blue"}}
>
{this.props.name}
</button>
);
}
}
/**
* PlayerDetails
*/
importReact, { Component } from "react";
exportdefaultclassPlayerDetails extends Component {
render() {
return <h2>{this.props.name}</h2>;
}
}
⚝
↥
Q. What is "Children" in React?
In React, children refer to the generic box whose contents are unknown until they're passed from the parent component. Children
allows to pass components as data to other components, just like any other prop you use.
The special thing about children is that React provides support through its ReactElement API and JSX. XML children translate
perfectly to React children!
Example:
/**
* Children in React
*/
constPicture = (props) => {
return (
<div>
<img src={props.src}/>
{props.children}
</div>
)
}
This component contains an <img> that is receiving some props and then it is displaying {props.children}. Whenever this
component is invoked {props.children} will also be displayed and this is just a reference to what is between the opening and
closing tags of the component.
/**
* App.js
*/
render() {
return (
<div className='container'>
<Picture key={picture.id} src={picture.src}>
{/** what is placed here is passed as props.children **/}
</Picture>
</div>
)
}
↥
Q. What is Compound Components in React?
A compound component is a type of component that manages the internal state of a feature while delegating control of the
rendering to the place of implementation opposed to the point of declaration. They provide a way to shield feature specific logic
from the rest of the app providing a clean and expressive API for consuming the component.
Internally they are built to operate on a set of data that is passed in through children instead of props. Behind the scenes they make
use of React's lower level API such as React.children.map(), and React.cloneElement(). Using these methods, the component is able
to express itself in such a way that promotes patterns of composition and extensibility.
Example:
function App() {
return (
<Menu>
<MenuButton>
Actions <span aria-hidden>▾</span>
</MenuButton>
<MenuList>
<MenuItem onSelect={()=> alert('Download')}>Download</MenuItem>
<MenuItem onSelect={()=> alert('Copy')}>Create a Copy</MenuItem>
<MenuItem onSelect={()=> alert('Delete')}>Delete</MenuItem>
</MenuList>
</Menu>
)
}
In this example, the <Menu> establishes some shared implicit state. The <MenuButton>, <MenuList>, and <MenuItem> components each
access and/or manipulate that state, and it's all done implicitly. This allows you to have the expressive API you're lookingfor.
↥
# 4.1. FUNCTIONAL COMPONENTS
Q. What are functional components in react?
A React functional component is a simple JavaScript function that accepts props and returns a React element. It also referred
asstateless components as it simply accept data and display them in some form.
After the introduction of React Hooks, writing functional components has become the standard way of writing React components in
modern applications.
Example:
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
constelement = <Welcome name="World!" />;
ReactDOM.render(
element,
document.getElementById('root')
);
⚝
↥
# 4.2. CLASS COMPONENTS
Q. What are class components in react?
The class component, a stateful/container component, is a regular ES6 class that extends the component class of the React library. It
is called a stateful component because it controls how the state changes and the implementation of the component logic. Aside
from that, they have access to all the different phases of a React lifecycle method.
Example:
classWelcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}
constelement = <Welcome name="World!" />;
ReactDOM.render(
element,
document.getElementById('root')
);
⚝
↥
Q. What is the recommended ordering of methods in
class component?
static methods
constructor()
getChildContext()
componentWillMount()
componentDidMount()
componentWillReceiveProps()
shouldComponentUpdate()
componentWillUpdate()
componentDidUpdate()
componentWillUnmount()
click handlers or event handlers like onClickSubmit()oronChangeDescription()
getter methods for render like getSelectReason()orgetFooterContent()
optional render methods like renderNavigation()orrenderProfilePicture()
render()
↥
Q. How to create a dynamic table in react?
/**
* Generate dynamic table in React
*/
classTable extendsReact.Component {
constructor(props) {
super(props)
this.state = {
employees: [
{ id: 20, name: 'Sarvesh Date', email: 'sarvesh@email.com' },
{ id: 30, name: 'Diksha Meka', email: 'diks[email protected]' }
]
}
}
renderTableHeader() {
let header = Object.keys(this.state.employees[0])
return header.map((key, index) => {
return <thkey={index}>{key.toUpperCase()}</th>
})
}
renderTableData() {
return this.state.employees.map((employee, index) => {
const { id, name, age,email } = employee
return (
<tr key={id}>
<td>{id}</td>
<td>{name}</td>
<td>{email}</td>
</tr>
)
})
}
render() {
return (
<div>
<h1 id='title'>React Dynamic Table</h1>
<table id='employees'>
<tbody>
<tr>{this.renderTableHeader()}</tr>
{this.renderTableData()}
</tbody>
</table>
</div>
)
}
}
⚝
↥
Q. How to prevent component from rendering in
React?
You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render
component.
In the example below, the <WarningBanner /> is rendered depending on the value of the prop called warn. If the value of the prop
isfalse, then the component does not render:
function WarningBanner(props) {
if (!props.warn) {
return null;
}
return (
<div className="warning">
Warning!
</div>
);
}
classPage extends React.Component {
constructor(props) {
super(props);
this.state = {showWarning: true};
this.handleToggleClick = this.handleToggleClick.bind(this);
}
handleToggleClick() {
this.setState(state => ({
showWarning: !state.showWarning
}));
}
render() {
return (
<div>
{ /* Prevent component render if value of the prop is false */}
<WarningBannerwarn={this.state.showWarning} />
<button onClick={this.handleToggleClick}>
{this.state.showWarning ? 'Hide' : 'Show'}
</button>
</div>
);
}
}
ReactDOM.render(
<Page/>,
document.getElementById('root')
);
⚝
↥
Q. How do you set a timer to update every second?
Using setInterval() inside React components allows us to execute a function or some code at specific intervals. A function or block
of code that is bound to an interval executes until it is stopped. To stop an interval, we can use the clearInterval() method.
Example:
classClock extendsReact.Component {
constructor(props) {
super(props)
this.state = {
time: new Date().toLocaleString()
}
}
componentDidMount() {
this.intervalID = setInterval(
() => this.tick(),
1000
)
}
componentWillUnmount() {
clearInterval(this.intervalID)
}
tick() {
this.setState({
time: new Date().toLocaleString()
})
}
render() {
return (
<p className="App-clock">
The time is {this.state.time}.
</p>
)
}
}
↥
Q. Differentiate between stateful and stateless
components?
Stateful and stateless components have many different names. They are also known as:
– Container vs Presentational components
– Smart vs Dumb components
The literal difference is that one has state, and the other does not. That means the stateful components are keeping track of
changing data, while stateless components print out what is given to them via props, or they always render the same thing.
Example: Stateful/Container/Smart component
classWelcome extends React.Component {
render() {
return <h1>This is a React Class Component</h1>;
}
}
Example: Stateless/Presentational/Dumb component
function welcome(props) {
return <h1>This is a React Functional Component</h1>;
}
Class Components
Functional Components
Class components need to extend the component from
"React.Component" and create a render function that returns the
required element.
Functional components are like normal functions which
take "props" as the argument and return the required
element.
They are also known as stateful components.
They are also known as stateless components.
They implement logic and the state of the component.
They accept some kind of data and display it in the UI.
Lifecycle methods can be used inside them.
Lifecycle methods cannot be used inside them.
It needs to store state therefore constructors are used.
Constructors are not used in it.
It has to have a "render()" method inside that.
It does not require a render method.
↥
Q. What is the purpose of using super constructor with
props argument?
The super() keyword is used to call the parent constructor. super(props) would pass props to the parent constructor.
/**
* super constructor
*/
classApp extends React.Component {
constructor(props) {
super(props)
this.state = {}
}
// React says we have to define render()
render() {
return <div>Hello world</div>
}
}
exportdefaultApp
Here, super(props) would call the React.Component constructor passing in props as the argument.
↥
Q. What is the difference between Element,
Component and Component instance in React?
1. React Elements:
A React Element is just a plain old JavaScript Object without own methods. It has essentially four properties:
type: a String representing an HTML tag or a reference referring to a React Component
key: a String to uniquely identify an React Element
ref: a reference to access either the underlying DOM node or React Component Instance)
props: (properties Object)
A React Element is not an instance of a React Component. It is just a simplified "description" of how the React Component Instance
to be created should look like.
2. React Components and React Component Instances:
A React Component is used by extending React.Component. If a React Component is instantiated it expects a props Object and
returns an instance, which is referred to as a React Component Instance.
A React Component can contain state and has access to the React Lifecycle methods. It must have at least a render method, which
returns a React Element(-tree) when invoked.
Example:
/**
* React Component Instances
*/
importReact from 'react'
importReactDOMfrom 'react-dom'
classMyComponent extends React.Component {
constructor(props) {
super(props)
console.log('This is a component instance:' + this)
}
render() {
const another_element = <div>Hello, World!</div>
console.log('This is also an element:' + another_element)
return another_element
}
}
console.log('This is a component:' + MyComponent)
constelement = <MyComponent/>
console.log('This is an element:' + element)
ReactDOM.render(element, document.getElementById('root'));
↥
Q. What does shouldComponentUpdate do and why is
it important?
The shouldComponentUpdate() method allows Component to exit the Update life cycle if there is no reason to apply a new render.
React does not deeply compare props by default. When propsorstate is updated React assumes we need to re-render the content.
The default implementation of this function returns true so to stop the re-render you need to return false here:
shouldComponentUpdate(nextProps, nextState) {
console.log(nextProps, nextState)
console.log(this.props, this.state)
return false
}
Preventing unnecessary renders
The shouldComponentUpdate() method is the first real life cycle optimization method that we can leverage in React. It checks the
current props and state, compares it to the next props and state and then returns true if they are different, or false if they are the
same. This method is not called for the initial render or when forceUpdate() is used.
↥
Q. What is the purpose of render() function in React?
The React class components uses render() function. It is used to update the UI.
Purpose of render():
React renders HTML to the web page by using a function called render().
The purpose of the function is to display the specified HTML code inside the specified HTML element.
In the render() method, we can read props and state and return our JSX code to the root component of our app.
In the render() method, we cannot change the state, and we cannot cause side effects ( such as making an HTTP request to
the webserver).
/**
* render() function
*
* React v18.0.0
*/
importReact from "react";
import { createRoot } from "react-dom/client";
classApp extends React.Component {
render() {
return <h1>Render() Method Example</h1>;
}
}
constcontainer = document.getElementById("root");
constroot = createRoot(container);
root.render(<App />);
⚝
↥
# 4.2.1. REACT LIFECYCLE
Q. What are the different phases of React component
lifecycle?
React provides several methods that notify us when certain stage of this process occurs. These methods are called the component
lifecycle methods and they are invoked in a predictable order. The lifecycle of the component is divided into four phases.
constructor()
getDerivedStateFromProps()
render()
componentDidMount()
2. Updating:
The next phase in the lifecycle is when a component is updated. A component is updated whenever there is a change in the
component's state or props.
React has five built-in methods that gets called, in this order, when a component is updated:
getDerivedStateFromProps()
shouldComponentUpdate()
render()
getSnapshotBeforeUpdate()
componentDidUpdate()
3. Unmounting:
The next phase in the lifecycle is when a component is removed from the DOM, or unmounting as React likes to call it.
componentWillUnmount()
↥
Q. How to make component to perform an action only
once when the component initially rendered?
1. Using Class Component:
The componentDidMount() lifecycle hook can be used with class components. Any actions defined within
a componentDidMount() lifecycle hook are called only once when the component is first mounted.
Example:
classHomepage extends React.Component {
componentDidMount() {
trackPageView('Homepage')
}
render() {
return <div>Homepage</div>
}
}
2. Using Function Component:
The useEffect() hook can be used with function components. The useEffect() hook is more flexible than the lifecycle methods used
for class components. It receives two parameters:
The first parameter it takes is a callback function to be executed.
The optional second parameter it takes is an array containing any variables that are to be tracked.
The value passed as the second argument controls when the callback is executed:
If the second parameter is undefined, the callback is executed every time that the component is rendered.
If the second parameter contains an array of variables, then the callback will be executed as part of the first render cycle
and will be executed again each time an item in the array is modified.
If the second parameter contains an empty array, the callback will be executed only once as part of the first render cycle.
Example:
constHomepage = ()=> {
useEffect(() => {
trackPageView('Homepage')
}, [])
return <div>Homepage</div>
}
↥
Q. What is the typical pattern for rendering a list of
components from an array of data?
The usual pattern for rendering lists of components often ends with delegating all of the responsibilities of each child component to
the entire list container component. But with a few optimizations, we can make a change in a child component not cause the parent
component to re-render.
Example: using custom shouldComponentUpdate()
/**
* shouldComponentUpdate()
*/
classAnimalTable extends React.Component<Props, never> {
shouldComponentUpdate(nextProps: Props) {
return !nextProps.animalIds.equals(this.props.animalIds);
}
...
Here, shouldComponentUpdate() will return false if the props its receiving are equal to the props it already has. And because the
AnimalTable is receiving just a List of string IDs, a change in the adoption status won't cause AnimalTable to receive a different set of
IDs.
↥
Q. What is difference between useEffect() vs
componentDidMount()?
In react when we use class based components we get access to lifecycle methods ( like componentDidMount(),
`componentDidUpdate(), etc ). But when we want use a functional component and also we want to use lifecycle methods, then using
useEffect() we can implement those lifecycle methods.
1. componentDidMount():
The componentDidMount() and useEffect() run after the mount. However useEffect() runs after the paint has been committed to the
screen as opposed to before. This means we would get a flicker if needed to read from the DOM, then synchronously set state to
make new UI.
The useLayoutEffect() was designed to have the same timing as componentDidMount(). So useLayoutEffect(fn, []) is a much
closer match to componentDidMount() than useEffect(fn, []) -- at least from a timing standpoint.
/**
* componentDidMount() in Class Component
*/
importReact, { Component } from "react";
exportdefaultclassSampleComponent extends Component {
componentDidMount() {
// code to run on component mount
}
render() {
return <>componentDidMount Example</>;
}
}
2. useEffect():
/**
* useEffect() in Functional Component
*/
importReact, { useEffect } from "react";
constSampleComponent = () => {
useEffect(() => {
// code to run on component mount
}, []);
return <>useEffect Example</>;
};
exportdefault SampleComponent;
When useEffect() is used to get data from server.
The first argument is a callback that will be fired after browser layout and paint. Therefore it does not block the painting
process of the browser.
The second argument is an array of values (usually props).
If any of the value in the array changes, the callback will be fired after every render.
When it is not present, the callback will always be fired after every render.
When it is an empty list, the callback will only be fired once, similar to componentDidMount.
↥
Q. Why is a component constructor called only once?
React's reconciliation algorithmassumes that without any information to the contrary, if a custom component appears in the same
place on subsequent renders, it's the same component as before, so reuses the previous instance rather than creating a new one.
If you give each component a unique key prop, React can use the key change to infer that the component has actually been
substituted and will create a new one from scratch, giving it the full component lifecycle.
renderContent() {
if (this.state.activeItem === 'item-one') {
return (
<Content title="First" key="first" />
)
} else {
return (
<Content title="Second" key="second" />
)
}
}
↥
Q. What is difference between componentDidMount()
and componentWillMount()?
componentDidMount()
The componentDidMount() is executed after the first render only on the client side. This is where AJAX requests and DOM or state
updates should occur. This method is also used for integration with other JavaScript frameworks and any functions with delayed
execution such assetTimeout()orsetInterval().
Example:
importReact, { Component } from 'react'
classApp extends Component {
constructor(props) {
super(props)
this.state = {
data: 'Alex Belfort'
}
}
getData(){
setTimeout(() => {
console.log('Our data is fetched')
this.setState({
data: 'Hello Alex'
})
}, 1000)
}
componentDidMount() {
this.getData()
}
render() {
return (
<div>
{this.state.data}
</div>
)
}
}
exportdefaultApp
componentWillMount()
The componentWillMount() method is executed before rendering, on both the server and the client
side. componentWillMount() method is the least used lifecycle method and called before any HTML element is rendered. It is useful
when we want to do something programatically right before the component mounts.
Example:
importReact, { Component } from 'react'
classApp extends Component {
constructor(props) {
super(props)
this.state = {
data: 'Alex Belfort'
}
}
componentWillMount() {
console.log('First this called')
}
getData() {
setTimeout(() => {
console.log('Our data is fetched')
this.setState({
data: 'Hello Alex'
})
}, 1000)
}
componentDidMount() {
this.getData()
}
render() {
return (
<div>
{this.state.data}
</div>
)
}
}
exportdefaultApp
↥
Q. Is it good to use setState() in
componentWillMount() method?
Avoid async initialization incomponentWillMount().
componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state in this
method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method.
Make async calls for component initialization in componentDidMount() instead of componentWillMount()
functioncomponentDidMount() {
axios.get(`api/messages`)
.then((result) => {
const messages = result.data
console.log("COMPONENT WILL Mount messages : ", messages);
this.setState({
messages: [...messages.content]
})
})
}
↥
Q. How to use componentWillUnmount() with
Functional Components in React?
The useEffect() can be used to manage API calls, as well as implementing componentWillMount(),
and componentWillUnmount().
If we pass an empty array as the second argument, it tells useEffect to fire on component load. This is the only time it will fire.
importReact, { useEffect } from 'react';
constComponentExample => ()=> {
useEffect( () => {
// Anything in here is fired on component mount.
}, []);
}
If you add a return function inside the useEffect() function, it is triggered when a component unmounts from the DOM.
importReact, { useEffect } from 'react';
constComponentExample => ()=> {
useEffect(() => {
return () => {
// Anything in here is fired on component unmount.
}
}, [])
}
↥
# 4.3. PURE COMPONENTS
Q. What are Pure Components in React?
Pure Components in React are the components which do not re-renders when the value of state and props has been updated with
the same values. Pure Components restricts the re-rendering ensuring the higher performance of the Component.
Features of React Pure Components:
Prevents re-rendering of Component if props or state is the same
Takes care of shouldComponentUpdate() implicitly
State() and Props are Shallow Compared
Pure Components are more performant in certain cases
Example:
/**
* React Pure Component
*/
importReact from "react";
exportdefaultclass App extends React.PureComponent {
constructor() {
super();
this.state = {
userArray: [1, 2, 3, 4, 5]
};
// Here we are creating the new Array Object during setState using "Spread" Operator
setInterval(() => {
this.setState({
userArray: [...this.state.userArray, 6]
});
}, 1000);
}
render() {
return <b>Array Length is: {this.state.userArray.length}</b>;
}
}
⚝
↥
Q. What is difference between Pure Component vs
Component?
PureComponent is exactly the same as Component except that it handles the shouldComponentUpdate()method. The major
difference between React.PureComponent and React.Component is PureComponent does a shallow comparison on state change. It
means that when comparing scalar values it compares their values, but when comparing objects it compares only references. It helps
to improve the performance of the app.
A component rerenders every time its parent rerenders, regardless of whether the component's props and state have changed. On
the other hand, a pure component will not rerender if its parent rerenders, unless the pure component's props (or state) have
changed.
When to use React.PureComponent:
State/Props should be an immutable object
State/Props should not have a hierarchy
We should call forceUpdate when data changes
Example:
// Regular class component
classApp extends React.Component {
render() {
return <h1>Component Example !</h1>
}
}
// React Pure class component
classMessage extends React.Component {
render() {
return <h1>PureComponent Example !</h1>
}
}
↥
Q. What are the problems of using render props with
PureComponent?
If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison
will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this
issue by defining the render function as instance method.
Example:
classMouse extendsReact.PureComponent {
// Mouse Component...
}
classMouseTracker extends React.Component {
// Defined as an instance method, `this.renderTheCat` always
// refers to *same* function when we use it in render
renderTheCat(mouse) {
return <Cat mouse={mouse} />;
}
render() {
return (
<div>
<h1>Move the mouse around!</h1>
{/* define the render function as instance method */}
<Mouse render={this.renderTheCat} />
</div>
);
}
}
↥
Q. When to use PureComponent over Component?
We want to avoid re-rendering cycles of component when its props and state are not changed
The state and props of component are immutable
We do not plan to implement own shouldComponentUpdate() lifecycle method.
On the other hand, we should not use PureComponent() as a base component if:
props or state are not immutable
Plan to implement own shouldComponentUpdate() lifecycle method.
↥
# 4.4. HIGHER ORDER COMPONENTS
Q. What is Higher Order Components in React.js?
A Higher-Order Component(HOC) is a function that takes a component and returns a new component. It is the advanced
technique in React.js for reusing a component logic.
Higher-Order Components are not part of the React API. They are the pattern that emerges from React's compositional nature. The
component transforms props into UI, and a higher-order component converts a component into another component. The examples
of HOCs are Redux's connect and Relay's createContainer.
/**
* Higher Order Component
*/
importReact, { Component } from "react";
exportdefault function Hoc(HocComponent) {
return class extends Component {
render() {
return (
<div>
<HocComponent></HocComponent>
</div>
);
}
};
}
/**
* App.js
*/
importReact, { Component } from "react";
importHoc from "./HOC";
exportdefaultclass App extends Component {
render() {
return <h2>Higher Order Component!</h2>;
}
}
App = Hoc(App);
Note:
A HOC do not modify or mutate components. It creates a new ones.
A HOC is used to compose components for code reuse.
A HOC is a pure function. It has no side effects, returning only a new component.
⚝
↥
Q. What are the benefits of using HOC?
Benefits:
Importantly they provided a way to reuse code when using ES6 classes.
No longer have method name clashing if two HOC implement the same one.
It is easy to make small reusable units of code, thereby supporting the single responsibility principle.
Apply multiple HOCs to one component by composing them. The readability can be improve using a compose function
like in Recompose.
Problems:
Boilerplate code like setting the displayName with the HOC function name e.g. (withHOC(Component)) to help with
debugging.
Ensure all relevant props are passed through to the component.
Hoist static methods from the wrapped component.
It is easy to compose several HOCs together and then this creates a deeply nested tree making it difficult to debug.
↥
Q. What are Higher Order Component factory
implementations?
Creating a higher order component basically involves manipulating WrappedComponent which can be done in two ways:
Props Proxy
Inheritance Inversion
Both enable different ways of manipulating the WrappedComponent.
1. Props Proxy:
In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass
through the props that the HOC receives, hence the name Props Proxy.
Example:
function ppHOC(WrappedComponent) {
return class PP extends React.Component {
render() {
return <WrappedComponent {...this.props}/>
}
}
}
Props Proxy can be implemented via a number of ways
Manipulating props
Accessing the instance via Refs
Abstracting State
Wrapping the WrappedComponent with other elements
2. Inheritance Inversion:
Inheritance Inversion allows the HOC to have access to the WrappedComponent instance via this keyword, which means it has
access to the state, props, component lifecycle hooks and the render method.
Example:
function iiHOC(WrappedComponent) {
return class Enhancer extends WrappedComponent {
render() {
return super.render()
}
}
}
Inheritance Inversion can be used in:
Conditional Rendering (Render Highjacking)
State Manipulation
↥
Q. Explain Inheritance Inversion (iiHOC) in react?
Inheritance Inversion gives the HOC access to the WrappedComponent instance via this, which means we can use the state, props,
component lifecycle and even the render method.
Example:
/**
* Inheritance Inversion
*/
classWelcome extends React.Component {
render() {
return (
<div> Welcome {his.props.user}</div>
)
}
}
constwithUser = (WrappedComponent) => {
return class extends React.Component {
render() {
if(this.props.user) {
return (
<WrappedComponent {...this.props} />
)
}
return <div>Welcome Guest!</div>
}
}
}
constwithLoader = (WrappedComponent) => {
return class extends WrappedComponent {
render() {
const { isLoader } = this.props
if(!isLoaded) {
return <div>Loading...</div>
}
return super.render()
}
}
}
exportdefault withLoader(withUser(Welcome))
↥
Q. How to create props proxy for Higher Order
Component component?
It's nothing more than a function, propsProxyHOC, that receives a Component as an argument (in this case we've called the
argument WrappedComponent) and returns a new component with the WrappedComponent within.
When we return the Wrapped Component we have the possibility to manipulate props and to abstract state, even passing state as a
prop into the Wrapped Component.
We can create propspassed to the component using props proxy pattern as below
constpropsProxyHOC = (WrappedComponent) => {
return class extends React.Component {
render() {
const newProps = {
user: currentLoggedInUser
}
return <WrappedComponent {...this.props} {...newProps} />
}
}
}
Props Proxy HOCs are useful to the following situations:
Manipulating props
Accessing the instance via Refs (be careful, avoid using refs)
Abstracting State
Wrapping/Composing the WrappedComponent with other elements
↥
Q. How to use decorators in React?
Decorators provide a way of calling Higher-Order functions. It simply take a function, modify it and return a new function with added
functionality. The key here is that they don't modify the original function, they simply add some extra functionality which means they
can be reused at multiple places.
Example:
exportconst withUniqueId = (Target) => {
return class WithUniqueId extends React.Component {
uid = uuid();
render() {
return <Target {...this.props} uuid={this.uid} />;
}
};
}
@withUniqueId
classUniqueIdComponentextends React.Component {
render() {
return <div>Generated Unique ID is: {this.props.uuid}</div>;
}
}
constApp = () => (
<div>
<h2>Decorators in React!</h2>
<UniqueIdComponent/>
</div>
);
Note: Decorators are an experimental feature in React that may change in future releases.
⚝
↥
Q. What is the purpose of displayName class property?
The displayName string is used in debugging messages. Usually, you don't need to set it explicitly because it's inferred from the
name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name
for debugging purposes or when you create a higher-order component.
Example:
functionwithSubscription(WrappedComponent) {
classWithSubscription extends React.Component {/* ... */}
WithSubscription.displayName = `WithSubscription(${getDisplayName(WrappedComponent)})`;
return WithSubscription;
}
functiongetDisplayName(WrappedComponent) {
return WrappedComponent.displayName || WrappedComponent.name || 'Component';
}
↥
# 4.5. LAZY LOADING
Q. How to set up lazy loading components in React?
1. REACT.LAZY():
React.lazy is a function that lets you load components lazily through what is called code splitting without help from any external
libraries. It makes possible for us to dynamically import components but they are rendered like regular components. This means that
the bundle containing the component will only be loaded when the component is rendered.
React.lazy() takes a function that returns a promise as it's argument, the function returns a promise by calling import() to load the
content. The returned Promise resolves to a module with a default containing the React Component.
// Without Lazy
importMyComponent from './MyComponent';
// With Lazy
constMyComponent = React.lazy(() => import('./MyComponent'));
2. SUSPENSE:
React.Suspense is a component that can be used to wrap lazy components. A React.Suspense takes a fallback prop that can be any
react element, it renders this prop as a placeholder to deliver a smooth experience and also give user feedback while the lazy
component is being loaded.
/**
* Suspense
*/
importReact, { Suspense } from 'react';
constMyComponent = React.lazy(() => import('./MyComponent'));
constApp = () => {
return (
<div>
<Suspense fallback={<div>Loading ... </div>}>
<MyComponent/>
</Suspense>
</div>
);
}
Example:
/**
* React Lazy Loading Routes
*/
importReact, { Suspense, lazy } from "react";
import { Switch, BrowserRouteras Router, Route, Link } from "react-router-dom";
constHome = lazy(() => import("./Home"));
constContactUs = lazy(()=> import("./ContactUs"));
constHelpPage = lazy(() => import("./Help"));
exportdefault function App() {
return (
<Router>
<ul>
<li><Linkto="/">Home</Link></li>
<li><Linkto="/contact-us">ContactUs</Link></li>
<li><Linkto="/help">HelpPage</Link></li>
</ul>
<hr />
<Suspense fallback={<h1>Loading...</h1>}>
<Switch>
<Route exact component={Home} path="/"/>
<Route component={ContactUs} path="/contact-us"/>
<Route component={HelpPage} path="/help" />
</Switch>
</Suspense>
</Router>
);
}
⚝
↥
# 5. REACT PROPS
Q. What is props in React?
Props is a special keyword in React, which stands for properties and is being used for passing data from one component to another.
However, callback functions can also be passed, which can be executed inside the child to initiate an update.
Props are immutable so we cannot modify the props from inside the component. These attributes are available in the class
component as this.props and can be used to render dynamic data in our render method.
Example:
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
constelement = <Welcome name="World!" />;
⚝
↥
Q. Why props in React are read only?
When you declare a component as a function or a class, it must never modify its own props.
Consider this sum function:
function sum(a, b) {
return a + b;
}
Such functions are called pure because they do not attempt to change their inputs, and always return the same result for the same
inputs. All React components must act like pure functions with respect to their props. A component should only manage its own
state, but it should not manage its own props.
In fact, props of a component is concretely "the state of the another component (parent component)". So props must be managed
by their component owner. That's why all React components must act like pure functions with respect to their props (not to mutate
directly their props).
⚝
↥
Q. What are default props?
The defaultProps is a React component property that allows you to set default values for the props argument. If the prop property is
passed, it will be changed.
The defaultProps can be defined as a property on the component class itself to set the default props for the class. defaultProps is
used for undefined props, not for null props.
/**
* Default Props
*/
classMessageComponentextendsReact.Component {
render() {
return (
<div>Hello, {this.props.value}.</div>
)
}
}
// Default Props
MessageComponent.defaultProps = {
value: 'World'
}
ReactDOM.render(
<MessageComponent />,
document.getElementById('default')
)
ReactDOM.render(
<MessageComponentvalue='Folks'/>,
document.getElementById('custom')
)
⚝
↥
Q. How to access props inside quotes in React JSX?
React JSX doesn't support variable interpolation inside an attribute value, but we can put any JS expression inside curly braces as the
entire attribute value.
Approach 1: Putting js expression inside curly braces
<img className="image" src={"images/" + this.props.image} />
Approach 2: Using ES6 template literals.
<imgclassName="image" src={`images/${this.props.image}`} />
Example:
/**
* Access Props
*/
classApp extends Component {
render() {
return (
<div>
<img
alt="React Logo"
// Using ES6 template literals
src={`${this.props.image}`}
/>
</div>
);
}
}
exportdefault App;
⚝
↥
Q. How to pass numbers to React component?
In react, numbers can be passed via curly braces({}) where as strings in quotes ("");
Example:
function App() {
return <Greetingsname="Nathan" age={27} occupation="Software Developer" />;
}
// Greetings Component
function Greetings(props) {
return (
<h2>
Hello! I'm {props.name}, a {props.age} years old {props.occupation}.
Pleased to meet you!
</h2>
);
}
⚝
↥
Q. How are boolean props used in React?
React JSX has exactly two ways of passing true, <MyComponent prop /> and <MyComponent prop={true} /> and exactly one way of
passing false <MyComponent prop={false} />.
Example:
/**
* Boolean Props
*/
constMyComponent = ({ prop1, prop2 }) => (
<div>
<div>Prop1: {String(prop1)}</div>
<div>Prop2: {String(prop2)}</div>
</div>
)
function App() {
return (
<div>
<MyComponent prop1={true}prop2={false} />
<MyComponent prop1 prop2/>
<MyComponent prop1={false} prop2 />
</div>
);
}
⚝
↥
Q. How to apply validation on Props in React?
Props are an important mechanism for passing the read-only attributes to React components. React provides a way to validate the
props using PropTypes. This is extremely useful to ensure that the components are used correctly.
Example:
/**
* Props Validation
*/
importReact from "react";
importPropTypes from "prop-types";
exportdefaultclass App extends React.Component {
render() {
return (
<>
<h3>Boolean: {this.props.propBool ? "True": "False"}</h3>
<h3>Array: {this.props.propArray}</h3>
<h3>Number: {this.props.propNumber}</h3>
<h3>String: {this.props.propString}</h3>
</>
);
}
}
App.defaultProps = {
propBool: true,
propArray: [10, 20, 30],
propNumber: 100,
propString: "Hello React!"
};
App.propTypes = {
propBool: PropTypes.bool.isRequired,
propArray: PropTypes.array.isRequired,
propNumber: PropTypes.number,
propString: PropTypes.string
};
⚝
↥
Q. How to specify the shape of an object with
PropTypes
The PropTypes.shape() validator can be used when describing an object whose keys are known ahead of time, and may represent
different types.
Example:
/**
* PropTypes.shape()
* @param {*} props
*/
importPropTypes from "prop-types";
constComponent = (props) => (
<div>
Component badge: {props.badge ? JSON.stringify(props.badge) : "none"}
</div>
);
// PropTypes validation for the prop object
Component.propTypes = {
badge: PropTypes.shape({
src: PropTypes.string.isRequired,
alt: PropTypes.string.isRequired
})
};
constApp = () => (
<div>
<Component badge={{ src: "horse.png", alt: "Running Horse"}} />
{/*<Component badge={{src:null, alt: 'this one gives an error'}}/>*/}
<Component/>
</div>
);
exportdefault App;
Output:
Component badge: {"src":"horse.png","alt":"Running Horse"}
Component badge: none
⚝
↥
Q. How PropTypes.objectOf is different from
PropTypes.shape?
The PropTypes.objectOf() validator is used when describing an object whose keys might not be known ahead of time, and often
represent the same type.
Example:
/**
* PropTypes
*/
importPropTypes from 'prop-types';
// Expected prop object - dynamic keys (i.e. user ids)
constmyProp = {
25891102: 'Shila Jayashri',
34712915: 'Employee',
76912999: '[email protected]'
};
// PropTypes validation for the prop object
MyComponent.propTypes = {
myProp: PropTypes.objectOf(PropTypes.number)
};
↥
Q. How React PropTypes allow different types for one
prop?
Using PropTypes.oneOfType() says that a prop can be one of any number of types. For instance, a phone number may either be
passed to a component as a string or an integer:
/**
* PropTypes.oneOfType()
*/
constComponent = (props) => <div>Phone Number: {props.phoneNumber}</div>
Component.propTypes = {
phoneNumber: PropTypes.oneOfType([
PropTypes.number,
PropTypes.string
]),
}
constApp = () => (
<div>
<Component phoneNumber={04403472916}/>
{/*<Component phoneNumber={"2823788557"}/>*/}
</div>
);
⚝
↥
Q. What are render props?
The term render props refers to a technique for sharing code between React components using a prop whose value is a function.
In simple words, render props are simply props of a component where you can pass functions. These functions need to return
elements, which will be used in rendering the components.
Example:
/**
* Render Props
*/
importReact from "react";
importWrapper from"./Wrapper";
classApp extends React.Component {
render() {
return (
<Wrapper
render={({ increment, count }) => (
<div>
<h3>Render Props Counter</h3>
<p>{count}</p>
<buttononClick={() => increment()}>Increment</button>
</div>
)}
/>
);
}
}
/**
* Wrapper Component
*/
classWrapper extends React.Component {
state = {
count: 0
};
// Increase count
increment = () => {
const { count } = this.state;
return this.setState({ count: count + 1 });
};
render() {
const { count } = this.state;
return (
<div>
{this.props.render({ increment: this.increment, count: count })}
</div>
);
}
}
⚝
↥
Q. What are the benefits of using Render Props?
Benefits:
Reuse code across components when using ES6 classes.
The lowest level of indirection - it's clear which component is called and the state is isolated.
No naming collision issues for props, state and class methods.
No need to deal with boiler code and hoisting static methods.
Problems:
Caution using shouldComponentUpdate() as the render prop might close over data it is unaware of.
There could also be minor memory issues when defining a closure for every render. But be sure to measure first before
making performance changes as it might not be an issue for your app.
Another small annoyance is the render props callback is not so neat in JSX as it needs to be wrapped in an expression.
Rendering the result of an HOC does look cleaner.
↥
Q. How do you create Higher Order Component using
render props?
It is possible to implement most higher-order components (HOC) using a regular component with a render prop. This way render
props gives the flexibility of using either pattern.
Example:
function withMouse(Component) {
return class extends React.Component {
render() {
return (
<Mouse render={mouse=> (
<Component {...this.props} mouse={mouse} />
)}/>
);
}
}
}
↥
Q. Explain HOC vs render props in react.js?
The Higher-Order Components, Render Props and Hooks are three patterns to implement state-orbehaviour-* sharing between
components. All three have their own use cases and none of them is a full replacement of the others.
1. Higher-order components:
Essentially HOC are similar to the decorator pattern, a function that takes a component as the first parameter and returns a new
component. This is where you apply your crosscutting functionality.
Example:
functionwithExample(Component) {
return function(props) {
// cross cutting logic added here
return <Component {...props} />;
};
}
2. Render Props:
A render prop is where a component's prop is assigned a function and this is called in the render method of the component. Calling
the function can return a React element or component to render.
Example:
render(){
<FetchDatarender={(data) =>{
return <p>{data}</p>
}} />
}
The React community is moving away from HOC (higher order components) in favor of render prop components (RPC). For the most
part, HOC and render prop components solve the same problem. However, render prop components provide are gaining popularity
because they are more declarative and flexible than an HOC.
↥
Q. What is children props?
The {this.props.children} is a special prop, automatically passed to every component, that can be used to render the content
included between the opening and closing tags when invoking a component.
Example:
/**
* React Children Props
*/
classMyComponent extends React.Component {
render() {
return (
<div>
<h1>React Children Props Example</h1>
{this.props.children}
</div>
);
}
}
classOtherComponentextends React.Component {
render() {
return <div>Other Component Props</div>;
}
}
ReactDOM.render(
<MyComponent>
<p>React DOM Props</p> {/* Children Props*/}
<OtherComponent/>
</MyComponent>,
document.getElementById("root")
);
⚝
↥
Q. When we should use React.cloneElement vs
this.props.children?
The React.cloneElement() works if child is a single React element.
For almost everything {this.props.children} is used. Cloning is useful in some more advanced scenarios, where a parent sends in
an element and the child component needs to change some props on that element or add things like ref for accessing the actual
DOM element.
React.Children:
Since {this.props.children} can have one element, multiple elements, or none at all, its value is respectively a single child node, an
array of child nodes or undefined. Sometimes, we want to transform our children before rendering them — for example, to add
additional props to every child. If we wanted to do that, we'd have to take the possible types of this.props.children into account.
For example, if there is only one child, we can not map it.
Example:
/**
* React Children Props
*/
exportdefaultclass App extends React.Component {
render() {
return (
<div>
<b>Children ({this.props.children.length}):</b>
{this.props.children}
</div>
);
}
}
classWidgetextends React.Component {
render() {
return
<div>
<h2>First Example:</h2>
<App>
<div>10</div>
<div>20</div>
<div>30</div>
</App>
<h2>Second Example:</h2>
<App>
<div>A</div>
<div>B</div>
</App>
</div>
}
}
Output
FirstExample:
Children (3):
10
20
30
Second Example:
Children (2):
A
B
⚝
↥
Q. What do these three dots in React do?
The ES6 Spread operator or Rest Parameters is use to pass props to a React component. Let us take an example for a component
that expects two props:
function App() {
return <Hello firstName="Pallav" lastName="Hegde"/>
}
Using the Spread operator, it become like this
function App() {
constprops = {firstName: 'Pallav', lastName: 'Hegde'}
return <Hello {...props} />
}
When we use the ...props syntax, actaully it expand the props object from the parent component, which means all its attributes are
passed down the child component that may not need them all. This will make things like debugging harder.
Using the Spread Operator with setState() for Setting the Nested State:
Let us suppose we have a state with a nested object in our component:
this.state = {
stateObj: {
attr1: '',
attr2: '',
},
}
We can use the Spread syntax to update the nested state object.
this.setState(state=>({
person: {
...state.stateObj,
attr1: 'value1',
attr2: 'value2',
},
}))
↥
Q. Why we need to be careful when spreading props
on DOM elements?
When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice.
Problem: This will to add the unknown HTML attribute flag to the DOM element.
constSample = () => (<Spread flag={true} className="content"/>);
constSpread = (props) => (<div {...props}>Test</div>);
Solution: By creating props specifically for DOM attribute, we can safely spread.
constSample = () => (<Spread flag={true} domProps={{className: "content"}}/>);
constSpread = (props) => (<div {...props.domProps}>Test</div>);
Or alternatively we can use prop destructuring with ...rest:
constSample = () => (<Spread flag={true} className="content"/>);
constSpread = ({ flag, ...domProps }) => (<div {...domProps}>Test</div>);
Note:
In scenarios where you use a PureComponent, when an update happens it re-renders the component even if domProps did not change.
This is because PureComponent only shallowly compares the objects.
↥
Q. What will happen if you use props in initial state?
Using props to generate state in getInitialState often leads to duplication of "source of truth", i.e. where the real data is. This is
because getInitialState is only invoked when the component is first created.
The danger is that if the props on the component are changed without the component being 'refreshed', the new prop value will
never be displayed because the constructor function (or getInitialState) will never update the current state of the component. The
initialization of state from props only runs when the component is first created.
Bad:
The below component won't display the updated input value
classApp extends React.Component {
// constructor function (or getInitialState)
constructor(props) {
super(props)
this.state = {
records: [],
inputValue: this.props.inputValue
}
}
render() {
return <div>{this.state.inputValue}</div>
}
}
Good:
Using props inside render method will update the value:
classApp extends React.Component {
// constructor function (or getInitialState)
constructor(props) {
super(props)
this.state = {
records: []
}
}
render() {
return <div>{this.props.inputValue}</div>
}
}
↥
Q. What is the difference between createElement and
cloneElement?
JSX elements will be transpiled to React.createElement() functions to create React elements which are going to be used for the
object representation of UI. Whereas cloneElement is used to clone an element and pass it new props.
The React.cloneElement() function returns a copy of a specified element. Additional props and children can be passed on in the
function. We shoul use this function when a parent component wants to add or modify the props of its children.
importReact from 'react'
exportdefaultclass App extends React.Component {
// rendering the parent and child component
render() {
return (
<ParentComp>
<MyButton/>
<br/>
<MyButton/>
</ParentComp>
)
}
}
/**
* The parent component
*/
classParentComp extends React.Component {
render() {
// The new prop to the added.
letnewProp = 'red'
// Looping over the parent's entire children,
// cloning each child, adding a new prop.
return (
<div>
{React.Children.map(this.props.children,
child => {
return React.cloneElement(child,
{newProp}, null)
})}
</div>
)
}
}
/**
* The child component
*/
classMyButton extends React.Component {
render() {
return <button style =
{{ color: this.props.newProp }}>
Hello World!</button>
}
}
↥
Q. When should I be using React.cloneElement vs
this.props.children?
The React.cloneElement only works if your child is a single React element.
Example:
<ReactCSSTransitionGroup
component="div"
transitionName="example"
transitionEnterTimeout={500}
transitionLeaveTimeout={500}
>
{React.cloneElement(this.props.children, {
key: this.props.location.pathname
})}
</ReactCSSTransitionGroup>
For almost everything {this.props.children} is used. Cloning is useful in some more advanced scenarios, where a parent sends in
an element and the child component needs to change some props on that element or add things like ref for accessing the actual
DOM element.
Example:
classUsers extendsReact.Component {
render() {
return (
<div>
<h2>Users</h2>
{this.props.children}
</div>
)
}
}
↥
Q. How to pass JSON Objects from Child to Parent
Component?
Example: Passing JSON Objects from Child to Parent Component using callback function
// Parent Component
exportdefaultclass App extends React.Component {
constructor() {
super();
this.state = {
message: ""
};
this.onSubmitMessage = this.onSubmitMessage.bind(this);
}
onSubmitMessage(message) {
this.setState({message: message });
}
render() {
const { message } = this.state;
return (
<div>
<h3>Parent component</h3>
<div>The message coming from the child component is : {message}</div>
<hr />
<Child
// passing as callback function
onSubmitMessage={this.onSubmitMessage}
/>
</div>
);
}
}
// Child Component
exportdefaultclass Child extends React.Component {
constructor() {
super();
this.state = {
greetingMessag: ""
};
this.onMessageChange = this.onMessageChange.bind(this);
this.onSubmit = this.onSubmit.bind(this);
}
onMessageChange(event) {
letmessage = event.target.value;
this.setState({greetingMessag: message });
}
// pass message to parent component using callback
onSubmit() {
this.props.onSubmitMessage(this.state.greetingMessag);
}
render() {
return (
<div>
<h3>Child Component</h3>
<input
type="text"
onChange={this.onMessageChange}
placeholder="Enter a message"
/>
<button onClick={this.onSubmit}>Submit</button>
</div>
);
}
}
⚝
↥
Q. What is the use of this props?
It is called spread operator (ES6 feature) and its aim is to make the passing of props easier.
Example:
<div {...this.props}>
Content Here
</div>
It is equal to Class Component
constperson = {
name: "Alex",
age: 26,
coun : "India"
}
classSpreadExampleextends React.Component {
render() {
const {name, age, coun } = {...this.props}
return (
<div>
<h3> Person Information: </h3>
<ul>
<li>name={name}</li>
<li>age={age}</li>
<li>coun ={coun}</li>
</ul>
</div>
)
}
}
ReactDOM.render(
<SpreadExample {...person}/>
, mountNode
)
↥
# 6. REACT STATE
Q. What is State in React?
The state is a built-in object that is used to contain data about the component. A component's state can change over time; whenever
it changes, the component re-renders.
Example:
/**
* React State
*/
exportdefaultclassEmployee extends React.Component {
constructor() {
super();
this.state = {
id: 100,
name: "Sarita Mangat"
};
}
render() {
return (
<div>
<div>ID: {this.state.id}</div>
<div>Name: {this.state.name}</div>
</div>
);
}
}
⚝
↥
Q. What does setState() do?
The component state can be updated in response to event handlers, server responses, or prop changes. This is done using
the setState() method. The setState() method enqueues all of the updates made to the component state and instructs React to re-
render the component and its children with the updated state.
Always use the setState() method to change the state object, since it will ensure that the component knows it's been updated and
calls the render() method.
Example:
/**
* React setState()
*/
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.state = {
color: "blue"
};
}
handleColor = () => {
this.setState({color: "red" });
};
render() {
return (
<div>
<h3 style={{ color: `${this.state.color}` }}>
Color: {this.state.color}
</h3>
<button type="button" onClick={this.handleColor}>
Change color
</button>
</div>
);
}
}
⚝
↥
Q. Why is setState() in React async instead of sync?
The setState() does not immediately mutate this.state() but creates a pending state transition. Accessing this.state() after
calling this method can potentially return the existing value. There is no guarantee of synchronous operation of calls to setState()
and calls may be batched for performance gains.
This is because setState() alters the state and causes rerendering. This can be an expensive operation and making it synchronous
might leave the browser unresponsive. Thus the setState() calls are asynchronous as well as batched for better UI experience and
performance.
↥
Q. What is the second argument that can optionally be
passed to setState() and what is its purpose?
A callback function which will be invoked when setState() has finished and the component is re-rendered. The setState() is
asynchronous, which is why it takes in a second callback function. Typically it's best to use another lifecycle method rather than
relying on this callback function, but it is good to know it exists.
Example:
this.setState(
{ username: 'Lila' },
() =>console.log('setState has finished and the component has re-rendered.')
)
The setState() will always lead to a re-render unless shouldComponentUpdate() returns false. To avoid unnecessary renders, calling
setState() only when the new state differs from the previous state makes sense and can avoid calling setState() in an infinite loop
within certain lifecycle methods like componentDidUpdate().
↥
Q. What are the possible ways of updating objects in
state?
Instead of directly modifying the state using this.state(), we use this.setState(). This is a function available to all React
components that use state, and allows us to let React know that the component state has changed. This way the component knows
it should re-render, because its state has changed and its UI will most likely also change.
Example:
this.state = {
user: { name: 'Vasuda Handa', age: 22 }
}
Using Object.assign()
this.setState(prevState =>{
let user = Object.assign({},prevState.user); // creating copy of state variable user
user.name = 'Sai Gupta'; // update the name property, assign a new value
return { user };// return new object user object
})
Using spread syntax
this.setState(prevState =>({
user: { // object that we want to update
...prevState.user, // keep all other key-value pairs
name: 'Niraj Gara'// update the value of specific key
}
}))
⚝
↥
Q. What will happen if you use setState() in
constructor?
When we use setState(), then apart from assigning to the object state react also rerenders the component and all it's children.
Which we don't need in the constructor, since the component hasn't been rendered anyway.
Inside constructor uses this.state = {} directly, other places use this.setState({ })
Example:
importReact, { Component } from 'react'
classFood extends Component {
constructor(props) {
super(props)
this.state = {
fruits: ['apple', 'orange'],
count: 0
}
}
render() {
return (
<divclassName = "container">
<h2> Hello!!!</h2>
<p> I have {this.state.count} fruit(s)</p>
</div>
)
}
}
⚝
↥
Q. Why should not we update the state directly?
The setState() does not immediately mutate this.state() but creates a pending state transition. Accessing this.state() after
calling this method can potentially return the existing value.
The setState() will always trigger a re-render unless conditional rendering logic is implemented in shouldComponentUpdate(). If
mutable objects are being used and the logic cannot be implemented in shouldComponentUpdate(), calling setState() only when the
new state differs from the previous state will avoid unnecessary re-renders.
Example:
importReact, { Component } from 'react'
classApp extends Component {
constructor(props) {
super(props)
this.state = {
list: [
{ id: '1', age: 42 },
{ id: '2', age: 33 },
{ id: '3', age: 68 },
],
}
}
onRemoveItem = id=> {
this.setState(state => {
const list = state.list.filter(item => item.id !== id)
return {
list,
}
})
}
render() {
return (
<div>
<ul>
{this.state.list.map(item => (
<li key={item.id}>
The person is {item.age} years old.
<button
type="button"
onClick={()=>this.onRemoveItem(item.id)}
>
Remove
</button>
</li>
))}
</ul>
</div>
)
}
}
exportdefaultApp
⚝
↥
Q. How to delete an item from state array?
When using React, we should never mutate the state directly. If an object is changed, we should create a new copy. The better
approach is to use Array.prototype.filter() method which creates a new array.
Example:
onDeleteByIndex(index) {
this.setState({
users: this.state.users.filter((item, i) => i !== index)
});
}
⚝
↥
Q. Why should not call setState() in
componentWillUnmount()?
We should not call setState()incomponentWillUnmount() because the component will never be re-rendered. Once a component
instance is unmounted, it will never be mounted again.
The componentWillUnmount() is invoked immediately before a component is unmounted and destroyed. This method can be used to
perform any necessary cleanup method, such as invalidating timers, canceling network requests, or cleaning up any subscriptions
that were created in componentDidMount().
↥
Q. How can you re-render a component without using
setState() function?
React components automatically re-render whenever there is a change in their state or props. A simple update of the state, from
anywhere in the code, causes all the User Interface (UI) elements to be re-rendered automatically.
However, there may be cases where the render() method depends on some other data. After the initial mounting of components, a
re-render will occur.
Using forceUpdate():
The following example generates a random number whenever it loads. Upon clicking the button, the forceUpdate() function is called
which causes a new, random number to be rendered:
/**
* forceUpdate()
*/
exportdefaultclass App extends React.Component {
constructor(){
super();
this.forceUpdateHandler = this.forceUpdateHandler.bind(this);
};
forceUpdateHandler(){
this.forceUpdate();
};
render(){
return(
<div>
<button onClick= {this.forceUpdateHandler} >FORCE UPDATE</button>
<h4>Random Number : { Math.random() }</h4>
</div>
);
}
}
⚝
Note: We should to avoid all uses of forceUpdate() and only read from this.props and this.state in render().
↥
Q. Why we need to pass a function to setState()?
The reason behind for this is that setState() is an asynchronous operation. React batches state changes for performance reasons, so
the state may not change immediately after setState() is called. That means we should not rely on the current state when
calling setState().
The solution is to pass a function to setState(), with the previous state as an argument. By doing this we can avoid issues with the
user getting the old state value on access due to the asynchronous nature of setState().
Problem:
// assuming this.state.count === 0
this.setState({count: this.state.count + 1});
this.setState({count: this.state.count + 1});
this.setState({count: this.state.count + 1});
// this.state.count === 1, not 3
⚝
Solution:
this.setState((prevState) =>({
count: prevState.count + 1
}));
this.setState((prevState) =>({
count: prevState.count + 1
}));
this.setState((prevState) =>({
count: prevState.count + 1
}));
// this.state.count === 3 as expected
⚝
↥
Q. How to update nested state properties in React.js?
We can pass the old nested object using the spread operator and then override the particular properties of the nested object.
Example:
// Nested object
state = {
name: 'Vyasa Agarwal',
address: {
colony: 'Old Cross Rds, Mehdipatnam',
city: 'Patna',
state: 'Jharkhand'
}
};
handleUpdate = () => {
// Overriding the city property of address object
this.setState({ address: { ...this.state.address, city: "Ranchi" } })
}
⚝
↥
Q. What is the difference between state and props?
Props
State
Props are read-only.
State changes can be asynchronous.
Props are immutable.
State is mutable.
Props allow you to pass data from one component to other
components as an argument.
State holds information about the components.
Props can be accessed by the child component.
State cannot be accessed by child components.
Props are used to communicate between components.
States can be used for rendering dynamic changes with
the component.
Stateless component can have Props.
Stateless components cannot have State.
Props make components reusable.
State cannot make components reusable.
Props are external and controlled by whatever renders the component.
The State is internal and controlled by the React
Component itself.
↥
Q. How to set state with a dynamic key name?
If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with computed property names.
inputChangeHandler : function (event) {
this.setState({ [event.target.id]: event.target.value });
// alternatively using template strings for strings
// this.setState({ [`key${event.target.id}`]: event.target.value });
}
⚝
↥
Q. How to listen state change in React.js?
The following lifecycle methods will be called when state changes. You can use the provided arguments and the current state to
determine if something meaningful changed.
componentWillUpdate(object nextProps, object nextState)
componentDidUpdate(object prevProps, object prevState)
In functional component, listen state changes with useEffect hook like this
exportfunction MyComponent(props) {
const [myState, setMystate] = useState('initialState')
useEffect(() => {
console.log(myState, '- Has changed')
},[myState]) // <-- here put the parameter to listen
}
↥
Q. How to access child's state in React?
Using Refs:
In React we can access the child's state using React.createRef(). We will assign a Refs for the child component in the parent
component, then using Refs we can access the child's state.
// App.js
classApp extends React.Component {
constructor(props) {
super(props);
this.ChildElement = React.createRef();
}
handleClick = () => {
const childelement = this.ChildElement.current;
childelement.getMsg("Message from Parent Component!");
};
render() {
return (
<div>
<Child ref={this.ChildElement} />
<button onClick={this.handleClick}>CLICK ME</button>
</div>
);
}
}
// Child.js
classChild extendsReact.Component {
state = {
name: "Message from Child Component!"
};
getMsg = (msg) => {
this.setState({
name: msg
});
};
render() {
return <h2>{this.state.name}</h2>;
}
}
⚝
↥
Q. How to change the state of a child component from
its parent in React?
To change child component's state from parent component with React, we can pass props.
/**
* Change Child state from its Parent
* @param {*} param0
*/
constChild = ({ open }) => {
return <h2>Child State: {open.toString()}</h2>;
};
constParent = () => {
const [isOpen, setIsOpen] = React.useState(false);
consttoggleChild = () => {
setIsOpen((prevValue) => !prevValue);
};
return (
<div>
<button onClick={toggleChild}>Click Me</button>
{/* Pass a callback to Child */}
<Child open={isOpen} />
</div>
);
};
exportdefault Parent;
⚝
↥
Q. Why is it advised to pass a callback function to
setState() as opposed to an object?
Because this.props andthis.state may be updated asynchronously, we should not rely on their values for calculating the next
state.
Example: setState Callback in a Class Component
importReact, { Component } from 'react'
classApp extends Component {
constructor(props) {
super(props)
this.state = {
age: 0,
}
}
// this.checkAge is passed as the callback to setState
updateAge = (value) => {
this.setState({age: value}, this.checkAge)
}
checkAge = () => {
const { age } = this.state
if (age !== 0 && age >=21) {
// Make API call to /beer
} else {
// Throw error 404, beer not found
}
}
render() {
const { age } = this.state
return (
<div>
<p>Drinking Age Checker</p>
<input
type="number"
value={age}
onChange={e => this.updateAge(e.target.value)}
/>
</div>
)
}
}
exportdefaultApp
Example: setState Callback in a Functional Component
importReact, { useEffect, useState } from'react'
function App() {
const [age, setAge] = useState(0)
updateAge(value) {
setAge(value)
}
useEffect(() => {
if (age !== 0 && age >=21) {
// Make API call to /beer
} else {
// Throw error 404, beer not found
}
}, [age])
return (
<div>
<p>Drinking Age Checker</p>
<input
type="number"
value={age}
onChange={e => setAge(e.target.value)}
/>
</div>
)
}
exportdefaultApp
↥
Q. How does the state differ from props in React?
1. State:
This is data maintained inside a component. It is local or owned by that specific component. The component itself will update the
state using the setState() function.
Example:
classAppComponent extends React.component {
state = {
msg : 'Hello World!'
}
render() {
return <div>Message {this.state.msg}</div>
}
}
2. Props:
Data passed in from a parent component. props are read-only in the child component that receives them. However, callback
functions can also be passed, which can be executed inside the child to initiate an update.
Example: The parent can pass a props by using this
<ChildComponent color='red'/>
Inside the ChildComponent constructor we could access the props
classChildComponentextends React.Component {
constructor(props) {
super(props)
console.log(props.color)
}
}
Props can be used to set the internal state based on a prop value in the constructor, like this
classChildComponentextends React.Component {
constructor(props) {
super(props)
this.state.colorName = props.color
}
}
Props should never be changed in a child component. Props are also used to allow child components to access methods defined in
the parent component. This is a good way to centralize managing the state in the parent component, and avoid children to have the
need to have their own state.
Difference between State and Props:
Props
State
Props are read-only.
State changes can be asynchronous.
Props allow to pass data from one component to other components
as an argument.
State holds information about the components.
Props can be accessed by the child component.
State cannot be accessed by child components.
Props are used to communicate between components.
States can be used for rendering dynamic changes with the
component.
Stateless component can have Props.
Stateless components cannot have State.
Props are external and controlled by whatever renders the
component.
The State is internal and controlled by the React
Component itself.
↥
# 7. REACT EVENTS
Q. What is meant by event handling in React?
Handling events with React elements is very similar to handling events on DOM elements. There are some syntax differences:
React events are named using camelCase, rather than lowercase.
With JSX you pass a function as the event handler, rather than a string.
Example:
/**
* Event Handling in React
*/
exportdefaultclass Toggle extends React.Component {
constructor(props) {
super(props);
this.state = { isToggleOn: true };
}
handleClick() {
this.setState((state) =>({
isToggleOn: !state.isToggleOn
}));
}
render() {
return (
<button onClick={() => this.handleClick()}>
{this.state.isToggleOn? "ON" : "OFF"}
</button>
);
}
}
⚝
↥
Q. How to pass a parameter to event handlers in React?
Example:
constmessage = "Hey there!";
exportdefaultclass App extends React.Component {
displayMessage(message) {
alert(message);
}
render() {
return (
<button onClick={() => this.displayMessage(message)}>CLICK ME</button>
);
}
}
⚝
↥
Q. How do you pass an event handler to a component?
Example:
importReact, {useState}from "react";
import"./styles.css";
exportdefault function App() {
return (
<Container/>
);
}
constContainer = () => {
const [counter, setCounter] = useState(0);
consthandleCustomClick = ()=> {
setCounter(counter + 1)
}
return (
<div>
<div>Counter: {counter}</div>
<CustomButtononCustomClick={handleCustomClick}/>
</div>
);
}
constCustomButton = ({onCustomClick}) => {
return (
<buttononClick={onCustomClick}>
My Custom Button
</button>
);
}
⚝
↥
Q. What is the difference between HTML and React
event handling?
In HTML, the attribute name is in all lowercase and is given a string invoking a function defined somewhere:
<buttononclick="handleClick()"></button>
In React, the attribute name is camelCase and are passed the function reference inside curly braces:
<buttononClick={handleClick} />
In HTML, false can be returned to prevent default behavior, whereas in React preventDefault() has to be called explicitly.
<a href="#" onclick="console.log('The link was clicked.'); return false" />
functionhandleClick(e) {
e.preventDefault()
console.log("The link was clicked.")
}
↥
Q. How to bind methods or event handlers in JSX
callbacks?
There are 3 possible ways to achieve this
1. Event Handler in Render Method:
We can bind the handler when it is called in the render method using bind() method.
handleClick() {
// ...
}
<buttononClick={this.handleClick.bind(this)}>Click</button>
⚝
2. Event Handler using Arrow Function:
In this approach we are binding the event handler implicitly. This approach is the best if you want to pass parameters to your event.
handleClick() {
// ...
}
<buttononClick={()=> this.handleClick()}>Click</button>
⚝
3. Event Handler in Constructor:
This has performance benefits as the events aren't binding every time the method is called, as opposed to the previous two
approaches.
constructor(props) {
// This binding is necessary to make `this` work in the callback
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
// ...
}
<buttononClick={this.handleClick}>Click</button>
⚝
↥
Q. Why do we need to bind methods inside class
component constructor?
In Class Components, when we pass the event handler function reference as a callback like this
<buttontype="button" onClick={this.handleClick}>Click Me</button>
the event handler method loses its implicitly bound context. When the event occurs and the handler is invoked, the this value falls
back to default binding and is set to undefined, as class declarations and prototype methods run in strict mode.
When we bind the this of the event handler to the component instance in the constructor, we can pass it as a callback without
worrying about it losing its context.
Arrow functions are exempt from this behavior because they use lexicalthis binding which automatically binds them to the scope
they are defined in.
Example:
/**
* Event Handling in React
*/
importReact from "react";
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.handleClick = this.handleClick.bind(this);
}
handleClick(event) {
alert("Click event triggered!");
}
render() {
return <button onClick={this.handleClick}>Click Me</button>;
}
}
⚝
↥
Q. How do I pass a parameter to an event handler or
callback?
You can use an arrow function to wrap around an event handler and pass parameters:
<buttononClick={()=> this.handleClick(id)}/>
This is equivalent to calling.bind
<buttononClick={this.handleClick.bind(this, id)} />
Example:
/**
* Pass parameter to an event handler
*/
const A = 65; // ASCII character code
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.state = {
justClicked: null,
letters: Array.from({ length: 26 }, (_, i) => String.fromCharCode(A + i))
};
}
handleClick(letter) {
this.setState({justClicked: letter });
}
render() {
return (
<>
Just clicked: {this.state.justClicked}
<ul>
{this.state.letters.map((letter) => (
<li key={letter} onClick={() => this.handleClick(letter)}>
{letter}
</li>
))}
</ul>
</>
);
}
}
⚝
↥
Q. When should we use arrow functions with React?
Arrows prevent thisbugs
Arrow functions don not redefine the value of thiswithin their function body. This makes it a lot easier to predict their behavior
when passed as callbacks, and prevents bugs caused by use of this within callbacks. Using inline arrow functions in function
components is a good way to achieve some decoupling.
Example:
importReact from 'react'
importReactDOMfrom 'react-dom'
classButtonextends React.Component {
render() {
return (
<button onClick={this.handleClick} style={this.state}>
Set background to red
</button>
)
}
handleClick = () => {
this.setState({backgroundColor: 'red' })
}
}
ReactDOM.render(
<Button />,
document.getElementById('root')
)
1.When we use thisit generates a new function on every render, which will obviously have a new reference.
2.If the component we pass this generated function to is extending PureComponent(), it will not be able to bail out on
rerendering, even if the actual data has not changed.
↥
Q. Is it good to use arrow functions in render methods?
Problem:
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of
arguments preceding any provided when the new function is called. When arrow functions and binds in render. It breaks
performance optimizations like shouldComponentUpdate() and PureComponent.
Example:
classApp extends React.Component {
constructor(props) {
super(props);
...
}
deleteUser = id => {
this.setState(prevState => {
return {
users: prevState.users.filter(user => user.id !== id)
};
});
};
render() {
return (
<div>
<ul>
{this.state.users.map(user => {
return (
<Userkey={user.id} name={user.name} id={user.id}
onDeleteClick={() => this.deleteUser(user.id)}
/>
);
})}
</ul>
</div>
);
}
}
⚝
Solution:
In below example, App Component has no arrow function in render. Instead, the relevant data is passed down to User Component.
In User Component, onDeleteClick() calls the onClick function passed in on props with the relevant user.id.
// User Component
classUser extends React.PureComponent {
onDeleteClick = () => {
// No bind needed since we can compose the relevant data for this item here
this.props.onClick(this.props.user.id);
};
render() {
console.log(`${this.props.user.name} just rendered`);
return (
<li>
<input type="button" value="Delete" onClick={this.onDeleteClick} />
{this.props.user.name}
</li>
);
}
}
// App Component
classApp extends React.Component {
constructor(props) {
super(props);
...
}
deleteUser = id => {
this.setState(prevState => {
return {
users: prevState.users.filter(user => user.id !== id)
};
});
};
renderUser = user=> {
return <User key={user.id} user={user} onClick={this.deleteUser} />;
}
render() {
return (
<div>
<ul>
{this.state.users.map(this.renderUser)}
</ul>
</div>
);
}
}
⚝
↥
Q. How to avoid the need for binding in React?
1. Use Arrow Function in Class Property:
Usually when we want to access this inside a class method we would need to bind it to method like so:
classButtonextendsComponent{
constructor(props) {
super(props)
this.state = { clicked: false }
}
handleClick = () => this.setState({ clicked: true})
render() {
return <button onClick={this.handleClick}>Click Me</button>
}
}
Binding thistohandleClick()in the constructor() allows us to use this.setState() from Component inside handleClick().
2. Bind in Render:
onChange={this.handleChange.bind(this)}
This approach is terse and clear, however, there are performance implications since the function is reallocated on every render.
3. Bind in Constructor:
One way to avoid binding in render is to bind in the constructor
constructor(props) {
super(props)
this.handleChange = this.handleChange.bind(this)
}
This is the approach currently recommended in the React docs for "better performance in your application".
↥
Q. How do I bind a function to a component instance?
There are several ways to make sure functions have access to component attributes like this.props and this.state, depending on
which syntax and build steps you are using.
1. Bind in Constructor (ES5):
classApp extends Component {
constructor(props) {
super(props)
this.handleClick = this.handleClick.bind(this)
}
handleClick() {
console.log('Click happened')
}
render() {
return <button onClick={this.handleClick}>Click Me</button>
}
}
2. Class Properties:
classApp extends Component {
// Note: this syntax is experimental and not standardized yet.
handleClick = () => {
console.log('Click happened')
}
render() {
return <button onClick={this.handleClick}>Click Me</button>
}
}
3. Bind in Render:
classApp extends Component {
handleClick() {
console.log('Click happened')
}
render() {
return <button onClick={this.handleClick.bind(this)}>Click Me</button>
}
}
Note: Using Function.prototype.bind in render creates a new function each time the component renders, which may have performance
implications
4. Arrow Function in Render:
classApp extends Component {
handleClick() {
console.log('Click happened')
}
render() {
return <button onClick={()=> this.handleClick()}>Click Me</button>
}
}
Note: Using an arrow function in render creates a new function each time the component renders, which may break optimizations
based on strict identity comparison.
↥
Q. How can I prevent a function from being called too
quickly?
1. Throttle:
Throttling prevents a function from being called more than once in a given window of time.
2. Debounce:
Debouncing ensures that a function will not be executed until after a certain amount of time has passed since it was last called. This
can be useful when you have to perform some expensive calculation in response to an event that might dispatch rapidly (eg scroll or
keyboard events).
Example:
/**
* Throttle and Debounce in React
*/
import * as React from "react";
import * as _ from "lodash";
exportdefaultclass App extends React.Component {
state = { count: 0 };
handleCount() {
this.setState((state) =>({
count: state.count + 1
}));
}
// You will run count() only once after 100ms
handleDebounce = _.debounce(() => this.handleCount(),100);
// You will run count() every 200ms
handleThrottle = _.throttle(() => this.handleCount(),200);
render() {
return (
<div>
{this.state.count}
<hr />
<button onClick={this.handleThrottle}>Click Me - Throttle </button>
<button onClick={this.handleDebounce}>Click Me - Debounce </button>
</div>
);
}
}
⚝
3. RequestAnimationFrame Throttling:
The requestAnimationFrame is a way of queuing a function to be executed in the browser at the optimal time for rendering
performance. A function that is queued with requestAnimationFrame will fire in the next frame. The browser will work hard to ensure
that there are 60 frames per second (60 fps). However, if the browser is unable to it will naturally limit the amount of frames in a
second.
For example, a device might only be able to handle 30 fps and so you will only get 30 frames in that second. Using
requestAnimationFrame for throttling is a useful technique in that it prevents you from doing more than 60 updates in a second. If
you are doing 100 updates in a second this creates additional work for the browser that the user will not see anyway.
/**
* RequestAnimationFrame Throttling
*/
importrafSchedule from "raf-schd";
importReact from "react";
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.handleScroll = this.handleScroll.bind(this);
// Create a new function to schedule updates.
this.scheduleUpdate = rafSchedule((point) => this.props.onScroll(point));
}
handleScroll(e) {
// When we receive a scroll event, schedule an update.
// If we receive many updates within a frame, we'll only publish the latest value.
this.scheduleUpdate({ x: e.clientX, y: e.clientY });
}
componentWillUnmount() {
// Cancel any pending updates since we're unmounting.
this.scheduleUpdate.cancel();
}
render() {
return (
<div style={{overflow: "scroll" }} onScroll={this.handleScroll}>
<img src="/my-huge-image.png" alt="Nature"/>
</div>
);
}
}
⚝
↥
Q. Explain synthetic event in React js?
Inside React event handlers, the event object is wrapped in a SyntheticEvent object. These objects are pooled, which means that the
objects received at an event handler will be reused for other events to increase performance. This also means that accessing the
event object's properties asynchronously will be impossible since the event's properties have been reset due to reuse.
The following piece of code will log null because event has been reused inside the SyntheticEvent pool:
functionhandleClick(event) {
setTimeout(function () {
console.log(event.target.name)
}, 1000)
}
To avoid this we need to store the event's property:
functionhandleClick(event) {
let name = event.target.name
setTimeout(function () {
console.log(name)
}, 1000)
}
SyntheticEvent Object
voidpreventDefault()
voidstopPropagation()
booleanisPropagationStopped()
booleanisDefaultPrevented()
voidpersist()
booleanbubbles
booleancancelable
DOMEventTarget currentTarget
booleandefaultPrevented
numbereventPhase
booleanisTrusted
DOMEventnativeEvent
DOMEventTarget target
numbertimeStamp
stringtype
↥
Q. What is Event Pooling in React?
The SyntheticEvent is pooled. This means that the SyntheticEvent object will be reused and all properties will be nullified after the
event callback has been invoked. This is for performance reasons. As such, you cannot access the event in an asynchronous way.
Example:
function onClick(event) {
console.log(event) // => nullified object.
console.log(event.type) // => "click"
consteventType = event.type// => "click"
setTimeout(function() {
console.log(event.type) // => null
console.log(eventType) // => "click"
}, 0)
// Won't work. this.state.clickEvent will only contain null values.
this.setState({clickEvent: event})
// You can still export event properties.
this.setState({eventType: event.type})
}
If we want to access the event properties in an asynchronous way, we should call event.persist() on the event, which will remove
the synthetic event from the pool and allow references to the event to be retained by user code.
↥
Q. How to trigger click event programmatically?
We can use ref prop to acquire a reference to the underlying HTMLInputElement object through a callback, store the reference as a
class property, then use that reference to later trigger a click from your event handlers using the HTMLElement.click method.
Example:
classMyComponent extends React.Component {
render() {
return (
<div onClick={this.handleClick}>
<input ref={input => this.inputElement = input} />
</div>
)
}
handleClick = (e) => {
this.inputElement.click()
}
}
Note: The ES6 arrow function provides the correct lexical scope for this in the callback.
↥
Q. How to listen for click events that are outside of a
component?
Example:
classOutsideAlerterextends Component {
// ...
componentDidMount() {
document.addEventListener("mousedown", this.handleClickOutside);
}
componentWillUnmount() {
document.removeEventListener("mousedown", this.handleClickOutside);
}
/**
* Set the wrapper ref
*/
setWrapperRef(node) {
this.wrapperRef = node;
}
/**
* Alert if clicked on outside of element
*/
handleClickOutside(event) {
if (this.wrapperRef&& !this.wrapperRef.contains(event.target)) {
alert("You clicked outside of me!");
}
}
render() {
return <div ref={this.setWrapperRef}>{this.props.children}</div>;
}
}
OutsideAlerter.propTypes = {
children: PropTypes.element.isRequired
};
⚝
↥
Q. How to convert text to uppercase on user input
entered?
importReact, { useState } from"react"
importReactDOMfrom "react-dom"
consttoInputUppercase = e => {
e.target.value = ("" + e.target.value).toUpperCase()
}
constApp = () => {
const [name, setName] = useState("")
return (
<input
name={name}
onChange={e =>setName(e.target.value)}
onInput={toInputUppercase} // apply on input which do you want to be capitalize
/>
)
}
ReactDOM.render(<App />, document.getElementById("root"))
↥
Q. How to set a dynamic key for state?
1. Dynamic Key:
onChange(e) {
constkey = e.target.name
constvalue = e.target.value
this.setState({ [key]: value})
}
2. Nested States:
handleSetState(cat, key, val) {
constcategory = {...this.state[cat]}
category[key] = val
this.setState({ [cat]: category })
}
↥
Q. What are the pointer events in React?
Pointer events, in essence, are very similar to mouse events (mousedown, mouseup, etc.) but are hardware-agnostic and thus can
handle all input devices such as a mouse, stylus or touch. This is great since it removes the need for separate implementations for
each device and makes authoring for cross-device pointers easier.
The API of pointer events works in the same manner as existing various event handlers. Pointer events are added as attributes to
React component and are passed a callback that accepts an event. Inside the callback we process the event.
The following event types are now available in React DOM
onPointerDown
onPointerMove
onPointerUp
onPointerCancel
onGotPointerCapture
onLostPointerCapture
onPointerEnter
onPointerLeave
onPointerOver
onPointerOut
Example: Drag and Drop using Point Events
// App Component
importReact, { Component } from 'react'
importlogo from './logo.svg'
import'./App.css'
importDragItemfrom'./DragItem'
classApp extends Component {
render() {
return (
<div className="App">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo"/>
<h1 className="App-title">Welcome to React sample of Point Events</h1>
</header>
<div className="App-intro">
<DragItem />
</div>
</div>
)
}
}
exportdefaultApp
DragItem Component
importReact from 'react'
constCIRCLE_DIAMETER = 100
exportdefaultclassDragItem extends React.Component {
state = {
gotCapture: false,
circleLeft: 500,
circleTop: 100
}
isDragging = false
previousLeft = 0
previousTop = 0
onDown = e => {
this.isDragging = true
e.target.setPointerCapture(e.pointerId)
this.getDelta(e)
}
onMove = e => {
if (!this.isDragging) {
return
}
const {left, top} = this.getDelta(e)
this.setState(({circleLeft, circleTop}) => ({
circleLeft: circleLeft + left,
circleTop: circleTop + top
}))
}
onUp = e => (this.isDragging = false)
onGotCapture = e => this.setState({gotCapture: true})
onLostCapture = e => this.setState({gotCapture: false})
getDelta = e => {
const left = e.pageX
const top = e.pageY
const delta = {
left: left - this.previousLeft,
top: top - this.previousTop,
}
this.previousLeft = left
this.previousTop = top
return delta
}
render() {
const {gotCapture, circleLeft, circleTop} = this.state
const boxStyle = {
border: '2px solid #cccccc',
margin: '10px 0 20px',
minHeight: 400,
width: '100%',
position: 'relative',
}
const circleStyle = {
width: CIRCLE_DIAMETER,
height: CIRCLE_DIAMETER,
borderRadius: CIRCLE_DIAMETER / 2,
position: 'absolute',
left: circleLeft,
top: circleTop,
backgroundColor: gotCapture ? 'red' : 'green',
touchAction: 'none',
}
return (
<div style={boxStyle}>
<div
style={circleStyle}
onPointerDown={this.onDown}
onPointerMove={this.onMove}
onPointerUp={this.onUp}
onPointerCancel={this.onUp}
onGotPointerCapture={this.onGotCapture}
onLostPointerCapture={this.onLostCapture}
/>
</div>
)
}
}
Note: It work only in browsers that support the Pointer Events specification
↥
Q. What is an alternative way to avoid having to bind
to this in event callback methods?
1. Bind in Constructor:
classApp extends Component {
constructor(props) {
super(props)
this.handleClick = this.handleClick.bind(this)
}
handleClick() {
console.log('Clicked !')
}
render() {
return <button onClick={this.handleClick}>Click Me</button>
}
}
2. Bind in Render:
classApp extends Component {
handleClick() {
console.log('Clicked !')
}
render() {
return <button onClick={this.handleClick.bind(this)}>Click Me</button>
}
}
3. Arrow Function in Render:
classApp extends Component {
handleClick() {
console.log('Clicked !')
}
render() {
return <button onClick={()=> this.handleClick()}>Click Me</button>
}
}
Using an arrow function in render creates a new function each time the component renders, which may break optimizations based
on strict identity comparison.
↥
Q. What is the alternative of bindingthis in the
constructor?
Arrow Function: This creates and binds the function all at once. Inside render (and elsewhere), the function is already bound
because the arrow function preserves the this binding.
Example:
classButtonextends React.Component {
// no binding
handleClick = (e) => {
console.log('clicked !');
}
render() {
return <button onClick={this.handleClick}>Click Me</button>;
}
}
↥
# 8. REACT LISTS
Q. Explain the Lists in React?
Using JSX we can show lists using JavaScript's built-inArray.map() method. The .map() method is often used to take one piece of
data and convert it to another.
Keys are unique identifiers that must be attached to the top-level element inside a map. Keys are used by React to know how to
update a list whether adding, updating, or deleting items. This is part of how React is so fast with large lists.
Example:
/**
* React List
*/
exportdefaultclass App extends Component {
state = {
lists: [
{ id: 0, context: "Success" },
{ id: 1, context: "Warning" },
{ id: 2, context: "Danger" }
]
};
render() {
return (
<>
<h2>React List</h2>
<ul className="list-group">
{this.state.lists.map((list) => (
<li key={list.id}>{list.context}</li>
))}
</ul>
</>
);
}
}
⚝
↥
Q. Why do I need Keys in React Lists?
Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the
array to give the elements a stable identity:
Example:
constnumbers = [10, 20, 30, 40, 50];
constlistItems = numbers.map((number) =>
<li key={number.toString()}>
{number}
</li>
);
React recommends that you do not use indexes as keys, if the order of items may change. It could impact performance negatively
and could lead to some unstable component behaviour.
↥
Q. How to display an array of strings in react
component?
constdata = ["this is line #1", "this is line #2","this is line #3"];
exportdefault function App() {
return (
<div>
{data.map((item, index) => (
<div key={index}>{item}</div>
))}
</div>
);
}
⚝
↥
Q. How do you render Array, Strings and Numbers in
React?
/**
* Array Component
*/
constitems = [
{ name: "AngularJS", description: "" },
{ name: "React", description: "" },
{ name: "Vue.js", description: "" }
];
constArrayList = (props) => (
<div>
<h2>Render Array List</h2>
{items.map((item, index) => (
<div key={index}>{item.name}</div>
))}
</div>
);
/**
* String Component
*/
constStringList = (props) => (
<div>
<h2>Render String List</h2>
{["test", "render", "array", "list"]}
</div>
);
/**
* Number Component
*/
constnumbers = [10, 20, 30];
constNumberList = (props) => (
<div>
<h2>Render Number List</h2>
{numbers.map((item, index) => (
<div key={index}>{item}</div>
))}
</div>
);
⚝
↥
# 9. React RESTful API
Q. How to make a API calls in React?
Consuming REST APIs in a React Application can be done in various ways. Some popular are Axios, fetch etc.
Example:
/**
* API call using fetch()
*/
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.state = {
users: []
};
}
componentDidMount() {
fetch("https://api.github.com/users")
.then((res) => res.json())
.then((result) => {
this.setState({ users: result });
});
}
render() {
const { users } = this.state;
return (
<ul>
{users.map((user) => (
<li key={user.id}>
<img src={user.avatar_url} alt={user.login} width="100px"/>
</li>
))}
</ul>
);
}
}
⚝
↥
Q. How to display API data using Axios in React?
Axios is a promise based HTTP client for making HTTP requests from a browser to any web server.
Example:
/**
* GET Request using Axios
*/
importReact, { useEffect, useState } from "react";
importaxios from "axios";
exportdefault function App() {
const [users, setUsers] = useState([]);
constfetchData = () => {
axios.get("https://jsonplaceholder.typicode.com/users").then((response) => {
setUsers(response.data);
});
};
useEffect(() => {
fetchData();
}, []);
return (
<div>
{users.length > 0 && (
<ul>
{users.map((user) =>(
<li key={user.id}>{user.name}</li>
))}
</ul>
)}
</div>
);
}
⚝
↥
Q. How to make a post call using Axios in React?
Example:
/**
* POST Request using Axios
*/
importReact from "react";
importaxios from "axios";
exportdefaultclassPersonList extends React.Component {
state = {
name: "",
result: []
};
handleChange = (event) => {
this.setState({name: event.target.value });
};
handleSubmit = (event) => {
event.preventDefault();
const user = {
name: this.state.name
};
axios
.post(`https://jsonplaceholder.typicode.com/users`, { user })
.then((res) => {
this.setState({ result: res.data.user.name});
});
};
render() {
return (
<div>
<form onSubmit={this.handleSubmit}>
<label>
Person Name:
<input type="text"name="name" onChange={this.handleChange} />
</label>
<button type="submit">Add</button>
</form>
<h4>Result: {this.state.result}</h4>
</div>
);
}
}
⚝
↥
Q. What are the benefits of using Axios() over Fetch()
for making http requests?
1. Fetch():
The Fetch API provides a fetch() method defined on the window object. It also provides a JavaScript interface for accessing and
manipulating parts of the HTTP pipeline (requests and responses). The fetch method has one mandatory argument- the URL of the
resource to be fetched. This method returns a Promise that can be used to retrieve the response of the request.
Example:
fetch('path-to-the-resource-to-be-fetched')
.then((response) => {
// Code for handling the response
})
.catch((error) => {
// Error Handling
});
2. Axios():
Axios is a Javascript library used to make HTTP requests from node.js or XMLHttpRequests from the browser and it supports the
Promise API that is native to JS ES6. It can be used intercept HTTP requests and responses and enables client-side protection against
XSRF. It also has the ability to cancel requests.
Example:
axios.get('url')
.then((response) => {
// Code for handling the response
})
.catch((error) => {
// Error Handling
});
Differences:
Axios()
Fetch()
Axios has url in request object.
Fetch has no url in request object.
Axios is a stand-alone third party package
that can be easily installed.
Fetch is built into most modern browsers
Axios has built-in XSRF protection.
Fetch does not.
Axios uses the data property.
Fetch uses the body property.
Axios data contains the object.
Fetch's body has to be stringified.
Axios request is ok when status is 200 and
statusText is 'OK'.
Fetch request is ok when response object contains the ok property.
Axios performs automatic transforms of
JSON data.
Fetch is a two-step process when handling JSON data- first, to make the actual
request; second, to call the .json() method on the response.
Axios allows cancelling request and request
timeout.
Fetch does not.
Axios has the ability to intercept HTTP
requests.
Fetch, by default, doesn't provide a way to intercept requests.
Axios has built-in support for download
progress.
Fetch does not support upload progress.
↥
Q. How does Axios Interceptors work in React?
Axios interceptors are the default configurations that are added automatically to every request or response that a user receives. It is
useful to check response status code for every response that is being received.
Interceptors are methods which are triggered before or after the main method. There are two types of interceptors:
1. Request Interceptor:
This is called before the actual call to the endpoint is made. One common use case for a request handler is to modify or add new
HTTP headers. For example, an authentication token could be injected into all requests.
Example:
// Request Handler
constrequestHandler = (request) => {
consttoken = localStorageService.getAccessToken()
if (token) {
request.headers['Authorization'] = 'Bearer ' + token
}
return request
}
// Request Interceptor
axios.interceptors.request.use(
request => requestHandler(request)
)
2. Response and error Interceptor:
This is called before the promise is completed and the data is received by the then callback.
Example:
// Response Handlers
consterrorHandler = (error) => {
if (isHandlerEnabled(error.config)) {
// Handle errors
}
return Promise.reject({ ...error })
}
constsuccessHandler = (response) => {
if (isHandlerEnabled(response.config)) {
// Handle responses
}
return response
}
// Response Interceptors
axios.interceptors.response.use(
response => successHandler(response),
error=> errorHandler(error)
)
↥
Q. How to do caching in React?
In React, caching data can be achieved in multiple ways
Local Storage
Redux Store
Keep data between mouting and unmounting
useMemo()
1. Memoizing Fetched Data:
Memoization is a technique we would use to make sure that we don't hit the API if we have made some kind of request to fetch it at
some initial phase. Storing the result of expensive fetch calls will save the users some load time, therefore, increasing overall
performance.
Example:
constcache = {}
constuseFetch = (url) => {
const [status, setStatus] = useState('idle')
const [data, setData] = useState([])
useEffect(() => {
if (!url) return
const fetchData = async()=> {
setStatus('fetching')
if (cache[url]) {
const data = cache[url]
setData(data)
setStatus('fetched')
} else {
const response = awaitfetch(url)
const data = await response.json()
cache[url] = data // set response in cache
setData(data)
setStatus('fetched')
}
}
fetchData()
}, [url])
return { status, data }
}
Here, we're mapping URLs to their data. So, if we make a request to fetch some existing data, we set the data from our local cache,
else, we go ahead to make the request and set the result in the cache. This ensures we do not make an API call when we have the
data available to us locally.
2. Memoizing Data With useRef():
With useRef(), we can set and retrieve mutable values at ease and its value persists throughout the component's lifecycle.
constuseFetch = (url) => {
constcache = useRef({})
const [status, setStatus] = useState('idle')
const [data, setData] = useState([])
useEffect(() => {
if (!url) return
const fetchData = async()=> {
setStatus('fetching')
if (cache.current[url]) {
const data = cache.current[url]
setData(data)
setStatus('fetched')
} else {
const response = awaitfetch(url)
const data = await response.json()
cache.current[url] = data // set response in cache
setData(data)
setStatus('fetched')
}
}
fetchData()
}, [url])
return { status, data }
}
3. Using localStorage():
constInitialState = {
someState: 'a'
}
classApp extends Component {
constructor(props) {
super(props)
// Retrieve the last state
this.state = localStorage.getItem("appState") ? JSON.parse(localStorage.getItem("appState")) : InitialState
}
componentWillUnmount() {
// Remember state for the next mount
localStorage.setItem('appState', JSON.stringify(this.state))
}
render() {
...
}
}
exportdefaultApp
4. Keep data between Mouting and Unmounting:
importReact, { Component } from 'react'
// Set initial state
letstate = { counter: 5 }
classCounter extends Component {
constructor(props) {
super(props)
// Retrieve the last state
this.state = state
this.onClick = this.onClick.bind(this)
}
componentWillUnmount() {
// Remember state for the next mount
state = this.state
}
onClick(e) {
e.preventDefault()
this.setState(prev => ({ counter: prev.counter + 1 }))
}
render() {
return (
<div>
<span>{ this.state.counter }</span>
<button onClick={this.onClick}>Increase</button>
</div>
)
}
}
exportdefault Counter
↥
Q. In which lifecycle event do you make AJAX requests
in React?
According to official React docs, the recommended place to do Ajax requests is in componentDidMount() which is a lifecycle
method that runs after the React component has been mounted to the DOM. This is so you can use setState() to update your
component when the data is retrieved.
Example:
/**
* API call in componentDidMount()
*/
importReact from "react";
exportdefaultclass App extends React.Component {
constructor(props) {
super(props);
this.state = {
error: null,
isLoaded: false,
users: []
};
}
componentDidMount() {
fetch("https://dummy.restapiexample.com/api/v1/employees")
.then((res) => res.json())
.then((result) => {
this.setState({
isLoaded: true,
users: result.data
});
})
.catch((error) => {
this.setState({
isLoaded: false,
error
});
});
}
render() {
const { error, isLoaded, users } = this.state;
if (error) {
return <div>Error: {error.message}</div>;
} else if (!isLoaded) {
return <div>Loading...</div>;
} else {
return (
<ul>
{users.length > 0 &&
users.map((user) => <li key={user.id}>{user.employee_name}</li>)}
</ul>
);
}
}
}
⚝
↥
Q. How to use async await in React?
Example:
classApp extends Component {
// ...
asynccomponentDidMount() {
{
const response = await fetch(`https://api.github.com/users`);
if (!response.ok) {
throw Error(response.statusText);
} else {
const json = await response.json();
this.setState({ data: json });
}
} catch (error) {
console.log(error);
}
}
render() {
return (
<div>
<ul>
{this.state.data.map((el) => (
<li key={el.id}>
<img src={el.avatar_url} alt={el.avatar_url} />
<spanclassName="UserLogin">{el.login}</span>
</li>
))}
</ul>
</div>
);
}
}
⚝
↥
Q. How RxJS is used in React for state management?
RxJS is a library for reactive programming using Observables, to make it easier to compose asynchronous or callback-based code.
Reactive programming is an event-based paradigm that allows us to run asynchronous sequences of events as soon as data is
pushed to a consumer.
RxJS Terminology
Observable: An Observable is a data stream that houses data that can be passed through different threads.
Observer: An Observer consumes the data supplied by an Observable
Subscription: In order for Observer to consume data from Observable, Observer has to subscribe it to the Observable.
Subject: An RxJS Subject can act as both an Observable and an Observer at the same time. In this way, values can be
multicasted to many Observers from it so that when a Subject receives any data, that data can be forwarded to every
Observer subscribed to it.
Operators: Operators are methods that can use on Observables and subjects to manipulate, filter or change the
Observable in a specified manner into a new Observable.
BehaviorSubject: It allows multiple observers to listen on stream and events multicasted to the observers,
BehaviorSubject stores the latest value and broadcasts it to any new subscribers.
Example:
// messageService.js
import { BehaviourSubject } from 'rxjs'
constsubscriber = new BehaviourSubject(0)
constmessageService = {
send: function(msg) {
subscriber.next(msg)
}
}
export {
messageService,
subscriber
}
The messageService object has a send function, which takes a msg parameter which holds the data we need to broadcast all
listening components, in the function body we call the emit method in the subscriber object it multicasts the data to the subscribing
components.
importReact, { Component } from 'react'
import { render } from 'react-dom'
import'./style.css'
import { subscriber, messageService } from './messageService'
classConsumerA extends React.Component {
constructor() {
this.state = {
counter: 0
}
}
componentDidMount() {
subscriber.subscribe((v)=> {
let { counter } = this.state
counter = counter + v
this.setState({ counter })
})
}
render() {
let { counter } = this.state
return (
<div>
<hr/>
<h3> Counter for Consumer A </h3>
<div> Counter: {counter} </div>
<hr/>
</div>
)
}
}
classConsumerB extends React.Component {
constructor() {
this.state = {
counter: 0
}
}
componentDidMount() {
subscriber.subscribe((v)=> {
let { counter } = this.state
counter = counter + v
this.setState({ counter })
})
}
render() {
let { counter } = this.state
return (
<div>
<hr/>
<h3>Counter for Consumer B</h3>
<div> Counter: { counter } </div>
<hr/>
<ProducerB />
</div>
)
}
}
classProducerA extends React.Component {
render() {
return (
<div>
<h3>ProducerA</h3>
<butto onClick={(e) =>subscriber.next(1)}>Increment Counter</button>
<ConsumerA />
</div>
)
}
}
classProducerB extends React.Component {
render() {
return (
<div>
<h3>ProducerB</h3>
<button onClick={(e)=> subscriber.next(-1)}>Decrement Counter</button>
</div>
)
}
}
classApp extends Component {
render() {
return (
<div>
<ProducerA />
<hr/>
<ConsumerB />
</div>
)
}
}
render(<App/>, document.getElementById('root'));
The ConsumerA and ConsumerB components keep a state counter individual. In their componentDidMount they subscribe to the
same stream subscriber, anytime an event is published they both update the counter. The ProducerA and ProducerB have buttons
Increment Counter and Decrement Counter when clicked they emit 1or-1.
Reference:
https://www.learnrxjs.io/
↥
# 10. REACT FORMS
Q. How dynamically generate menu options
for <select> from an array?
Example:
classApp extends React.Component {
constructor(props) {
super(props);
this.state = {
colors: {}
};
this.selectColor = this.selectColor.bind(this);
}
selectColor = (e) => {
console.log("Selected Color: ", e.target.value);
};
componentDidMount() {
this.setState({
colors: {
"#ff0000": "Red",
"#00ff00": "Green",
"#0000ff": "Blue"
}
});
}
render() {
const { colors } = this.state;
letcolorsList = Object.keys(colors).map((k) => {
return (
<option key={k} value={k}>
{colors[k]}
</option>
);
}, this);
return (
<div>
<select onChange={this.selectColor}>{colorsList}</select>
</div>
);
}
}
⚝
↥
Q. How would you create a form in React?
Example:
importReact from "react";
import"./styles.css";
constcountries = ["Austria", "India", "Italy", "Russia", "United States"];
exportdefault function App() {
const [email, setEmail] = React.useState("");
const [password, setPassword] = React.useState("");
const [coun , setCoun ] = React.useState("");
const [acceptedTerms, setAcceptedTerms] = React.useState(false);
consthandleSubmit = (event) => {
console.log(`
Email: ${email}
Password: ${password}
Coun : ${coun }
Accepted Terms: ${acceptedTerms}
`);
event.preventDefault();
};
return (
<form onSubmit={handleSubmit}>
<h1>Create Account</h1>
<label>Email:
<input name="email" type="email" value={email}
onChange={(e) => setEmail(e.target.value)}
required
/>
</label>
<label>Password:
<input name="password"type="password" value={password}
onChange={(e) => setPassword(e.target.value)}
required
/>
</label>
<label>Coun :
<select name="coun " value={coun }
onChange={(e) => setCoun (e.target.value)}
required
>
<option key=""></option>
{countries.map((coun) => (
<optionkey={coun }>{coun }</option>
))}
</select>
</label>
<label><inputname="acceptedTerms"type="checkbox"
onChange={(e) => setAcceptedTerms(e.target.value)}
required
/>
I accept the terms of service
</label>
<button>Submit</button>
</form>
);
}
Output:
Installation:
npminstall react-hook-form
npminstallmaterial-ui
Example:
importReact from "react";
import { makeStyles } from "@material-ui/core";
importTextField from "@material-ui/core/TextField";
importButton from "@material-ui/core/Button";
import { useForm, Controller } from "react-hook-form";
constuseStyles = makeStyles((theme) =>({
root: {
display: "flex",
flexDirection: "column",
justifyContent: "center",
alignItems: "center",
padding: theme.spacing(2),
"& .MuiTextField-root": {
margin: theme.spacing(1),
width: "300px"
},
"& .MuiButtonBase-root": {
margin: theme.spacing(2)
}
}
}));
constForm = ({ handleClose })=> {
constclasses = useStyles();
const { handleSubmit, control } = useForm();
constonSubmit = (data) => {
console.clear();
console.log(data);
};
return (
<formclassName={classes.root} onSubmit={handleSubmit(onSubmit)}>
<Controller
name="email"
control={control}
defaultValue=""
render={({ field: { onChange, value }, fieldState: { error } }) => (
<TextField
label="Email"
variant="filled"
value={value}
onChange={onChange}
error={!!error}
helperText={error? error.message : null}
type="email"
/>
)}
rules={{ required: "Email required" }}
/>
<Controller
name="password"
control={control}
defaultValue=""
render={({ field: { onChange, value }, fieldState: { error } }) => (
<TextField
label="Password"
variant="filled"
value={value}
onChange={onChange}
error={!!error}
helperText={error? error.message : null}
type="password"
/>
)}
rules={{ required: "Password required"}}
/>
<div>
<Button variant="contained" onClick={handleClose}>
Cancel
</Button>
<Button type="submit" variant="contained" color="primary">
Signup
</Button>
</div>
</form>
);
};
⚝
↥
Q. Why You Should Choose React Hook Form Over
Formik and Redux-Form?
Below are the main reasons to recommend React Hook Form Over Formik and Redux-Form,
1. Isolate Component Re-Rendering:
React Hook Form isolates the component and avoids the other components from re-rending. This feature will improve the
performance by avoiding unwanted rendering in other child components. However, libraries like Formik and Redux-Form re-render
the other child components along with the form component.
2. Reduce Rendering:
Besides isolating the component, it also limits its own (form components) form re-rendering on particular events such as onChange,
onBlur, etc.
3. Faster Mounting:
Mounting time is about 13% faster than Formik and 25% faster than Redux-Form. In other words, the form's DOM elements will be
inserted into the DOM tree quicker, leading to fast rendering compared with other libraries.
4. Input Change Subscriptions:
React Hook Form allows you to subscribe to each input element without going through the re-rendering of each element inside the
form component.
5. Typescript Support:
React Hook Form is built with TypeScript and can define a FormData type to support form values.
6. Less Code to Maintain:
The React Hook Form provides a hook called useForm(), consisting of methods and props handleSubmit, register, and errors. They
would handle the submit events, the input via refs using register, and display any errors. However, in the other two libraries, you
need to write your custom handlers for events and validations.
↥
Q. What are controlled and uncontrolled components
in React?
In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form
data is handled by the DOM itself.
1. Controlled Components:
In a controlled component, the form data is handled by the state within the component. The state within the component serves as
“the single source of truth” for the input elements that are rendered by the component.
Example:
/**
* Controlled Components
*/
importReact, { Component } from "react";
exportdefaultclass App extends Component {
state = {
message: ""
};
updateMessage = (newText)=> {
console.log(newText);
this.setState(() => ({
message: newText
}));
};
render() {
return (
<divclassName="App">
<div className="container">
<input
type="text"
placeholder="Your message here.."
value={this.state.message}
onChange={(event) => this.updateMessage(event.target.value)}
/>
<h3>Message: {this.state.message}</h3>
</div>
</div>
);
}
}
⚝
2. Uncontrolled Components:
Uncontrolled components act more like traditional HTML form elements. The data for each input element is stored in the DOM, not
in the component. Instead of writing an event handler for all of your state updates, It uses ref to retrieve values from the
DOM. Refs provide a way to access DOM nodes or React elements created in the render method.
/**
* Uncontrolled Component
*/
importReact, { Component } from "react";
exportdefaultclass App extends Component {
constructor(props) {
super(props);
this.handleChange = this.handleChange.bind(this);
this.input = React.createRef();
}
handleChange = (newText)=> {
console.log(newText);
};
render() {
return (
<divclassName="App">
<div className="container">
<input
type="text"
placeholder="Your message here.."
ref={this.input}
onChange={(event) => this.handleChange(event.target.value)}
/>
</div>
</div>
);
}
}
⚝
↥
Q. How do you set default value for uncontrolled
components?
React providesdefaultValue attribute that pre-populate the input field with the default Value without overriding any value input by
the user.
Example:
/**
* React defaultValue
*/
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
Name:
<input
defaultValue="Samir Chahal"
type="text"
ref={this.input} />
</label>
<input type="submit" value="Submit" />
</form>
);
}
⚝
↥
# 11. REACT HOOKS
Q. What are React Hooks?
React Hooks are in-built functions that allow to use state and lifecycle methods inside functional components, they also work
together with existing code, so they can easily be adopted into a codebase.
Rules of Hooks:
Hooks should not be called inside loops, conditions, or nested functions.
Hooks should be used inside React function components
Built-in Hooks:
Hooks
Description
useState()
To manage states. Returns a stateful value and an updater function to update it.
useEffect()
To manage side-effects like API calls, subscriptions, timers, mutations, and more.
useContext()
To return the current value for a context.
useReducer()
A useState alternative to help with complex state management.
Hooks
Description
useCallback()
It returns a memorized version of a callback to help a child component not re-render unnecessarily.
useMemo()
It returns a memoized value that helps in performance optimizations.
useRef()
It returns a ref object with a .current property. The ref object is mutable. It is mainly used to access a child
component imperatively.
useImperativeHandle()
It customizes the instance value that is exposed to parent components when using ref.
useLayoutEffect()
It fires at the end of all DOM mutations. It's best to use useEffect as much as possible over this one as the
useLayoutEffect fires synchronously.
useDebugValue()
Helps to display a label in React DevTools for custom hooks.
Example:
/**
* useState() Hooks
*/
importReact, { useState } from "react";
exportdefault function App() {
const [isButtonClicked, setIsButtonClickedStatus] = useState(false);
return (
<buttononClick={() => setIsButtonClickedStatus(!isButtonClicked)}>
{isButtonClicked ? "Clicked" : "Click Me, Please"}
</button>
);
}
⚝
↥
Q. What are advantages of using React Hooks?
Hooks are easier to work with and to test (as separated functions from React components*) and make the code look
cleaner, easier to read — a related logic can be tightly coupled in a custom hook.
Hooks allow to do by breaking the logic between components into small functions and using them inside the
components.
Improved code reuse
Better code composition
Better defaults
Sharing non-visual logic with the use of custom hooks
Flexibility in moving up and down the components tree.
↥
Q. How to create custom Hooks?
React also allows us to create custom Hooks with unique features that extracts component logic into reusable functions.
A custom Hooks has following features:
As a function, it takes input and returns output.
Its name starts with uselike useQuery, useMedia…
Unlike functional components, custom hooks return a normal, non-jsx data.
Unlike normal functions, custom hooks can use other hooks such as useState, useRef… and other custom hooks.
Example: Custom Hook - useFetch()
/**
* Custom Hook
*/
import { useState, useEffect } from "react";
constuseFetch = (url) => {
const [data, setData] = useState(null);
useEffect(() => {
fetch(url)
.then((res) => res.json())
.then((data) =>setData(data));
}, [url]);
return [data];
};
exportdefault useFetch;
/**
* App Component
*/
import"./styles.css";
importuseFetchfrom"./useFetch";
exportdefault function App() {
// custom hook
const [data] = useFetch("https://jsonplaceholder.typicode.com/todos");
return (
<>
{data&&
data.map((item) => {
return <p key={item.id}>{item.title}</p>;
})}
</>
);
}
⚝
↥
Q. How to fetch data with React Hooks?
The useState() is a hook used to maintain local states in function components and useEffect() hook is used to execute functions
after a component gets rendered (to "perform side effects").
Example:
/**
* useState() and useEffect() Hooks
*/
importReact, { useState, useEffect } from "react";
exportdefault function App() {
const [users, setUsers] = useState([]);
useEffect(() => {
fetch("https://api.github.com/users")
.then((response) => response.json())
.then((data) => {
setUsers(data);
});
}, []);
return (
<div>
{users.map((user) => (
<div key={user.id}>
<span>
<img src={user.avatar_url} width={"30px"} alt={user.avatar_url} />
</span>
<span> {user.login.toUpperCase()}</span>
</div>
))}
</div>
);
}
⚝
↥
Q. Do Hooks replace render props and higher-order
components?
1. Using React Hooks:
Hooks were designed to replace class and provide another great alternative to compose behavior into your components. Higher
Order Components are also useful for composing behavior. Hooks encapsulate the functionality to easily reusable functions
const [active, setActive] = useState(defaultActive)
There are few build-in Hooks
import {
useState,
useReducer,
useEffect,
useCallback,
useMemo,
useRef,
...
} from 'react'
2. Using Higher Order Components:
A Higher Order Component (HOC) is a component that takes a component and returns a component. HOCs are composable using
point-free, declarative function composition.
Example: logger API
importReact, { useEffect } from 'react'
constwithLogging = Component=> props => {
useEffect(() => {
fetch(`/logger?location=${window.location}`)
}, [])
return <Component {...props } />
}
exportdefault withLogging
To use it, you can mix it into an HOC that you\’ll wrap around every page:
importReact from 'react'
importwithAuthfrom'./with-auth.js'
importwithLogging from './with-logging.js'
importwithLayout from './with-layout.js'
constpage = compose(
withRedux,
withAuth,
withLogging,
withLayout('default'),
)
exportdefaultpage
To use this for a page
importpage from '../hocs/page.js'
importMyPageComponent from './my-page-component.js'
exportdefault page(MyPageComponent)
↥
Q. How to compare oldValues and newValues on React
Hooks useEffect?
We can store old values in a ref since assigning values to them won't trigger a re-rendering of the component but the value will
persist after each render cycle.
Example:
// To store old values
constusePrevious = (value) =>{
constref = useRef();
useEffect(() => {
ref.current = value;
});
return ref.current;
};
exportdefault function App() {
const [count, setCount] = useState(0);
constprevCount = usePrevious(count);
useEffect(() => {
console.log("prevCount: ", prevCount, "count: ", count);
}, [prevCount, count]);
return (
<div>
<button onClick={() => setCount((c) => c + 10)}>Increment</button>
<p>{count}</p>
</div>
);
}
Here, We create the usePrevioushook with the value parameter which is state we want to get the previous value from, In the hook,
we create a ref with the useRef hook to create a non-reactive property. Then we add the useEffect hook with a callback that sets
the ref.current to value to set the previous value.
⚝
↥
Q. How to re-render the view when the browser is
resized?
1. Using React Hooks:
importReact, { useLayoutEffect, useState } from 'react'
functionuseWindowSize() {
const [size, setSize] = useState([0, 0])
useLayoutEffect(() => {
function updateSize(){
setSize([window.innerWidth, window.innerHeight])
}
window.addEventListener('resize', updateSize)
updateSize()
return () => window.removeEventListener('resize', updateSize)
}, [])
return size
}
functionShowWindowDimensions(props) {
const [width, height] = useWindowSize()
return <span>Window size: {width} x {height}</span>
}
2. Using React Class:
importReact from 'react'
classShowWindowDimensions extends React.Component {
state = { width: 0, height: 0 }
updateDimensions = () => {
this.setState({width: window.innerWidth, height: window.innerHeight })
}
/**
* Add event listener
*/
componentDidMount() {
window.addEventListener('resize', this.updateDimensions)
}
/**
* Remove event listener
*/
componentWillUnmount() {
window.removeEventListener('resize', this.updateDimensions)
}
render() {
return (
<span>Window size: {this.state.width} x {this.state.height}</span>
)
}
}
↥
Q. How can I force a component to re-render with
hooks in React?
The useState()oruseReducer() hooks can be used to force a React component to rerender.
The example below is equivalent to forceUpdate() method in class-based components. This hook works in the following way:
The useState() hook returns an array with two elements, a value and an updater function.
Here, we are instantly calling the updater function, which in this case is called with undefined, so it is the same as
calling updater(undefined).
Example:
/**
* Rerender Component with useState()
*/
importReact, { useState } from "react";
constuseForceUpdate = () => useState()[1];
exportdefault function App() {
constforceUpdate = useForceUpdate();
console.log("Rendered");
return <button onClick={forceUpdate}>Update Me</button>;
}
⚝
↥
Q. What is useState() in React?
The useState() is a Hook that allows to have state variables in functional components.
importReact, { useState } from'react'
constApp = () => {
const [count, setCount] = React.useState(0)
consthandleIncrease = ()=>{
setCount(count + 1)
}
consthandleDecrease = ()=>{
setCount(count - 1)
}
return (
<div>
Count: {count}
<hr/>
<div>
<button type="button" onClick={handleIncrease}>
Increase
</button>
<button type="button" onClick={handleDecrease}>
Decrease
</button>
</div>
</div>
)
}
The useState() function takes as argument a value for the initial state. In this case, the count starts out with 0. In addition, the hook
returns an array of two values: count and setCount. It's up to you to name the two values, because they are destructured from the
returned array where renaming is allowed.
↥
Q. Why do we use array destructuring in useState?
The useState hook allows us to make our function components stateful. When called, useState() returns an array of two items. The
first being our state value and the second being a function for setting or updating that value. The useState hook takes a single
argument, the initial value for the associated piece of state, which can be of any Javascript data type.
importReact, { useState } from 'react';
constComponent = () => {
const [value, setValue] = useState(initial value)
...
Example: State with Various Data Types
const [count, setCount] = useState(0)
const [color, setColor] = useState('#526b2d')
const [isHidden, setIsHidden] = useState(true)
const [products, setProducts] = useState([])
const [user, setUser] = useState({
username: '',
avatar: '',
email: '',
})
↥
Q. What is useReducer() in React?
It accepts a reducer function with the application initial state, returns the current application state, then dispatches a function.
Although useState() is a Basic Hook and useReducer() is an Additional Hook, useState() is actually implemented with useReducer().
This means useReducer() is primitive and we can use useReducer() for everything can do with useState(). Reducer is so powerful that
it can apply for various use cases.
Example:
importReact, { useReducer } from 'react'
constinitialState = 0
constreducer = (state, action)=> {
switch (action) {
case 'increment': return state + 1
case 'decrement': return state - 1
case 'reset': return 0
default: throw new Error('Unexpected action')
}
}
constReducerExample = () => {
const [count, dispatch] = useReducer(reducer, initialState)
return (
<div>
{count}
<button onClick={() => dispatch('increment')}>+1</button>
<button onClick={() => dispatch('decrement')}>-1</button>
<button onClick={() => dispatch('reset')}>reset</button>
</div>
)
}
exportdefault ReducerExample
Here, we first define an initialState and a reducer. When a user clicks a button, it will dispatch an action which updates the count and
the updated count will be displayed. We could define as many actions as possible in the reducer, but the limitation of this pattern is
that actions are finite.
↥
Q. How does React renderer work exactly when we call
setState?
The state allows React components to change their output over time in response to user actions, network responses, and anything
else, without violating this rule. Components defined as classes have some additional features. Local state is a feature available only
to class Components.
The setState() is the API method provided with the library so that the user is able to define and manipulate state over
time. setState() is the only legitimate way to update state after the initial state setup.
Example:
importReact, { Component } from 'react'
classSearchextendsComponent{
constructor(props) {
super(props)
this.state = {
searchString: ''
}
}
}
we are passing an empty string as a value and, to update the state of searchString, we have to call setState().
setState({ searchString: event.target.value })
Here, we are passing an object to setState(). The object contains the part of the state we want to update which, in this case, is the
value of searchString. This is basically kicking off a process that React calls reconciliation. The reconciliation process is the way React
updates the DOM, by making changes to the component based on the change in state.
When the request to setState() is triggered, React creates a new tree containing the reactive elements in the component (along
with the updated state). This tree is used to figure out how the Search component's UI should change in response to the state
change by comparing it with the elements of the previous tree.
↥
Q. What is a Webhook in React?
Web hooks provide a mechanism where by a server-side application can notify a client-side application when a new event (that the
client-side application might be interested in) has occurred on the server.
Webhooks are also sometimes referred to as "Reverse APIs". In APIs, the client-side application calls (consumes) the server-side
application. Whereas, in case of web hooks it is the server-side that calls (consumes) the web hook (the end-point URL provided by
the client-side application), i.e. it is the server-side application that calls the client-side application.
↥
Q. Exlain is useCallback(), useMemo(),
useImperativeHandle(), useLayoutEffect(),
useDebugValue() in React?
1. useCallback()
React's useCallback() Hook can be used to optimize the rendering behavior of your React function components.
The useCallback will return a memoized version of the callback that only changes if one of the dependencies has changed.
This is useful when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders
(e.g. shouldComponentUpdate).
Example:
/**
* useCallback()
*/
importReact, { useState, useCallback, useEffect } from "react";
constcount = new Set();
exportdefault function App() {
const [counter, setCounter] = useState(0);
constincrement = useCallback(() => {
setCounter(counter + 1);
}, [counter]);
useEffect(() => {
count.add(increment);
console.log(count.size);
}, [increment]);
return (
<>
<h1>useCallback()</h1>
<h2>Function Call: {count.size}</h2>
<button onClick={increment}>Increment</button>
</>
);
}
⚝
2. useMemo()
The useMemo() is similar to useCallback() except it allows you to apply memoization to any value type. It does this by accepting a
function which returns the value and then that function is only called when the value needs to be retrieved.
Example:
React application which renders a list of users and allows us to filter the users by their name. The filter happens only when a user
explicitly clicks a button; not already when the user types into the input field.
importReact from 'react'
constusers = [
{ id: 'a', name: 'Robin' },
{ id: 'b', name: 'Dennis' },
]
constApp = () => {
const [text, setText] = React.useState('')
const [search, setSearch] = React.useState('')
consthandleText = (event) => {
setText(event.target.value)
}
consthandleSearch = () => {
setSearch(text)
}
// useMemo Hooks
constfilteredUsers = React.useMemo(
() =>
users.filter((user) => {
console.log('Filter function is running ...');
return user.name.toLowerCase().includes(search.toLowerCase());
}),
[search]
);
return (
<div>
<input type="text" value={text} onChange={handleText} />
<button type="button" onClick={handleSearch}>
Search
</button>
<List list={filteredUsers} />
</div>
)
}
constList = ({ list }) => {
return (
<ul>
{list.map((item) => (
<ListItem key={item.id}item={item} />
))}
</ul>
)
}
constListItem = ({item })=> {
return <li>{item.name}</li>
}
exportdefaultApp
Here, the filteredUsers function is only executed once the search state changes. It doesn't run if the text state changes, because
that's not a dependency for this filter function and thus not a dependency in the dependency array for the useMemo hook.
3. useImperativeHandle()
useImperativeHandle() customizes the instance value that is exposed to parent components when using ref. As always, imperative
code using refs should be avoided in most cases. useImperativeHandle should be used with forwardRef.
function FancyInput(props, ref) {
constinputRef = useRef()
useImperativeHandle(ref, ()=> ({
focus: () => {
inputRef.current.focus()
}
}))
return <input ref={inputRef} ... />
}
FancyInput = forwardRef(FancyInput)
4. useLayoutEffect()
This runs synchronously immediately after React has performed all DOM mutations. This can be useful if you need to make DOM
measurements (like getting the scroll position or other styles for an element) and then make DOM mutations or trigger a
synchronous re-render by updating state.
As far as scheduling, this works the same way as componentDidMount and componentDidUpdate. Your code runs immediately after the
DOM has been updated, but before the browser has had a chance to "paint" those changes (the user doesn't actually see the
updates until after the browser has repainted).
Example:
importReact, { useState, useLayoutEffect } from 'react'
importReactDOMfrom 'react-dom'
constBlinkyRender = () => {
const [value, setValue] = useState(0)
useLayoutEffect(() => {
if (value === 0) {
setValue(10 + Math.random() * 200)
}
}, [value])
console.log('render', value)
return (
<divonClick={() => setValue(0)}>
value: {value}
</div>
)
}
ReactDOM.render( <BlinkyRender/>, document.querySelector('#root'))
useLayoutEffect vs useEffect
useLayoutEffect: If you need to mutate the DOM and/or do need to perform measurements
useEffect: If you don't need to interact with the DOM at all or your DOM changes are unobservable (seriously, most of the
time you should use this).
5. useDebugValue()
useDebugValue() can be used to display a label for custom hooks in React DevTools.
Example:
functionuseFriendStatus(friendID) {
const [isOnline, setIsOnline] = useState(null)
// ...
// Show a label in DevTools next to this Hook
// e.g. "FriendStatus: Online"
useDebugValue(isOnline ? 'Online' : 'Offline')
return isOnline
}
↥
Q. How to Memoize Components in React?
If your component renders the same result given the same props, you can wrap it in a call to React.memo for a performance boost in
some cases by memoizing the result. This means that React will skip rendering the component, and reuse the last rendered result.
React.memo only checks for prop changes. If your function component wrapped in React.memo has
a useState, useReduceroruseContext Hook in its implementation, it will still rerender when state or context change.
Example: Let's take an example of search functionality. In the example below, the App component contains:
Search input for the fruit name
A button and a child component where the user search will be displayed
A count of the number of times a user has clicked the button
exportdefault function App() {
const fruits = ["apple", "orange", "banana"];
const [fruitName, setFruitName] = useState("");
const [searchedFruit, setSearchedFruit] = useState(
"Search your favorite fruit"
);
const [count, setCount] = useState(0);
const searchFruitName = ()=> {
if (fruits.includes(fruitName)) {
setSearchedFruit(fruitName);
} else {
setSearchedFruit("No results Found");
}
setCount(count+1);
};
const showAllFruits = ()=> {
return fruits.map((fruit, index) => {
return (
<span key={index} className="fruitname">
{fruit}
</span>
);
});
};
return (
<divclassName="App">
<h3>Count: {count}</h3>
<div className="fruits">{showAllFruits()}</div>
<div>
<input
type="text"
placeholder="Search.."
onChange={event => setFruitName(event.target.value)}
value={fruitName}
/>
<button onClick={searchFruitName}>Search</button>
</div>
<SearchComponent searchedFruitName={searchedFruit} />
</div>
);
}
⚝
↥
Q. How to prevent unnecessary updates using
React.memo()?
The React.memo() is a higher-order component that will memoize your component, very similar to PureComponent. It will
shallowly compare current and new props of the component, and if nothing changes, React will skip the rendering of that
component.
// Memo.js
constText = (props) => {
console.log(`Text Component`);
return <div>Text Component re-render: {props.count} times </div>;
};
constMemoText = React.memo(
(props) => {
console.log(`MemoText Component`);
return <div>MemoText Component re-render: {props.count} times </div>;
},
(preprops, nextprops)=> true
);
// App.js
constApp = () => {
console.log(`App Component`);
const [count, setCount] = useState(0);
return (
<>
<h2>This is function component re-render: {count} times </h2>
<Text count={count} />
<MemoText count={count} />
<br/>
<button
onClick={()=> {
setCount(count + 1);
}}
>
CLICK ME
</button>
</>
);
};
⚝
↥
# 12. REACT CONTEXT
Q. What is Context API in React?
The React Context API allows to easily access data at different levels of the component tree, without having to pass data down
through props.
Example:
/**
* Counter Component
*/
const { useState, useContext } = React;
constCountContext = React.createContext();
function Counter() {
const { count, increase, decrease } = useContext(CountContext);
return (
<h2>
<button onClick={decrease}>Decrement</button>
<spanclassName="count">{count}</span>
<button onClick={increase}>Increment</button>
</h2>
);
}
/**
* App Component
*/
exportdefault function App() {
const [count, setCount] = useState(0);
constincrease = () => {
setCount(count + 1);
};
constdecrease = () => {
setCount(count - 1);
};
return (
<div>
<CountContext.Provider value={{ count, increase, decrease }}>
<Counter />
</CountContext.Provider>
</div>
);
}
⚝
↥
Q. How do you solve performance corner cases while
using context?
Context provides a way to pass data or state through the component tree without having to pass props down manually through
each nested component. It is designed to share data that can be considered as global data for a tree of React components, such as
the current authenticated user or theme (e.g. color, paddings, margins, font-sizes).
Context API uses Context. Provider and Context. Consumer Components pass down the data but it is very cumbersome to write the
long functional code to use this Context API. So useContext hook helps to make the code more readable, less verbose and removes
the need to introduce Consumer Component. The useContext hook is the new addition in React 16.8.
Syntax:
constauthContext = useContext(initialValue);
The useContext accepts the value provided by React.createContext and then re-render the component whenever its value changes
but you can still optimize its performance by using memorization.
↥
Q. What is the purpose of default value in context?
The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be
helpful for testing components in isolation without wrapping them. Passing undefined as a Provider value does not cause
consuming components to use defaultValue.
/**
* Default value in Context API
*/
import { createContext, useContext } from "react";
constContext = createContext("Default Value");
/**
* Child1 Component
*/
function Child1() {
constcontext = useContext(Context);
return <h2>Child1: {context}</h2>;
}
/**
* Child2 Component
*/
function Child2() {
constcontext = useContext(Context);
return <h2>Child2: {context}</h2>;
}
/**
* App Component
*/
exportdefault function App() {
return (
<>
<Context.Provider value={"Initial Value"}>
<Child1 /> {/* Child inside Provider will get "Initial Value" */}
</Context.Provider>
<Child2 /> {/* Child outside Provider will get "Default Value" */}
</>
);
}
⚝
↥
Q. How to use contextType react?
The ContextType property on a class component can be assigned a Context object created by React.createContext() method. This
property lets you consume the nearest current value of the context using this.context. We can access this.context in any lifecycle
method including the render functions.
Example:
/**
* ContextType()
*/
importReact, { Component } from "react";
importReactDOMfrom "react-dom";
constMyContext = React.createContext({
name: "Context Type"
});
const { Provider, Consumer } = MyContext;
/**
* StoreProvider Component
*/
classStoreProviderextends Component {
state = {
count: 0
};
incrementCount = () => {
console.log("Increment");
const { count } = this.state;
this.setState({count: count + 1});
};
render() {
return (
<Provider
value={{
state: this.state,
incrementCount: this.incrementCount
}}
>
{this.props.children}
</Provider>
);
}
}
/**
* VoteCount Component
*/
classVoteCount extends Component {
static contextType = MyContext;
render() {
const { state, incrementCount } = this.context;
return (
<div>
<button onClick={incrementCount}>Click Me</button> {state.count}
</div>
);
}
}
constrootElement = document.getElementById("root");
ReactDOM.render(
<StoreProvider>
<VoteCount/>
</StoreProvider>,
rootElement
);
⚝
↥
Q. How to update React Context from inside a child
component?
The Context API allows data storage and makes it accessible to any child component who want to use it. This is valid whatever level
of component graph the children is in.
Example:
/**
* React Context API
*/
importReact, { useState, useContext } from"react";
constMyContext = React.createContext();
/**
* Child Component
*/
constMyComponent = () => {
const { count, increment } = useContext(MyContext);
return (
<div>
<button onClick={increment}>Click Me</button> {count}
</div>
);
};
/**
* App Component
*/
exportdefault function App() {
const [count, updateCount] = useState(0);
function increment() {
updateCount(count + 1);
}
return (
<MyContext.Provider value={{ count, increment }}>
<div>
<MyComponent/>
</div>
</MyContext.Provider>
);
}
Here, We are storing data in component state in which we want to use context and we created a function that modify this state. We
pass the state and function as context values. It then become possible from the child to get the function and to use it to update your
context.
⚝
↥
Q. What is prop drilling and how can you avoid it?
React passes data to child components via props from top to bottom. While there are few props or child components, it is easy to
manage and pass down data. But when the application grows, and want to pass data from the top level component to a 3rd or 4th
level level component but we end up passing these data to components on each level of the tree. This is called Prop-drilling.
Using Context API:
The Context API solves some of these prop drilling problems. It let pass data to all of the components in the tree without writing
them manually in each of them. Shared data can be anything: state, functions, objects, we name it, and it is accessible to all nested
levels that are in the scope of the context.
Example:
/**
* Prop Drilling
*/
importReact, { useContext, createContext } from "react";
// It returns an object with 2 values:
// { Provider, Consumer }
constNumberContext = createContext();
function Display() {
constvalue = useContext(NumberContext);
return <h3>Contex Value: {value}</h3>;
}
exportdefault function App() {
// Use the Provider to make a value available to all
// children and grandchildren
return (
<NumberContext.Provider value={100}>
<div>
<Display />
</div>
</NumberContext.Provider>
);
}
⚝
↥
# 13. REACT ROUTER
Q. What is React Router?
React router enables the navigation among views of various components in a React Application, allows changing the browser URL,
and keeps the UI in sync with the URL. It has a simple API with powerful features like lazy loading, dynamic route matching, and
location transition handling.
/**
* React Router v6
*/
import { BrowserRouter, Route, Routes, NavLink } from "react-router-dom";
/**
* Home Component
*/
constHome = () => {
return <h1>Home Page</h1>;
};
/**
* Contacts Component
*/
constContacts = ()=> {
return <h1>Contact Page</h1>;
};
/**
* App Component
*/
exportdefault function App() {
return (
<div className="App">
<BrowserRouter>
<div className="navbar">
<NavLink to={"/"}>Home</NavLink>
<NavLink to={"/contact"}>Contact Us</NavLink>
</div>
<Routes>
<Route path="/" element={<Home />} />
<Route path="/contact" element={<Contacts />} />
</Routes>
</BrowserRouter>
</div>
);
}
⚝
↥
Q. What are the components of react router?
The main components of React router are
1. BrowserRouter:
BrowserRouter is a router implementation that uses the HTML5 history API (pushState, replaceState and the popstate event) to keep
your UI in sync with the URL. It is the parent component that is used to store all of the other components.
2. Routes:
It's a new component introduced in the v6 and a upgrade of the component. The main advantages of Routes over Switch that routes
are chosen based on the best match instead of being traversed in order.
3. Route:
Route is the conditionally shown component that renders some UI when its path matches the current URL.
4. Link:
Link component is used to create links to different routes and implement navigation around the application. It works like HTML
anchor tag.
↥
Q. What is the difference between NavLink and Link?
The <Link> component is used to navigate the different routes on the site. But <NavLink> is used to add the style attributes to the
active routes.
Link
<Linkto="/">Home</Link>
NavLink
<NavLink to="/" activeClassName="active">Home</NavLink>
Example:
index.css
.active {
color: blue;
}
Routes.js
importReactDOMfrom 'react-dom'
import'./index.css'
import { Route, NavLink, BrowserRouter as Router, Switch } from 'react-router-dom'
importApp from './App'
importUsers from './users'
importContact from'./contact'
importNotfoundfrom'./notfound'
constRoutes = (
<Router>
<div>
<ul>
<li>
<NavLink exact activeClassName="active" to="/">
Home
</NavLink>
</li>
<li>
<NavLink activeClassName="active" to="/users">
Users
</NavLink>
</li>
<li>
<NavLink activeClassName="active" to="/contact">
Contact
</NavLink>
</li>
</ul>
<hr/>
<Switch>
<Route exactpath="/" component={App} />
<Route path="/users" component={Users} />
<Route path="/contact"component={Contact} />
<Route component={Notfound} />
</Switch>
</div>
</Router>
)
ReactDOM.render(Routes, document.getElementById('root'))
↥
Q. What is withRouter for in react-router-dom?
withRouter() is a higher-order component that allows to get access to the history object's properties and the closest <Route>'s
match. withRouter will pass updated match, location, and history props to the wrapped component whenever it renders.
Example:
importReact from "react"
importPropTypes from "prop-types"
import { withRouter } from "react-router"
// A simple component that shows the pathname of the current location
classShowTheLocationextends React.Component {
static propTypes = {
match: PropTypes.object.isRequired,
location: PropTypes.object.isRequired,
history: PropTypes.object.isRequired
}
render() {
const { match, location, history } = this.props
return <div>You are now at {location.pathname}</div>
}
}
constShowTheLocationWithRouter = withRouter(ShowTheLocation)
↥
Q. How React Router is different from history library?
React Router is a wrapper around the history library which handles interaction with the browser's window.history with its browser
and hash histories. React Router provides two API's
BrowserRouter
HashRouter
// <BrowserRouter>
http://example.com/about
// <HashRouter>
http://example.com/#/about
The <BrowserRouter> is the more popular of the two because it uses the HTML5 History API to keep your UI in sync with the URL,
whereas the <HashRouter> uses the hash portion of the URL (window.location.hash). If you need to support legacy browsers that
don't support the History API, you should use <HashRouter>. Otherwise <BrowserRouter> is the better choice for most use cases.
Example:
// src/index.js
importReact from "react";
importReactDOMfrom "react-dom";
importApp from "./App";
import { BrowserRouter } from "react-router-dom";
ReactDOM.render(
<BrowserRouter>
<App/>
</BrowserRouter>,
document.getElementById("root")
);
The above code creates a history instance for our entire <App> component. Each <Router>component creates a history object that
keeps track of the current location (history.location) and also the previous locations in a stack. The history object has methods
such as history.push, history.replace, history.goBack, history.goForward etc.
↥
Q. How to use useNavigate() in React Router v6?
The useNavigate() hook is introduced in React Router v6 to replace the useHistory() hook. In the earlier version,
the useHistory() hook accesses the React Router history object and navigates to the other routers using the push() or replace()
methods.
The useNavigate() hook returns a function that lets you navigate programmatically, for example after a form is submitted. If
using replace: true, the navigation will replace the current en in the history stack instead of adding a new one.
/**
* useNavigate()
*/
importReact from "react";
import { NavLink, Link, Routes,Route, useParams, useNavigate } from "react-router-dom";
import"./styles.css";
function Home() {
return <h1>Home Page</h1>;
}
function Users() {
return (
<ul>
<li><Link to={"/users/1"}>User 1</Link></li>
</ul>
);
}
function UserDetail() {
let { id } = useParams();
let navigate = useNavigate();
function handleClick() {
navigate("/users");
}
return (
<>
<h1>User Details Page: {id}</h1>
<button onClick={handleClick}>Back</button>
</>
);
}
functionAppRoutes() {
return (
<Routes>
<Route path="/" element={<Home />} />
<Route path="users" element={<Users />} />
<Route path="users/:id" element={<UserDetail/>} />
</Routes>
);
}
exportdefault function App() {
return (
<div className="App">
<nav>
<ul>
<li><NavLink to="/"end>Home Page</NavLink></li>
<li><NavLink to="/users">Users Page</NavLink></li>
</ul>
</nav>
<AppRoutes/>
</div>
);
}
⚝
↥
Q. How to get parameter value from query string?
In order to get query parameters from the URL, we can use URLSearchParams. In simple words, URLSearchParams is a defined
interface, implemented by modern browsers, that allows us to work with the query string. It does not require React Router or even
React itself.
Example:
// http://localhost:3000/?id=100&name=react
constqueryParams = new URLSearchParams(window.location.search);
constid = queryParams.get('id');
constname = queryParams.get('name');
console.log(id, name); // 100 react
⚝
↥
Q. How to access history object in React Router v6?
The useNavigate() hook has been added to React Router v6 to replace the useHistory() hook.
Example:
/**
* React Router
*/
import { BrowserRouter, Routes,NavLink, Route, useParams, useNavigate } from "react-router-dom";
exportdefault function App() {
return (
<BrowserRouter>
<div>
<ul>
<li><NavLink to="/">Home</NavLink></li>
<li><NavLink to="/user/Bhavya/bhavyasingh@email.com">User Profile</NavLink></li>
</ul>
<Routes>
<Route path="/user/:name/:email"element={<User/>} />
<Route path="/" element={<Home />} />
</Routes>
<HomeButton/>
</div>
</BrowserRouter>
);
}
function Home() {
return <h2>Welcome Home</h2>;
}
function User() {
let { name, email } = useParams();
return (
<h2>Name: {name} <br /> Email: {email}</h2>
);
}
function HomeButton() {
consthistory = useNavigate();
function handleClick() {
history("/");
}
return (
<>
<button type="button" onClick={handleClick}>Go Home</button>
</>
);
}
⚝
↥
Q. How to perform automatic redirect in React.js?
In contrast to the Navigate component and its declarative redirect, we can perform a programmatic redirect by using React
Router's useNavigate() Hook:
Example:
/**
* Automatic Redirect in router-v6
*/
import { NavLink, BrowserRouter, Routes, Route, Navigate } from"react-router-dom";
exportdefault function App() {
return (
<BrowserRouter>
<nav style={{display: "flex", flexDirection: "row", gap: "1em" }}>
<NavLink to="/" children="Home" />
<NavLink to="/about" children="About" />
<NavLink to="/help" children="Help" />
</nav>
<Routes>
<Route indexelement={<Navigate replace to="home"/>} />
<Route path="home" element={<h1>Home Page</h1>} />
<Route path="about" element={<h1>About Page</h1>} />
<Route path="help" element={<h1>Help Page</h1>} />
</Routes>
</BrowserRouter>
);
}
Note: To keep the history clean, you should set replace prop. This will avoid extra redirects after the user click back.
⚝
↥
Q. How to pass additional data while redirecting to a
route in React?
Using Link:
Example:
/**
* Pass additional data while redirecting
*/
import { BrowserRouter, Link, Route, Routes, useLocation } from "react-router-dom";
/**
* View User Component
*/
function ViewUser() {
constlocation = useLocation();
return (
<>
<h2>User Details</h2>
<div>Name:{location.state.name}</div>
<div>Email:{location.state.email}</div>
</>
);
}
/**
* User Component
*/
function User() {
return (
<div>
<h2>Pass additional data while redirecting</h2>
<Link
to="/view-user"
state={{
name: "Kalini Khalsa",
email: "kalini.khalsa@email.com"
}}
>
<button>View User</button>
</Link>
</div>
);
}
/**
* App Component
*/
exportdefault function App() {
return (
<BrowserRouter>
<Routes>
<Route exactpath="/" element={<User />} />
<Route exact path="/user" element={<User />} />
<Route exact path="/view-user" element={<ViewUser/>} />
</Routes>
</BrowserRouter>
);
}
⚝
↥
Q. How to pass props in React router v6?
React Router uses a declarative, component-based approach to routing. Route allows you to map URL paths to different React
components.
Example:
/**
* Pass props in React Router-v6
*/
importReact from "react";
import { BrowserRouter, Routes,Route, NavLink } from "react-router-dom";
exportfunction Greeting(props) {
const { text } = props;
return (
<>
<h2>Greetings Page</h2>
<p>{text}</p>
</>
);
}
constRouterExample = () => <h2>Home Page</h2>;
constApp = () => (
<BrowserRouter>
<ul>
<li><NavLink to="/">Home</NavLink></li>
<li><NavLink to="/greeting/pradeep">Greeting</NavLink></li>
</ul>
<hr/>
<Routes>
<Route exact path="/" element={<RouterExample />} />
<Route path="/greeting/:name" element={<Greeting text="Hello World" />} />
</Routes>
</BrowserRouter>
);
exportdefault App;
⚝
↥
Q. How to get query parameters in react routing?
Using useParams()
Example:
/**
* useParams()
*/
importReact from "react";
import { BrowserRouter, Route, Routes, Link, useParams } from"react-router-dom";
exportdefault function App() {
return (
<BrowserRouter>
<div>
<ul>
<li><Linkto="/home">Home</Link></li>
<li><Linkto="/contact-us">Contact Us</Link></li>
<li><Linkto="/help">Help</Link></li>
</ul>
<Routes>
<Route path="/:id" element={<Child />} />
</Routes>
</div>
</BrowserRouter>
);
}
function Child() {
// `useParams` hook used here to access parameters
let { id } = useParams();
return <h2>Parameter: {id}</h2>;
}
⚝
↥
Q. What is the difference between HashRouter and
BrowserRouter in React?
1. BrowserRouter:
The widely popular router and a router for modern browsers which user HTML5 pushState API.
(i.e. pushState, replaceState and popState API).
It routes as normal URL in browser, you can't differentiate whether it is server rendered page or client rendered page
through the URL.
It assumes, your server handles all the request URL (eg., /, /about) and points to root index.html. From there,
BrowserRouter take care of routing the relevant page.
It accepts forceRefresh props to support legacy browsers which doesn't support HTML5 pushState API
Syntax:
/**
* https://example.com/home
* https://example.com/about
*/
<BrowserRouter
basename={optionalString}
forceRefresh={optionalBool}
getUserConfirmation={optionalFunc}
keyLength={optionalNumber}
>
<App />
</BrowserRouter>
Example:
/**
* BrowserRouter()
*/
import { Link, BrowserRouter, Routes, Route } from"react-router-dom";
constHomePage = ()=> {
return <h2>Home Page</h2>;
};
constAboutPage = () => {
return <h2>About Page</h2>;
};
exportdefault function App() {
return (
<section className="App">
<BrowserRouter>
<ul>
<li><Linkto="/home">Home</Link></li>
<li><Linkto="/about">About</Link></li>
</ul>
<Routes>
<Route exact path="/home" element={<HomePage />} />
<Route exact path="/about" element={<AboutPage />} />
</Routes>
</BrowserRouter>
</section>
);
}
⚝
2. HashRouter:
A router which uses client side hash routing.
Whenever, there is a new route get rendered, it updated the browser URL with hash routes. (eg., /#/about)
Hash portion of the URL won't be handled by server, server will always send the index.html for every request and ignore
hash value. Hash value will be handled by react router.
It is used to support legacy browsers which usually doesn't support HTML pushState API
Syntax:
/**
* https://example.com/#/home
* https://example.com/#/about
*/
<HashRouter
basename={optionalString}
getUserConfirmation={optionalFunc}
hashType={optionalString}
>
<App />
</HashRouter>
Example:
/**
* HashRouter()
*/
import { Link, HashRouter, Routes, Route } from "react-router-dom";
constHomePage = ()=> {
return <h2>Home Page</h2>;
};
constAboutPage = () => {
return <h2>About Page</h2>;
};
exportdefault function App() {
return (
<section className="App">
<HashRouter>
<ul>
<li><Linkto="/home">Home</Link></li>
<li><Linkto="/about">About</Link></li>
</ul>
<Routes>
<Route exact path="/home" element={<HomePage />} />
<Route exact path="/about" element={<AboutPage />} />
</Routes>
</HashRouter>
</section>
);
}
⚝
↥
Q. What is route based code splitting?
Route based code splitting is essential during the page transitions on the web, which takes some amount of time to load. Here is an
example of how to setup route-based code splitting into the app using React Router with React.lazy.
Example:
/**
* Lazy Loading
*/
importReact, { Suspense, lazy } from "react";
import { BrowserRouter, Routes,Route } from "react-router-dom";
constHome = lazy(() => import("./Home"));
constAbout = lazy(() => import("./About"));
exportdefault function App() {
return (
<BrowserRouter>
<Suspense fallback={<div>Loading...</div>}>
<Routes>
<Route exact path="/" element={<Home />} />
<Route path="/about"element={<About />} />
</Routes>
</Suspense>
</BrowserRouter>
);
}
⚝
↥
Q. What is lazy function in React?
React.lazy() makes it easy to create components that are loaded using dynamic import() but are rendered like regular
components. This will automatically cause the bundle containing the component to be loaded when the component is rendered.
React.lazy() takes a function as its argument that must return a promise by calling import() to load the component. The returned
Promise resolves to a module with a default export containing the React component.
Example:
importReact, { lazy } from 'react'
constMyComponent = React.lazy(() => import('./MyComponent'))
constApp = () => {
<div>
<MyComponent/>
</div>
}
↥
Q. What is code-splitting?
Code-Splitting is a feature supported by bundlers like Webpack, Rollup and Browserify ( via factor-bundle ) which can create
multiple bundles that can be dynamically loaded at runtime.
Code splitting uses React.lazy and Suspense library, which helps to load a dependency lazily and only load it when needed by the
user. The code splitting improves:
The performance of the app
The impact on memory
The downloaded Kilobytes (or Megabytes) size
React.lazy and Suspense
The React.lazy function allows us to render a dynamic import as a regular component. The suspense component is responsible for
handling the output when the lazy component is fetched and rendered.
Example:
importReact, { Suspense } from 'react';
constUsersComponent = React.lazy(() => import('./UsersComponent'));
functionMyComponent() {
return (
<div>
<Suspense fallback={<div>Loading...</div>}>
<UsersComponent/>
</Suspense>
</div>
);
}
↥
Q. What is dynamic import in react?
React.lazy takes a function that must call a dynamic import(). This must return a Promise which resolves to a module with a default
export containing a React component.
The lazy component should then be rendered inside a Suspense component, which allows us to show some fallback content (such as
a loading indicator) while we're waiting for the lazy component to load.
Example:
importReact, { Suspense } from 'react';
constUsersComponent = React.lazy(() => import('./UsersComponent'));
functionMyComponent() {
return (
<div>
<Suspense fallback={<div>Loading...</div>}>
<UsersComponent/>
</Suspense>
</div>
);
}
↥
Q. What are loadable components?
If you want to do code-splitting in a server rendered app, it is recommend to use Loadable Components
because React.lazy andSuspense is not available for server-side rendering. Loadable lets you render a dynamic import as a regular
component.
Installation:
npminstall @loadable/component
Example:
importloadablefrom'@loadable/component'
constUsersComponent = loadable(() => import('./UsersComponent'))
functionMyComponent() {
return (
<div>
<UsersComponent/>
</div>
)
}
↥
# 14. REACT ERROR BOUNDARIES
Q. What are error boundaries in React?
Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and
display a fallback UI instead of the component tree that crashed. Error boundaries catch errors during rendering, in lifecycle
methods, and in constructors of the whole tree below them.
Use static getDerivedStateFromError() to render a fallback UI after an error has been thrown. Use componentDidCatch() to log
error information.
Syntax:
<ErrorBoundary>
<User/>
</ErrorBoundary>
Example:
/**
* ErrorBoundary Component
*/
importReact, {Component} from'react'
exportdefaultclassErrorBoundary extends Component {
state = {
isErrorOccured: false,
errorMessage: ''
}
componentDidCatch = (error,info) => {
this.setState({
isErrorOccured: true,
errorMessage: error
})
}
render() {
if(this.state.isErrorOccured) {
return <p>Something went wrong</p>
} else {
return <div>{this.props.children}</div>
}
}
}
Here, We have a state object having two variables isErrorOccured and errorMessage which will be updated to true if any error occurs.
⚝
↥
Q. What is the difference between catch block and
error boundaries?
1. …catch is used in specific code blocks where you program the functionality of the application.
{
// Some Calculation
} catch (error) {
console.log(`Error: ${error}`);
}
2. Error Boundaries deal with declarative code. Imperative programming is how you do something and declarative programming is
what you do.
With error boundary, if there is an error, you can trigger a fallback UI; whereas, with …catch, you can catch errors in your code.
importErrorBoundary from "error-boundary";
function Users() {
return (
<div>
<ErrorBoundary>
<Users />
</ErrorBoundary>
</div>
)
}
⚝
↥
Q. What is the benefit of component stack trace from
error boundary?
Component Stack Trace prints all errors that occurred during rendering to the console in development, even if the application
accidentally swallows them. It also display the filenames and line numbers in the component stack trace.
Example:
↥
Q. What are the methods invoked during error
handling?
To create an error boundary, we simply have to create a class component and define a state variable for determining whether the
error boundary has caught an error. Our class component should also have at least three methods:
A static method called getDerivedStateFromError(), which is used to update the error boundary's state
A componentDidCatch() lifecycle method for performing operations when our error boundaries catch an error, such as
logging to an error logging service
A render() method for rendering our error boundary's child or the fallback UI in case of an error
Example:
/**
* Error Boundary in React
*/
classErrorBoundaryextends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
static getDerivedStateFromError(error) {
// Update state so the next render will show the fallback UI.
return { hasError: true};
}
componentDidCatch(error, errorInfo) {
// You can also log the error to an error reporting service
logErrorToMyService(error, errorInfo);
}
render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}
↥
# 15. REACT REFS
Q. What do you understand by refs in React?
The Refs provide a way to access DOM nodes or React elements created in the render method. React Refs are a useful feature that
act as a means to reference a DOM element or a class component from within a parent component.
Refs also provide some flexibility for referencing elements within a child component from a parent component, in the form of ref
forwarding.
Example:
/**
* Refs
*/
classApp extends React.Component {
constructor(props) {
super(props)
// create a ref to store the textInput DOM element
this.textInput = React.createRef()
this.state = {
value: ''
}
}
// Set the state for the ref
handleSubmit = e => {
e.preventDefault()
this.setState({value: this.textInput.current.value })
}
render() {
return (
<div>
<h1>React Ref - createRef</h1>
{/** This is what will update **/}
<h3>Value: {this.state.value}</h3>
<form onSubmit={this.handleSubmit}>
{/** Call the ref on <input> so we can use it to update the <h3> value **/}
<input type="text" ref={this.textInput} />
<button>Submit</button>
</form>
</div>
)
}
}
When to Use Refs:
Managing focus, text selection, or media playback.
Triggering imperative animations.
Integrating with third-party DOM libraries.
When not to use refs:
Should not be used with functional components because they dont have instances.
Not to be used on things that can be done declaritvely.
↥
Q. How can I use multiple refs for an array of elements
with hooks?
Example:
/**
* Multiple Refs
*/
importReact, { useRef } from "react";
exportdefault function App() {
constarr = [10, 20, 30];
// multiple refs
constrefs = useRef([]);
return (
<div>
{arr.map((item, index) => {
return (
<div
key={index}
ref={(element) => {
refs.current[index] = element;
}}
>
{item}
</div>
);
})}
</div>
);
}
⚝
↥
Q. What is the difference between useRef() and
createRef()?
1. useRef():
The useRef is a hook that uses the same ref throughout. It saves its value between re-renders in a functional component and doesn't
create a new instance of the ref for every re-render. It persists the existing ref between re-renders.
Example:
/**
* useRef()
*/
exportdefault function App() {
const [count, setCount] = useState(0);
constref = useRef();
useEffect(() => {
ref.current = "SomeInitialValue";
}, []);
useEffect(() => {
console.log(count, ref.current);
}, [count]);
return (
<div className="App">
<button onClick={() => setCount((c) => c + 1)}>Increment</button>
<p>{count}</p>
</div>
);
}
⚝
2. createRef():
The createRef is a function that creates a new ref every time. Unlike the useRef, it does not save its value between re-renders, instead
creates a new instance of the ref for every re-render. Thus implying that it does not persist the existing ref between re-renders.
Example:
/**
* createRef()
*/
exportdefault function App() {
const [count, setCount] = useState(0);
constref = createRef();
useEffect(() => {
ref.current = "SomeInitialValue";
}, []);
useEffect(() => {
console.log(count, ref.current);
}, [count]);
return (
<div className="App">
<button onClick={() => setCount((c) => c + 1)}>Increment</button>
<p>{count}</p>
</div>
);
}
⚝
↥
Q. Why are inline ref callback or function not
recommended?
Ifref callbackis defined as an inline function, it will get called twice during updates, first with null and then again with the DOM
element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the
new one.
Example:
/**
* Inline Ref Callback()
*/
importReact from "react";
exportdefaultclass App extends React.Component {
handleSubmit = (e) => {
e.preventDefault();
console.log("Input Value is: " + this.input.value);
};
render() {
return (
<formonSubmit={(e) => this.handleSubmit(e)}>
<input type="text"ref={(input) => (this.input = input)} />
<button type="submit">Submit</button>
</form>
);
}
}
Here, When the <input>element is rendered, React calls the function defined in the ref attribute, passing that function
the <input> element as an argument.
⚝
↥
Q. Which is the preferred option callback refs or
findDOMNode()?
It is preferred to use callback refs over findDOMNode() API. Because findDOMNode() prevents certain improvements in React in the
future.
The legacy approach of using findDOMNode():
classMyComponent extends Component {
componentDidMount() {
findDOMNode(this).scrollIntoView()
}
render() {
return <div />
}
}
The recommended approach is:
classMyComponent extends Component {
componentDidMount() {
this.node.scrollIntoView()
}
render() {
return <div ref={node=> this.node = node} />
}
}
↥
Q. How to set focus on an input field after rendering?
Refs can be used to access DOM nodes or React components that are rendered in the render method. Refs are created
with React.createRef() function. Refs can then be assigned to an element with ref-attribute. Following example shows a component
that will focus to the text input when rendered.
classAutoFocusTextInput extends React.Component {
constructor(props) {
super(props)
this.textInput = React.createRef()
}
componentDidMount() {
this.textInput.current.focus()
}
render() {
return <input ref={this.textInput} />
}
}
↥
Q. Why are string refs considered legacy in React?
Although string refs are not deprecated, they are considered legacy, and will likely be deprecated at some point in the future.
Callback refs are preferred.
Callback Refs:
Instead of passing a ref attribute created by createRef(), you pass a function. The function receives the React component instance
or HTML DOM element as its argument, which can be stored and accessed elsewhere.
Example:
// Ref.js
classCustomTextInputextends React.Component {
constructor(props) {
super(props);
this.textInput = null;
this.setTextInputRef = (element) => {
this.textInput = element;
};
}
handleSubmit = (e) => {
e.preventDefault();
console.log(this.textInput.value);
};
render() {
return (
<div>
<form onSubmit={(e) =>this.handleSubmit(e)}>
<input type="text" ref={this.setTextInputRef} />
<button>Submit</button>
</form>
</div>
);
}
}
// App.js
constApp = () => (
<div style={styles}>
<Hello name="React Refs"/>
<CustomText/>
</div>
)
⚝
↥
Q. What is forwardRef() in React?
Ref forwarding is a technique for passing a ref through a component to one of its children. It is very useful for cases like reusable
component libraries and Higher Order Components (HOC).
We can forward a ref to a component by using the React.forwardRef() function. Ref forwarding allows components to take a ref
they receive and pass it further down (in other words, "forward" it) to a child.
Example:
// Ref.js
constTextInput = React.forwardRef((props, ref) => (
<inputtype="text" placeholder="Hello World" ref={ref} />
))
constinputRef = React.createRef()
classCustomTextInputextends React.Component {
handleSubmit = e => {
e.preventDefault()
console.log(inputRef.current.value)
}
render() {
return (
<div>
<form onSubmit={e=> this.handleSubmit(e)}>
<TextInput ref={inputRef} />
<button>Submit</button>
</form>
</div>
)
}
}
In the example above, we have a component called TextInput that has a child which is an input field. First, we start by creating a ref
with the line of code below:
constinputRef = React.createRef()
We pass our ref down to <TextInput ref={inputRef}> by specifying it as a JSX attribute. React then forwards the ref to
the forwardRef() function as a second argument. Next, We forward this ref argument down to <input ref={ref}>. The value of the
DOM node can now be accessed at inputRef.current.
↥
Q. How to debug forwardRefs() in DevTools?
React.forwardRef accepts a render function as parameter and DevTools uses this function to determine what to display for the ref
forwarding component.
Problem: If you don't name the render function or not using displayName property then it will appear as "ForwardRef" in the
DevTools,
constWrappedComponent = React.forwardRef((props, ref) => {
return <LogProps {...props} forwardedRef={ref} />;
});
Solution: If you name the render function then it will appear as "ForwardRef(myFunction)"
constWrappedComponent = React.forwardRef(functionmyFunction(props, ref) {
return <LogProps {...props} forwardedRef={ref} />;
});
Example:
constForwardP = React.forwardRef(functionForwardP(props, ref) {
return (
<>
<p>I'm a real component too</p>
<p>
Especially with <code>useImperativeMethods</code>
</p>
<p {...props} ref={ref} />
</>
);
});
function App() {
return (
<div className="App">
<ForwardP style={{ opacity: 0.5 }}>
But my props are <code>null</code> in DevTools
</ForwardP>
</div>
);
}
⚝
↥
# 16. REACT COMPOSITION
Q. Explain Inheritance in React?
Inheritance uses the keywordextends to allow any component to use the properties and methods of another component connected
with the parent. A class that is used as the basis for inheritance is called a superclass or base class. A class that inherits from a
superclass is called a subclass or derived class.
Using the extends keyword, it allows the current component to access all the component's properties, including the function, and
trigger it from the child component.
Example:
importReact from "react";
/**
* Parent Class
*/
exportclass ParentClass extends React.Component {
constructor(props) {
super(props);
this.callMe = this.callMe.bind(this);
}
// ParentClass function
callMe() {
console.log("This is a method from parent class");
}
render() {
return false;
}
}
/**
* Child Class
*/
exportdefaultclass App extends ParentClass {
render() {
return <button onClick={()=> this.callMe()}>Call Parent</button>;
}
}
⚝
Note: React does not use inheritance except in the initial component class, which extends from thereact package.
↥
Q. Explain Composition in React?
Composition is also a familiar concept in Object Oriented Programming. Instead of inheriting properties from a base class, it
describes a class that can reference one or more objects of another class as instances.
Example:
/**
* Composition in React
*/
importReact, { useState } from "react";
importName from "./Name";
exportdefault function App() {
const [name, setName] = useState("");
return (
<form>
<h2>React Composition Example</h2>
<Name name={name} setName={setName} />
<h3>{name}</h3>
</form>
);
}
/**
* Name Component
* @param {*} param0 - name
* @param {*} param1 - setName
*/
exportdefault function Name({name, setName }) {
return (
<div>
<label>Name: </label>
<input value={name} onChange={(event) => setName(event.target.value)}/>
</div>
);
}
⚝
↥
Q. How Composition is different from Inheritance in
React?
Inheritance used the is-a relationship method. Derived components had to inherit the properties of the base component and it was
quite complicated while modifying the behavior of any component.
Composition does not inherit properties, only the behavior. In inheritance, it was difficult to add new behavior because the derived
component was inheriting all the properties of parent class and it was quite difficult to add new behavior. But in composition, we
only inherit behavior and adding new behavior is fairly simple and easy.
React recommends use of Composition over Inheritance, here is why. Everything in React is a component, and it follows a strong
component based model. This is one of the primary reasons that composition is a better approach than inheritance for code reuse.
↥
# 17. REACT CSS STYLING
Q. How to use styles in React.js?
React Components can add styling in the following ways:
1. Inline Styling:
In JSX, JavaScript expressions are written inside curly braces, and since JavaScript objects also use curly braces, the styling in the
example above is written inside two sets of curly braces {{}}. Since the inline CSS is written in a JavaScript object, properties with
two names, like background-color, must be written with camel case syntax:
Example:
/**
* Inline Styling
*/
classHeaderComponentextends React.Component {
render() {
return (
<div>
<h1 style={{backgroundColor: "lightblue"}}>Header Component Style!</h1>
<p>Add a little style!</p>
</div>
);
}
}
⚝
2. JavaScript Object:
We can also create an object with styling information, and refer to it in the style attribute:
Example:
/**
* JavaScript Object
*/
classHeaderComponentextends React.Component {
render() {
const mystyle = {
color: "white",
backgroundColor: "DodgerBlue",
padding: "10px",
fontFamily: "Arial"
};
return (
<div>
<h1 style={mystyle}>Header Component Style!</h1>
<p>Add a little style!</p>
</div>
);
}
}
⚝
3. CSS Stylesheet:
You can write your CSS styling in a separate file, just save the file with the .css file extension, and import it in your application.
Example:
/**
* App.css
*/
body {
background-color: #282c34;
color: white;
padding: 40px;
font-family: Arial;
text-align: center;
}
/**
* CSS Stylesheet
*/
import'./App.css';
classHeaderComponentextends React.Component {
render() {
return (
<div>
<h1>Header Component Style!</h1>
<p>Add a little style!.</p>
</div>
);
}
}
4. CSS Modules:
CSS Modules are convenient for components that are placed in separate files
Example:
/**
* mystyle.module.css
*/
.bigblue {
color: DodgerBlue;
padding: 40px;
font-family: Arial;
text-align: center;
}
/**
* CSS Modules
*/
importstyles from './mystyle.module.css';
classHeaderComponentextends React.Component {
render() {
return (
<div>
<h1 className={styles.bigblue}>Header Component Style!</h1>
<p>Add a little style!.</p>
</div>
);
}
}
↥
Q. How to conditionally apply CSS classes in React JS?
Example: Using the ternary operator
classApp extends Component {
constructor() {
super()
this.state = { isRed: true }
}
render() {
const isRed = this.state.isRed
return <p style={{ color: isRed ? 'red' : 'blue' }}>Example Text</p>
}
}
⚝
↥
Q. How to combine multiple inline style objects?
Using Spread operator:
constbox = {
color: "green",
fontSize: '23px'
}
constshadow = {
background: "orange",
boxShadow: "1px 1px 1px 1px #cccd"
}
exportdefault function App(){
return (
<div style={{...box, ...shadow}}>
<h1>Hello React</h1>
</div>
)
}
⚝
↥
Q. What are the popular package for animation in
React JS?
React Transition Group
React Spring
Framer Motion
React Motion
React Move
React Animations
React Reveal
↥
Q. What is the benefit of styles modules?
CSS module is a CSS file in which all class names and animation names are scoped locally by default. In the React application, we
usually create a single .cssfile and import it to the main file so the CSS will be applied to all the components.
But using CSS modules helps to create separate CSS files for each component and is local to that particular file and avoids class
name collision.
Benefits of CSS modules:
Using CSS modules avoid namespace collision for CSS classes
You can use the same CSS class in multiple CSS files
You can confidently update any CSS file without worrying about affecting other pages
Using CSS Modules generates random CSS classes when displayed in the browser
↥
Q. What are styled components?
The Styled-components is a CSS-in-JS styling framework that uses tagged template literals in JavaScript and the power of CSS to
provide a platform that allows you to write actual CSS to style React components.
The styled-components comes with a collection of helper methods, each corresponding to a DOM node for
example <h1>, <header>, <button>, and SVG elements like line and path. The helper methods are called with a chunk of CSS, using an
obscure JavaScript feature known as “tagged template literals”.
Example:
importstyled from 'styled-components'
constButton = styled.button`
color: black;
//...
`
constWhiteButton = Button.extend`
color: white;
//...
`
render(
<div>
<Button>A black button, like all buttons</Button>
<WhiteButton>A white button</WhiteButton>
</div>
)
↥
Q. How to use useSpring() for animation?
React Spring is a spring-physics based animation library that powers most UI related animation in React. It is a bridge on the two
existing React animation libraries; React Motion and Animated. It inherits animated powerful interpolations and performance while
maintaining react-motion's ease of use.
There are 5 hooks in react-spring currently:
useSpring a single spring, moves data from a -> b
useSpringsmultiple springs, for lists, where each spring moves data from a -> b
useTrail multiple springs with a single dataset, one spring follows or trails behind the other
useTransition for mount/unmount transitions (lists where items are added/removed/updated)
useChain to queue or chain multiple animations together
1. useSpring()
It turns defined values into animated values. It does this in two ways, either by overwriting the existing props with a different set of
props on component re-render or by passing an updater function that returns a different set of props that is then used to update
the props using set.
Example:
/**
* useSpring()
*/
importReact from "react";
import"./styles.css";
import { useSpring, animated } from "react-spring";
exportdefault function App() {
const [message, setMessage] = React.useState(false);
constcontentProps = useSpring({
opacity: message ? 1 : 0,
marginTop: message ? 0 : -500
});
return (
<div className="container">
<divclassName="button-container">
<button onClick={()=>setMessage((a) => !a)} className="button">
Click Here
</button>
</div>
<animated.divclassName="box" style={contentProps}>
<h1>React Spring</h1>
</animated.div>
</div>
);
}
⚝
2. useSpring()
It works kind of like a mix between useSpring() and useTransition() in that it takes an array, maps over it, and uses the from and to
properties to assign the animation. For our styles we can just pass in the values from each item in our array.
Example:
/**
* useSprings()
*/
importReact, { useState } from "react";
import { animated, useSprings } from "react-spring";
exportdefault function App() {
const [on, toggle] = useState(false);
constitems = [
{ color: "red", opacity: 1 },
{ color: "blue", opacity: 0.6 },
{ color: "green", opacity: 0.2 }
];
constsprings = useSprings(
items.length,
items.map((item) => ({
from: { color: "#fff", opacity: 0 },
to: {
color: on ? item.color: "#fff",
opacity: on? item.opacity : 0
}
}))
);
return (
<div>
{springs.map((animation) => (
<animated.h1 style={animation}>Hello World</animated.h1>
))}
<button onClick={() => toggle(!on)}>Click Here</button>
</div>
);
}
⚝
3. useTrail()
It allows to create an effect similar to both useSpring() and useSprings(), it will allow us to attach an animation to multiple items but
instead of being executed at the same time, they will be executed one after the other. It just takes a number for how many we want
and the style object.
Example:
/**
* useTrail()
*/
importReact, { useState } from "react";
import { animated, useTrail } from "react-spring";
exportdefault function App() {
const [on, toggle] = useState(false);
constsprings = useTrail(3, {
to: { opacity: on ? 1 : 0 },
config: { tension: 250 }
});
return (
<div>
{springs.map((animation, index) => (
<animated.h1 style={animation} key={index}>
Hello World
</animated.h1>
))}
<button onClick={() => toggle(!on)}>Click Here</button>
</div>
);
}
⚝
4. useTransition()
It allows to create an animated transition group. It takes in the elements of the list, their keys, and lifecycles. The animation is
triggered on appearance and disappearance of the elements.
Example:
importReact, { useState } from'react'
import { animated, useTransition } from 'react-spring'
const [on, toggle] = useState(false)
consttransition = useTransition(on, null, {
from: { opacity: 0 },
enter: { opacity: 1 },
leave: { opacity: 0 }
});
return (
<div>
{transition.map(({ item, key,props }) => (
item && <animated.div style={props} >Hello world</animated.div>
))}
<button onClick={() => toggle(!on)}>Change</button>
</div>
)
5. useChain()
It allows to set the execution sequence of previously defined animation hooks. To do this, we use refs, which will subsequently
prevent the independent execution of the animation.
Example:
importReact, { useState, useRef } from'react'
import { animated, useSpring, useTrail, useChain} from 'react-spring'
constApp = () => {
const [on, toggle] = useState(false)
constspringRef = useRef()
constspring = useSpring({
ref: springRef,
from: { opacity: .5 },
to: { opacity: on ? 1 : .5},
config: { tension: 250 }
})
consttrailRef = useRef()
consttrail = useTrail(5, {
ref: trailRef,
from: { fontSize: '10px' },
to: { fontSize: on ? '45px' : '10px' }
})
useChain(on ? [springRef, trailRef] : [trailRef, springRef])
return (
<div>
{trail.map((animation, index) => (
<animated.h1style={{ ...animation, ...spring }} key={index}>Hello World</animated.h1>
))}
<button onClick={() => toggle(!on)}>Change</button>
</div>
)
}
⚝
↥
Q. How many ways can we style the React Component?
1. CSS Stylesheet
.DottedBox {
margin: 40px;
border: 5px dotted pink;
}
.DottedBox_content {
font-size: 15px;
text-align: center;
}
importReact from 'react'
import'./DottedBox.css'
constDottedBox = () => (
<div className="DottedBox">
<p className="DottedBox_content">Get started with CSS styling</p>
</div>
)
exportdefault DottedBox
2. Inline styling
In React, inline styles are not specified as a string. Instead they are specified with an object whose key is the camelCased version of
the style name, and whose value is the style's value, usually a string.
importReact from 'react'
constdivStyle = {
margin: '40px',
border: '5px solid pink'
}
constpStyle = {
fontSize: '15px',
textAlign: 'center'
}
constBox = () => (
<div style={divStyle}>
<p style={pStyle}>Get started with inline style</p>
</div>
)
exportdefaultBox
We can create a variable that stores style properties and then pass it to the element like style={nameOfvariable}
We can also pass the styling directly style={{color: 'pink'}}
3. CSS Modules
A CSS Module is a CSS file in which all class names and animation names are scoped locally by default.
:local(.container) {
margin: 40px;
border: 5px dashed pink;
}
:local(.content) {
font-size: 15px;
text-align: center;
}
importReact from 'react'
importstyles from './DashedBox.css'
constDashedBox = () => (
<div className={styles.container}>
<p className={styles.content}>Get started with CSS Modules style</p>
</div>
)
exportdefault DashedBox
we import css file import styles './DashedBox.css', then we access to className as we access to object.
:local(.className)- this when you use create-react-app because of webpack configurations
.className - this if you use your own react boilerplate.
4. Styled-components
Styled-components is a library for React and React Native that allows to use component-level styles in component application that
are written with a mixture of JavaScript and CSS
npm install styled-components --save
importReact from 'react'
importstyled from 'styled-components'
constDiv = styled.div`
margin: 40px;
border: 5px outset pink;
&:hover {
background-color: yellow;
}
`;
constParagraph = styled.p`
font-size: 15px;
text-align: center;
`;
constOutsetBox = () => (
<Div>
<Paragraph>Get started with styled-components</Paragraph>
</Div>
)
exportdefault OutsetBox
↥
Q. How do you apply vendor prefixes to inline styles in
React?
In React, inline styles keys are camelCased in order to be consistent with accessing the properties on DOM nodes from JS
(e.g. node.style.backgroundImage). Vendor prefixes other than ms should begin with a capital letter.
constdivStyle = {
background: "#eee",
color: "blue",
padding: "20px",
margin: "20px",
WebkitTransition: "all", // note the capital 'W' here
msTransition: "all" // 'ms' is the only lowercase vendor prefix
};
⚝
↥
Q. What are the popular animation package in React?
ReactCSSTransitionGroup
ReactCSSTransitionGroup is a high-level API based on ReactTransitionGroup and is an easy way to perform CSS transitions and
animations when a React component enters or leaves the DOM. It has four components that display transitions from one component
state to another using a declarative API used for mounting and unmounting of components:
1.Transition
2.CSSTransition
3.SwitchTransition
4.TransitionGroup
Example:
importReactCSSTransitionGroup from 'react-transition-group'
classAnimationComponent extends React.Component {
constructor(props) {
super(props)
this.state = {items: ['hello', 'world', 'click', 'me']}
this.handleAdd = this.handleAdd.bind(this)
}
handleAdd() {
const newItems = this.state.items.concat([
prompt('Enter some text')
])
this.setState({items: newItems})
}
handleRemove(i) {
letnewItems = this.state.items.slice()
newItems.splice(i, 1)
this.setState({items: newItems})
}
render() {
const items = this.state.items.map((item, i) => (
<div key={item} onClick={() => this.handleRemove(i)}>
{item}
</div>
))
return (
<div>
<button onClick={this.handleAdd}>Add Item</button>
<ReactCSSTransitionGroup
transitionName="example"
transitionEnterTimeout={500}
transitionLeaveTimeout={300}>
{items}
</ReactCSSTransitionGroup>
</div>
)
}
}
In this component, when a new item is added to ReactCSSTransitionGroup it will get the example-enter CSS class and the example-
enter-active CSS class added in the next tick. This is a convention based on the transitionName prop.
↥
Q. How to display style based on props value?
importstyled from 'styled-components'
constButton = styled.button`
background: ${props => props.primary ? 'palevioletred' : 'white'}
color: ${props =>props.primary ? 'white' : 'palevioletred'}
`;
functionMyPureComponent(props) {
return (
<div>
<Button>Normal</Button>
<Button primary>Primary</Button>
</div>
)
}
↥
# 18. REACT INTERNATIONALIZATION
Q. How to translate your React app with react-i18next?
Installing dependencies
npm install react-i18next i18next i18next-browser-languagedetector --save
Configure i18next
Create a new file i18n.js beside your index.jscontaining following content:
/**
* i18next Component
*/
importi18n from "i18next";
importLanguageDetectorfrom "i18next-browser-languagedetector";
import { initReactI18next } from "react-i18next";
i18n
.use(LanguageDetector)
.use(initReactI18next)
.init({
// we init with resources
resources: {
en: {
translations: {
"Welcome to React": "Welcome to React and react-i18next"
}
},
hi: {
translations: {
"Welcome to React": "React react-i18next "
}
}
},
fallbackLng: "en",
debug: true,
// have a common namespace used around the full app
ns: ["translations"],
defaultNS: "translations",
keySeparator: false, // we use content as keys
interpolation: {
escapeValue: false
}
});
exportdefault i18n;
We pass the i18n instance to react-i18nextwhich will make it available for all the components via the context api.
/**
* useTranslation() in React
*/
importReact from "react";
import { useTranslation } from"react-i18next";
exportdefault function App() {
const { t, i18n } = useTranslation();
constchangeLanguage = (lng) => {
i18n.changeLanguage(lng);
};
return (
<>
<h2>{t("Welcome to React")}</h2>
<button onClick={() => changeLanguage("en")}>English</button>
<button onClick={() => changeLanguage("hi")}>Hindi</button>
</>
);
}
⚝
Reference:
https://react.i18next.com/guides/quick-start
↥
# 19. REACT TESTING
Q. Why should we use Test-Driven Development (TDD)
for ReactJS?
Test-driven development is an approach when developers create a product backwards. TDD requires developers to write tests first
and only then start to write the code. TDD is a development method that utilizes repetition of a short development cycle called Red-
Green-Refactor.
Process:
1.Add a test
2.Run all tests and see if the new test fails (red)
3.Write the code to pass the test (green)
4.Run all tests
5.Refactor
6.Repeat
Pros:
1.Design before implementation
2.Helps prevent future regressions and bugs
3.Increases confidence that the code works as expected
Cons:
1.Takes longer to develop (but it can save time in the long run)
2.Testing edge cases is hard
3.Mocking, faking, and stubbing are all even harder
↥
Q. Explain react unit testing using Jest and Enzyme?
1. Jest
Jest is a JavaScript unit testing framework, used by Facebook to test services and React applications. Jest acts as a test
runner, assertion library, and mocking library.
Jest also provides Snapshot testing, the ability to create a rendered snapshot of a component and compare it to a previously
saved snapshot. The test will fail if the two do not match.
2. Enzyme
Enzyme is a JavaScript Testing utility for React that makes it easier to assert, manipulate, and traverse your React Components
output. Enzyme, created by Airbnb, adds some great additional utility methods for rendering a component (or multiple
components), finding elements, and interacting with elements.
Setup with Create React App:
# Set up a React application
npx create-react-app counter-app
# for rendering snapshots
npm install react-test-renderer --save-dev
# for dom testing
npm install enzyme --save-dev
/**
* App.js
*/
importReact, { useState } from "react";
exportdefault function Counter() {
const [counter, setCounter] = useState(0);
constincrementCounter = ()=> {
setCounter((prevCounter)=> prevCounter + 1);
};
return (
<>
<button onClick={incrementCounter}>Click Me</button>
<h2 data-testid="counter">{counter}</h2>
</>
);
}
Writing Test Cases:
/**
* App.test.js
*/
importReact from "react";
import { shallow } from "enzyme";
importApp from "./App";
// Testing App Component
describe("App component", ()=> {
it("starts with a count of 0", () => {
const wrapper = shallow(<App />);
const text = wrapper.find("h2").text();
expect(text).toEqual("0");
});
});
// Testing Button Event
describe("App component", ()=> {
it("increments count by 1 when the increment button is clicked", () => {
const mockCallBack = jest.fn();
const button = shallow(<button onClick={mockCallBack}>Click Me</button>);
button.find("button").simulate("click");
expect(mockCallBack.mock.calls.length).toEqual(1);
});
});
⚝
Reference:
https://jestjs.io/docs/en/tutorial-react
https://enzymejs.github.io/enzyme/
↥
Q. Explain unit test structure in React?
describe('Component Description', () => {
beforeAll(() => {
/* Runs before all tests */
})
afterAll(() => {
/* Runs after all tests */
})
beforeEach(() => {
/* Runs before each test */
})
afterEach(() => {
/* Runs after each test */
})
test('test case decription', () => {
const actual = fn(['one', 'Two', 'Three'])
expect(actual).toEqual(['1 => One', '2 => Two','3 => Three'])
})
})
↥
Q. What are the matchers available in jest framework?
1. Basic matchers:
Method
Example
Description
toBe()
expect(42).toBe(42)
Strict equality (===)
not.toBe()
expect(42).not.toBe(3)
Strict equality (!==)
toEqual()
expect({ a: undefined, b: 2 }).toEqual({ b: 2 })
Deep equality
not.toStrictEqual()
expect({ a: undefined, b: 2 }).not.toStrictEqual({ b: 2 })
Strict equality
2. Truthiness:
Method
Example
Description
toBeTruthy()
expect('foo').toBeTruthy()
Matches anything that an if statement treats as true
(not false, 0, '', null, undefined, NaN)
toBeFalsy()
expect('').toBeFalsy()
Matches anything that an if statement treats as false
(false, 0, '', null, undefined, NaN)
toBeNull()
expect(null).toBeNull()
Matches only null
toBeUndefined()
expect(undefined).toBeUndefined()
Matches only undefined
toBeDefined()
expect(7).toBeDefined()
The opposite of toBeUndefined
toEqual()
expect(true).toEqual(expect.any(Boolean))
Matches true or false
toBeInTheDocument()
expect(getByText(/React/i)).toBeInTheDocument()
Return true/false
3. Numbers:
Method
Example
Description
toBeGreaterThan()
expect(2).toBeGreaterThan(1)
toBeGreaterThanOrEqual()
expect(1).toBeGreaterThanOrEqual(1)
toBeLessThan()
expect(1).toBeLessThan(2)
toBeLessThanOrEqual()
expect(1).toBeLessThanOrEqual(1)
toBeCloseTo()
expect(0.2 + 0.1).toBeCloseTo(0.3, 5)
toEqual()
expect(NaN).toEqual(expect.any(Number))
4. Strings:
Method
Example
Description
toMatch()
expect('long string').toMatch('str')
toEqual()
expect('string').toEqual(expect.any(String))
toMatch()
expect('coffee').toMatch(/ff/)
not.toMatch()
expect('pizza').not.toMatch('coffee')
toEqual()
expect(['pizza', 'coffee']).toEqual([expect.stringContaining('zz'), expect.stringMatching(/ff/)])
5. Arrays:
Method
Example
Description
toEqual()
expect([]).toEqual(expect.any(Array))
toHaveLength()
expect(['Alice', 'Bob', 'Eve']).toHaveLength(3)
toContain()
expect(['Alice', 'Bob', 'Eve']).toContain('Alice')
toContainEqual()
expect([{ a: 1 }, { a: 2 }]).toContainEqual({ a: 1 })
toEqual()
expect(['Alice', 'Bob', 'Eve']).toEqual(expect.arrayContaining(['Alice', 'Bob']))
6. Objects:
Method
Example
Description
toHaveProperty()
expect({ a: 1 }).toHaveProperty('a')
toMatchObject()
expect({ a: 1, b: 2 }).toMatchObject({ a: 1 })
7. Exceptions:
constfn = () => { throw new Error('Throw some custom error!') }
Method
Example
Description
toThrow()
expect(fn).toThrow()
toThrow()
expect(fn).toThrow('Out of cheese')
toThrowErrorMatchingSnapshot()
expect(fn).toThrowErrorMatchingSnapshot()
↥
Q. What is TestRenderer package in React?
This package provides a React renderer that can be used to render React components to pure JavaScript objects, without depending
on the DOM or a native mobile environment.
This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or
React Native without using a browser or jsdom.
Example:
importTestRendererfrom'react-test-renderer';
function Link(props) {
return <a href={props.page}>{props.children}</a>;
}
consttestRenderer = TestRenderer.create(
<Linkpage="https://www.facebook.com/">Facebook</Link>
);
console.log(testRenderer.toJSON());
// { type: 'a',
// props: { href: 'https://www.facebook.com/' },
// children: [ 'Facebook' ] }
↥
Q. What is the purpose of the ReactTestUtils package?
ReactTestUtils is used to test React-based components. It can simulate all the JavaScript-based events, which ReactJS supports.
Some of its frequently methods are
act()
mockComponent()
isElement()
isElementOfType()
isDOMComponent()
renderIntoDocument()
Simulate()
act()
To prepare a component for assertions, wrap the code rendering it and performing updates inside an act() call. This makes your test
run closer to how React works in the browser.
classCounter extends React.Component {
constructor(props) {
super(props)
this.state = {count: 0}
this.handleClick = this.handleClick.bind(this)
}
componentDidMount() {
document.title = `You clicked ${this.state.count} times`
}
componentDidUpdate() {
document.title = `You clicked ${this.state.count} times`
}
handleClick() {
this.setState(state => ({
count: state.count + 1,
}))
}
render() {
return (
<div>
<p>You clicked {this.state.count} times</p>
<button onClick={this.handleClick}>
Click me
</button>
</div>
)
}
}
importReact from 'react'
importReactDOMfrom 'react-dom'
import { act } from'react-dom/test-utils'
importCounter from'./Counter'
letcontainer
beforeEach(() => {
container = document.createElement('div')
document.body.appendChild(container)
})
afterEach(() => {
document.body.removeChild(container)
container = null
})
it('can render and update a counter', () => {
// Test first render and componentDidMount
act(() => {
ReactDOM.render(<Counter />, container)
})
constbutton = container.querySelector('button')
constlabel = container.querySelector('p')
expect(label.textContent).toBe('You clicked 0 times')
expect(document.title).toBe('You clicked 0 times')
// Test second render and componentDidUpdate
act(() => {
button.dispatchEvent(new MouseEvent('click', {bubbles: true}))
})
expect(label.textContent).toBe('You clicked 1 times')
expect(document.title).toBe('You clicked 1 times')
})
↥
Q. What is react-test-renderer package in React?
This package provides a React renderer that can be used to render React components to pure JavaScript objects, without depending
on the DOM or a native mobile environment.
Essentially, this package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a React
DOM or React Native component without using a browser or jsdom.
Example:
importReact from 'react'
importrendererfrom 'react-test-renderer'
importApp from './app.js' // The component being tested
/**
* Snapshot tests are a useful when UI does not change frequently.
*
* A typical snapshot test case for a mobile app renders a UI component, takes a snapshot,
* then compares it to a reference snapshot file stored alongside the test.
*/
describe('APP Component', ()=> {
test('Matches the snapshot', () => {
const tree = renderer.create(<App />).toJSON()
expect(tree).toMatchSnapshot()
})
}
↥
Q. What are the benefits of using data-test selector
over className or Id selector in Jest?
HTML structure and css classes tend to change due to design changes. Which will cause to re-write tests quite often. Also, if we are
using css-modules we can not rely on class names. Because of that, React provides data-test attribute for selecting elements in jsx.
// APP Component
importReact from 'react'
import'./App.scss'
function App() {
return (
<div data-testid='app-header'>
Hello React
</div>
)
}
exportdefaultApp
importReact from 'react'
import { cleanup, render, screen } from "@testing-library/react";
importApp from './App'
afterEach(cleanup);
describe('APP Component', ()=> {
let wrapper
beforeEach(() => {
wrapper = render(<Header/>)
})
test('should check for the title', () => {
expect(screen.getByText(/Hello React/i)).toBeInTheDocument();
})
})
↥
# 20. REACT MISCELLANEOUS
Q. What is React.cloneElement?
The React.cloneElement() function returns a copy of a specified element. Additional props and children can be passed on in the
function. This function is used when a parent component wants to add or modify the prop(s) of its children.
React.cloneElement(element, [props], [...children])
The react.cloneElement() method accepts three arguments.
element: Element we want to clone.
props: props we need to pass to the cloned element.
children: we can also pass children to the cloned element (passing new children replaces the old children).
Example:
importReact from 'react'
exportdefaultclass App extends React.Component {
// rendering the parent and child component
render() {
return (
<ParentComp>
<MyButton/>
<br></br>
<MyButton/>
</ParentComp>
)
}
}
// The parent component
classParentComp extends React.Component {
render() {
// The new prop to the added.
letnewProp = 'red'
// Looping over the parent's entire children,
// cloning each child, adding a new prop.
return (
<div>
{React.Children.map(this.props.children,
child => {
return React.cloneElement(child,
{newProp}, null)
})}
</div>
)
}
}
// The child component
classMyButton extends React.Component {
render() {
return <button style =
{{ color: this.props.newProp }}>
Hello World!</button>
}
}
⚝
↥
Q. What is render hijacking in React?
The phenomena of render hijacking is the ability to control what a component will output from another component. It actually
means that, the users decorate their component by wrapping it into a Higher-Order component. By wrapping, they can inject
additional props or make other changes, which can cause changing logic of rendering. It does not actually allow hijacking, but by
using HOC, users can make the component behave in a different way.
In Render Highjacking we can:
Read, add, edit, remove props in any of the React Elements outputted by render
Read, and modify the React Elements tree outputted by render
Conditionally display the elements tree
Wrapping the element's tree for styling purposes.
↥
Q. What is windowing technique in react?
WindowingorList virtualization is a concept of only rendering or write the visible portion in the current "window" to the DOM.
The number of items that rendered at first time are smaller than the original one. The remaining items are rendered when you scroll
down to it. The DOM nodes of items that exit the window are replaced by the new ones. This improves the performance of rendering
a large list.
react-window and react-virtualized are popular windowing libraries. They provide several reusable components for displaying lists,
grids, and tabular data.
Example: react-window
function renderRow(props) {
const { index, style } = props;
return (
<ListItembutton style={style} key={index}>
<ListItemTextprimary={`Item ${index + 1}`} />
</ListItem>
);
}
exportdefault function VirtualizedList() {
return (
<div>
<FixedSizeListheight={400} width={200} itemSize={46} itemCount={100000}>
{renderRow}
</FixedSizeList>
</div>
);
}
⚝
↥
Q. How to optimize React Performance?
React uses many techniques to minimize the number of DOM operations for us already. For many applications, if you are using the
production build, you may already meet or surpass your performance expectations. Nevertheless, there are several ways you can
speed up your application.
1. React DevTools Profiler
Experience performance problems with a specific component, the React DevTools Profiler is usually the first place to look.
2.shouldComponentUpdate() method
React provides a simple lifecycle method to indicate if a component needs re-rendering and that is, shouldComponentUpdate()
which is triggered before the re-rendering process starts. The default implementation of this function returns true.
If you know that in some situations your component doesn't need to update, you can return false from shouldComponentUpdate()
instead, to skip the whole rendering process, including calling render() on component.
shouldComponentUpdate(nextProps, nextState) {
return false;
}
3. Functional Components and PureComponent
Functional components reduce the bundle size as they are better at minifying than classes and they prevent construction of class
instances.
Pure Components in React are the components which do not re-renders when the value of state and props has been updated with
the same values. Pure Components restricts the re-rendering ensuring the higher performance of the Component.
4. React.memo()
React.memo is a higher order component. It's similar to React.PureComponent but for function components instead of classes.
constMyComponent = React.memo(functionMyComponent(props) {
/* render using props */
})
If your function component renders the same result given the same props, you can wrap it in a call to React.memo for a performance
boost in some cases by memoizing the result. This means that React will skip rendering the component, and reuse the last rendered
result.
React.memo only checks for prop changes. If your function component wrapped in React.memo has a useStateoruseContext Hook in
its implementation, it will still rerender when stateorcontext change.
5. Virtualizing Long Lists
In order to address the issue with our long chat feed, the React team recommends a technique called windowing. This technique
only renders the portion of the list that is visible to the user (+/- a given offset) in order to reduce the time to render. As the user
scrolls, new list items are retrieved and rendered. react-window and react-virtualized are two libraries that provide components to
help with list virtualization.
6. Use the Production Build:
React's production build improves react apps performance. The file size of the production build is significantly smaller, which means
that the page loads faster as the browser has to download, parse, and execute fewer elements.
Reference:
https://reactjs.org/docs/optimizing-performance.html
↥
Q. What do you understand with the term polling in
React?
The setInterval() inside React components allows us to execute a function or some code at specific intervals. This will schedule once
the React component mounts for the first time. To properly clear the interval, we return clearInterval() from the useEffect() Hook,
passing in the interval.
Example:
/**
* Polling in React
*/
importReact, { useState, useEffect } from "react";
exportdefault function App() {
const [seconds, setSeconds] = useState(0);
useEffect(() => {
const interval = setInterval(() => {
setSeconds((seconds) => seconds + 1);
}, 1000);
return () => clearInterval(interval);
}, []);
return <h2>{seconds} seconds have elapsed since mounting.</h2>;
}
The example above shows a React component, IntervalExample, scheduling a new interval once it mounts to the DOM. The interval
increments the seconds state value by one, every second.
⚝
↥
Q. What are the drawbacks of MVW pattern?
MVW stands for Model-View-Whatever
MVC - Model-View-Controller
MVP - Model-View-Presenter
MVVM - Model-View-ViewModel
MVW / MV* / MVx - Model-View-Whatever
HMVC - Hierarchical Model-View-Controller
MMV - Multiuse Model View
MVA - Model-View-Adapter
MVW is easy to manage in a simple application, with few models/controllers. But we can easily start to witness problems as we grow
in size with the following problems:
1.There is need when models/controllers communicate with each others (through a service layer probably), and these
modules changes the states of each others, and the more controllers, the more easy to lose control of who changed the
state of a controller.
2.Asynchronous network calls to retrieve data add uncertainty of when the model will be changed or modified, and imagine
the user changing the UI while a callback from asynchronous call comeback, then we will have "nondeterministic" status of
the UI.
3.Change state/model has another layer of complexity which is the mutation. When to consider the state or model is
changed and how to build tools to help recognize the mutation.
4.Adding to that if the application is a collaborative applications, (like google docs for examples) where lots of data changes
happening in real-time.
5.No way to do undo (travel back in time) easily without adding so much extra code.
↥
Q. How do you update render elements?
React Elements are immutable i.e. once an element is created it is impossible to update its children or attribute. Thus, in order to
update an element, we must use the render() method several times to update the value over time.
Example:
function showTime() {
constelement = (
<div>
<h2>Current Time is: {newDate().toLocaleTimeString()}</h2>
</div>
);
ReactDOM.render(element, document.getElementById("root"));
}
setInterval(showTime, 1000);
⚝
↥
Q. What is the difference between rendering and
mounting in ReactJS?
Rendering is any time a function component gets called (or a class-based render method gets called) which returns a set of
instructions for creating DOM. render() function will be invoked every time rerendering happens in the component. It may happen
either through a statechange or a propchange.
Mounting is when React renders the component for the first time and actually builds the initial DOM from those instructions.
Mounting a react component means the actual addition of the DOM elements created by the react component into the browser
DOM for the first time.
A re-render is when React calls the function component again to get a new set of instructions on an already mounted component.
Example:
/**
* React Render()
*/
importReact from "react";
/**
* Message Component
* @param {*} props
*/
function Message(props) {
return <h2>{props.name}</h2>;
}
/**
* App Component
*/
exportdefaultclass App extends React.Component {
state = {
showMessage: false
};
render() {
return (
<div>
<button onClick={()=>this.setState({ showMessage: true })}> Show Message </button>
<button onClick={()=>this.setState({ showMessage: false })}> Hide Message </button>
{this.state.showMessage && <Message name="Hello React!" />}
</div>
);
}
}
Internally, React will create an instance of App and will eventually call the render() method to get the first set of instructions for what
it needs to build in the DOM. Anytime React calls the render method of a class-based component, we call that a render.
⚝
↥
Q. What is Flow in react?
Type Checking
Type checking means ensuring that the type of a property (variable, object, function, string) in a programming language is being
used as it should be. It is the process of verifying and enforcing the constraints of types, and it can occur either at compile time or at
runtime. It helps to detect and report errors.
Type checking can be divided into two: static type checking and dynamic type checking.
1. Static Type Checking
Static type checking is used in static-typed languages where the type of the variable is known at the compile time. This means that
the type of the variable must be declared beforehand. Static typing usually results in compiled code that executes more quickly
because the compiler already knows the exact data types that are in use.
2. Dynamic type checking
Dynamic type checking is used in dynamic-typed languages where the type is usually known at runtime. This means that the type of
the variable doesn't need to be explicitly defined.
Flow
Flow is a static type checker for JavaScript apps that aims to find and eliminate problems as you code. Designed by the Facebook
team for JavaScript developers, it's a static type checker that catches common errors in your application before they run.
Integrating Flow
# Create React App with Flowchecker
npx create-react-app flowchecker
# Add Dependency
npm install --save-dev flow-bin
The next thing to do is add Flow to the "scripts" section of your package.json so that Flow can be used in the terminal. In the
package.json file, add the code snippet below.
"scripts": {
"flow": "flow",
}
Finally, for the Flow setup, run any of the commands below:
npm run flow init
This will help to create a Flow configuration file that should be committed. The Flow config file helps to determine the files that Flow
should work with and what should be ignored.
↥
Q. What are the recommended way for static type
checking?
Static type checkers like Flow andTypeScript identify certain types of problems before you even run your code. They can also
improve developer workflow by adding features like auto-completion. For this reason, we should use FloworTypeScript instead
ofPropTypes for larger code bases.
↥
Q. What is the difference between Flow and
PropTypes?
Flow is a static analysis tool which uses a superset of the language, allows to add type annotations to all of your code and catch an
entire class of bugs at compile time.
PropTypes is a basic type checker which has been patched onto React. It can't check anything other than the types of the props being
passed to a given component.
↥
Q. What is React Fiber?
React Fiber is the new reconciliation algorithm. Reconciliation is the process of comparing or diffing old trees with a new tree in
order to find what is changed or modified. In the original reconciliation algorithm (now called Stack Reconciler), the processing of
component trees was done synchronously in a single pass, so the main thread was not available for other UI related tasks like
animation, layouts, and gesture handling. Fiber Reconciler has different goals:
Ability to split interruptible work in chunks.
Ability to prioritize, rebase, and reuse work in progress.
Ability to yield back and forth between parents and children to support layout in React.
Ability to return multiple elements from render().
A fiber is a JavaScript object that contains information about a component, its input, and output. At any time, a component instance
has at most two fibers that correspond to it: the current fiber and the work-in-progress fiber. A fiber can be defined as a unit of
work.
React Fiber performs reconciliation in two phases: Render and Commit
1. Lifecycle methods called during render phase:
UNSAFE_componentWillMount()
UNSAFE_componentWillReceiveProps()
getDerivedStateFromProps()
shouldComponentUpdate()
UNSAFE_componentWillUpdate()
render()
2. Lifecycle methods called during commit phase:
getSnapshotBeforeUpdate()
componentDidMount()
componentDidUpdate()
componentWillUnmount()
The earlier whole reconciliation process was synchronous (recursive), but in Fiber, it is divided into two phases. Render phase (a.k.a.
Reconciliation phase) is asynchronous, so three of the lifecycle methods were marked unsafe because putting the code with side-
effects inside these methods can cause problems, as lifecycle methods of different components are not guaranteed to fire in a
predictable order.
React Fiber uses requestIdleCallback() to schedule the low priority work and requestAnimationFrame() to schedule high priority
work.
Problems with Current Implementation:
Long-running tasks cause frame drops.
Different tasks have different priorities.
How React Fiber works
It makes apps more fluid and responsible.
In the future, it could parallelize work a.k.a. Time Slicing.
It would improve startup time while rendering components using React Suspense.
Fiber is currently available for use but it runs in compatibility mode with the current implementation.
↥
Q. Does the static object work with ES6 classes in
React?
Although statics only works for React.createClass(), you can still write static methods in ES6 notation. If you are using ES7, then you
can also write static properties.
classComponent extends React.Component {
static propTypes = {
...
}
static someMethod(){
}
}
↥
Q. How do you access imperative API of web
components?
Web Components often expose an imperative API to implement its functions. To access the imperative APIs of a web component,
you will need to attach a ref to the component and interact with the DOM node directly. If you are using third-party web
components, the recommended solution is to write a React component that behaves as a wrapper for your web component.
Example: Using React in your Web Components
varproto = Object.create(HTMLElement.prototype, {
createdCallback: {
value: function() {
var mountPoint = document.createElement('span');
this.createShadowRoot().appendChild(mountPoint);
var name = this.getAttribute('name');
var url = 'https://www.google.com/search?q=' + encodeURIComponent(name);
ReactDOM.render(<a href={url}>{name}</a>, mountPoint);
}
}
});
document.registerElement('x-search', {prototype: proto});
classSearchComponentextends React.Component {
render() {
return <div>Results: <x-search>{this.props.name}</x-search>!</div>;
}
}
↥
Q. What is the purpose of eslint plugin for hooks?
The ESLint plugin ( eslint-plugin-react-hooks) enforces rules of Hooks to avoid bugs. It assumes that any function starting with
"use" and a capital letter right after it is a Hook. In particular, the rule enforces that,
Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function
(assumed to be a custom Hook).
Hooks are called in the same order on every render.
// ESLint configuration
{
"plugins": [
// ...
"react-hooks"
],
"rules": {
// ...
"react-hooks/rules-of-hooks": "error", // Checks rules of Hooks
"react-hooks/exhaustive-deps": "warn" // Checks effect dependencies
}
}
Note: This plugin is included by default in Create React App.
↥
Q. What is service worker in React.js?
A service worker is a background worker that acts as a programmable proxy, allowing us to control what happens on a request-by-
request basis. We can use it to make (parts of, or even entire) React apps work offline.
Service workers depend on two APIs to work effectively: Fetch (a standard way to retrieve content from the network)
and Cache (content storage for application data. This cache is independent from the browser cache or network status).
Service Worker Lifecycle:
Each service worker goes through three steps in its lifecycle: registration, installation and activation.
Registration:
To install a service worker, you need to register it in script. Registration informs the browser where your service worker is located and
lets it know it can start installing in the background.
Example: Basic registration in your index.html could look like this
// Check for browser support of service worker
if ('serviceWorker'innavigator) {
navigator.serviceWorker.register('service-worker.js')
.then(function(registration) {
// Successful registration
console.log('Registration successful, scope is:', registration.scope);
}).catch(function(err) {
// Failed registration, service worker won\'t be installed
console.log('Service worker registration failed, error:', error);
});
}
Installation and activation:
Service workers are event driven. The installation and activation processes fire off corresponding install and activateevents to which
the service workers can respond.
With the service worker registered, the first time a user hits your PWA, the install event will be triggered and this is where you'll want
to cache the static assets for the page.
↥
Q. What is the purpose of registerServiceWorker in
React?
React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache
your assets and other files so that when the user is offline or on slow network, user can still see results on the screen.
Example: Enable service worker in react
// index.js
importReact from 'react';
importReactDOMfrom 'react-dom';
importApp from './App';
importregisterServiceWorker from './registerServiceWorker';
ReactDOM.render(<App />, document.getElementById('root'));
registerServiceWorker();
↥
Q. How Virtual-DOM is more efficient than Dirty
checking?
React Virtual DOM:
In React, Each time the DOM updates or data of page changes, a new Virtual DOM representation of the user interface is made. It is
just a lightweight copy or DOM.
Virtual DOM in React has almost same properties like a real DOM, but it can not directly change the content on the page. Working
with Virtual DOM is faster as it does not update anything on the screen at the same time. In a simple way, Working with Virtual DOM
is like working with a copy of real DOM nothing more than that.
Updating virtual DOM in ReactJS is faster because ReactJS uses
1.It is efficient diff algorithm.
2.It batched update operations
3.It efficient update of sub tree only
4.It uses observable instead of dirty checking to detect change
How Virtual DOM works in React:
When we render a JSX element, each virtual DOM updates. This approach updates everything very quickly. Once the Virtual DOM
updates, React matches the virtual DOM with a virtual DOM copy that was taken just before the update. By Matching the new virtual
DOM with pre-updated version, React calculates exactly which virtual DOM has changed. This entire process is called diffing.
When React knows which virtual DOM has changed, then React updated those objects. and only those object, in the real DOM. React
only updates the necessary parts of the DOM. React's reputation for performance comes largely from this innovation.
In brief, here is what happens when we update the DOM in React:
1.The entire virtual DOM gets updated.
2.The virtual DOM gets compared to what it looked like before you updated it. React matches out which objects have
changed.
3.The changed objects and the changed objects only get updated on the real DOM.
4.Changes on the real DOM cause the screen to change finally.
↥
Q. What is the difference between DOM and virtual
DOM?
1. DOM:
DOM stands for "Document Object Model". The HTML DOM provides an interface (API) to traverse and modify the nodes. It contains
methods like getElementById()orremoveChild().
The DOM is represented as a tree data structure. Because of that, the changes and updates to the DOM are fast. But after the
change, the updated element and it's children have to be re-rendered to update the application UI. The re-rendering or re-painting
of the UI is what makes it slow.
2. Virtual DOM:
The virtual DOM is only a virtual representation of the DOM. Everytime the state of our application changes, the virtual DOM gets
updated instead of the real DOM.
The Virtual DOM is an abstraction of the HTML DOM. It is lightweight and detached from the browser-specific implementation
details. Since the DOM itself was already an abstraction, the virtual DOM is, in fact, an abstraction of an abstraction.
Why Virtual DOM is faster:
When new elements are added to the UI, a virtual DOM, which is represented as a tree is created. Each element is a node on this
tree. If the state of any of these elements changes, a new virtual DOM tree is created. This tree is then compared or “diffed” with the
previous virtual DOM tree.
Once this is done, the virtual DOM calculates the best possible method to make these changes to the real DOM. This ensures that
there are minimal operations on the real DOM. Hence, reducing the performance cost of updating the real DOM.
Pros of Virtual DOM:
Updates process is optimized and accelerated.
JSX makes components/blocks code readable.
React data binding establishes conditions for creation dynamic applications.
Virtual DOM is ideal for mobile first applications.
Prompt rendering. Using comprises methods to minimize number of DOM operations helps to optimize updating process
and accelerate it.
Once the Virtual DOM is created, React compares this new representation with a snapshot of the previous version of the virtual DOM
to see exactly which elements have changed.
Once the difference is known, React updates only those objects that differ on the actual DOM and the browser re-paints the screen.
The next time state or props changes for a component in the application, a new virtual DOM tree of React elements will be created
and the process will repeat.
The process of checking the difference between the new Virtual DOM tree and the old Virtual DOM tree is called diffing. Diffing is
accomplished by a heuristic O(n) algorithm. During this process, React will deduce the minimum number of steps needed to update
the real DOM, eliminating unnecessary costly changes. This process is also referred to as reconciliation.
React implements a heuristic O(n) algorithm based on two assumptions:
1.Two elements of different types will produce different trees.
2.The developer can hint at which child elements may be stable across different renders with a key prop."
↥
Q. How Diff Algorithm is implemented in Reactjs?
The main work of a diff algorithmis to find a heuristic to change anything from a state to another. When diffing two trees, React
first compares the two root elements. The behavior is different depending on the types of the root elements.
1. Elements Of Different Types:
Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. When
tearing down a tree, old DOM nodes are destroyed. Component instances receive componentWillUnmount().
When building up a new tree, new DOM nodes are inserted into the DOM. Component instances
receive UNSAFE_componentWillMount() and then componentDidMount(). Any state associated with the old tree is lost.
2. DOM Elements Of The Same Type:
When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM
node, and only updates the changed attributes.
Example: By comparing these two elements, React knows to only modify the className on the underlying DOM node.
<divclassName="before" title="React JS" />
<divclassName="after" title="React JS" />
3. Component Elements Of The Same Type:
When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the
underlying component instance to match the new element, and
calls UNSAFE_componentWillReceiveProps(),UNSAFE_componentWillUpdate() and componentDidUpdate() on the underlying instance.
Recursing On Children
By default, when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and
generates a mutation whenever there's a difference.
For example, when adding an element at the end of the children, converting between these two trees works well:
<ul>
<li>first</li>
<li>second</li>
</ul>
<ul>
<li>first</li>
<li>second</li>
<li>third</li>
</ul>
React will match the two <li>first</li> trees, match the two <li>second</li> trees, and then insert the <li>third</li> tree.
Keys
When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example,
adding a key to our inefficient example above can make the tree conversion efficient:
<ul>
<li key="2015">Duke</li>
<li key="2016">Villanova</li>
</ul>
<ul>
<li key="2014">Connecticut</li>
<li key="2015">Duke</li>
<li key="2016">Villanova</li>
</ul>
↥
Q. What is reconciliation in React?
Reconciliation is the process through which React updates the DOM.
As a developer we are creating tree of components, react then takes this tree, process it and we get a Virtual DOM that it's kept in
memory. When there is an update in our application (e.g. change in stateorprops) react will take the updated Virtual DOM and
compares it with the old one Virtual DOM, then decides what and how should be changed. This procedure is repeated all over again.
Also synced versions between Virtual DOM and "real" DOM are served by libraries such as ReactDOM. React needs to be very fast at
comparing those trees, so it uses heuristic algorithm with complexity of O(n), so this says for 1000 nodes we need 1000
comparasions. This approach is used instead of state of the art algorithms, which have complexity of O(n^3) => for 1000 nodes we
need 1 bilion comparasions.
Example: Let's build a simple component that adds two numbers. The numbers will be entered in an input field.
classApp extends React.Component {
state = {
result: '',
en 1: '',
en 2: ''
}
handleEn 1 = (event) => {
this.setState({en 1: event.target.value})
}
handleEn 2 = (event) => {
this.setState({en 2: event.target.value})
}
handleAddition = (event) => {
const firstInt = parseInt(this.state.en 1)
const secondInt = parseInt(this.state.en 2)
this.setState({result: firstInt + secondInt })
}
render() {
const { en 1, en 2, result } = this.state
return(
<div>
<div>
Result: { result }
</div>
<span><input type='text' onChange={this.handleEn 1} /></span>
<br />
<br />
<span><input type='text' onChange={this.handleEn 2} /></span>
<div>
<button onClick={this.handleAddition} type='submit'>Add</button>
</div>
</div>
)
}
}
ReactDOM.render(<App />, document.getElementById("root"))
When an en is made in the first input field, React creates a new tree. The new tree which is the virtual DOM will contain the new
state for en 1. Then, React compares the virtual DOM with the old DOM and, from the comparison, it figures out the difference
between both DOMs and makes an update to only the part that is different. A new tree is created each time the state of App
component changes— when a value is entered in either of the inputs field, or when the button is clicked.
↥
Q. What are portals in React?
Portals provide a quick and seamless way to render children into a DOM node that exists outside the DOM hierarchy of the parent
component.
Normally, a functional or a class component renders a tree of React elements (usually generated from JSX). The React element
defines how the DOM of the parent component should look.
ReactDOM.createPortal(child, container)
Features:
It transports its children component into a new React portal which is appended by default to document.body.
It can also target user specified DOM element.
It supports server-side rendering
It supports returning arrays (no wrapper div's needed)
It uses <Portal /> and <PortalWithState /> so there is no compromise between flexibility and convenience.
Installation:
npm install react-portal --save
Example:
/**
* React Portal
*/
importPortalExample from "./PortalExample.js";
exportdefault function App() {
return (
<div>
<h2>React Component Example</h2>
<PortalExample/>
</div>
);
}
/**
* Portal Component
*/
importReact from "react";
importReactDOMfrom "react-dom";
exportdefault function PortalExample() {
return ReactDOM.createPortal(
<h2>React Portal Example</h2>,
document.getElementById("portal-root")
);
}
Now, open the Index.html file and add a <div id="portal-root"></div> element to access the child component outside the root
node.
<!-- index.html -->
<!DOCTYPE html>
<htmllang="en">
<head>
<meta charset="utf-8" />
<link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<meta name="theme-color" content="#000000" />
<link rel="manifest" href="%PUBLIC_URL%/manifest.json" />
<title>React App using Portal</title>
</head>
<body>
<noscript>It is required to enable JavaScript to run this app.</noscript>
<div id="root"></div>
<div id="portal-root"></div>
</body>
</html>
⚝
↥
Q. What is ReactDOMServer?
The ReactDOMServer object enables you to render components to static markup. Typically, it's used on a Node server:
// ES modules
importReactDOMServer from 'react-dom/server'
// CommonJS
varReactDOMServer = require('react-dom/server')
The Server-side rendering (SSR) is a popular technique for rendering a client-side single page application (SPA) on the server and
then sending a fully rendered page to the client. This allows for dynamic components to be served as static HTML markup.
It allows your site to have a faster first page load time, which is the key to a good user experience
This approach can be useful for search engine optimization (SEO) when indexing does not handle JavaScript properly.
It is great when people share a page of your site on social media, as they can easily gather the metadata needed to nicely
share the link (images, title, description..)
Example:
Creating an Express Server
npm install express
All the content inside the build folder is going to be served as-is, statically by Express.
// server/server.js
importpath from 'path'
importfs from 'fs'
importexpress from'express'
importReact from 'react'
importReactDOMServer from 'react-dom/server'
importApp from '../src/App'
constPORT = 8080
constapp = express()
constrouter = express.Router()
constserverRenderer = (req, res, next) => {
fs.readFile(path.resolve('./build/index.html'), 'utf8', (err, data) => {
if (err) {
console.error(err)
return res.status(500).send('An error occurred')
}
return res.send(
data.replace(
'<div id="root"></div>',
`<div id="root">${ReactDOMServer.renderToString(<App/>)}</div>`
)
)
})
}
router.use('^/$', serverRenderer)
router.use(
express.static(path.resolve(__dirname, '..', 'build'), { maxAge: '30d' })
)
// tell the app to use the above rules
app.use(router)
// app.use(express.static('./build'))
app.listen(PORT, ()=> {
console.log(`SSR running on port ${PORT}`)
})
Now, in the client application, in your src/index.js, instead of calling ReactDOM.render():
ReactDOM.render(<App />, document.getElementById('root'))
call ReactDOM.hydrate(), which is the same but has the additional ability to attach event listeners to existing markup once React
loads:
ReactDOM.hydrate(<App />, document.getElementById('root'))
All the Node.js code needs to be transpiled by Babel, as server-side Node.js code does not know anything about JSX, nor ES
Modules (which we use for the include statements).
Babel Package
npm install @babel/register @babel/preset-env @babel/preset-react ignore-styles
Let's create an en point in server/index.js:
require('ignore-styles')
require('@babel/register')({
ignore: [/(node_modules)/],
presets: ['@babel/preset-env', '@babel/preset-react']
})
require('./server')
Build the React application, so that the build/ folder is populated and run this:
# Build App
npm run build
# Run App on Express
node server/index.js
↥
Q. How can automated tooling be used to improve the
accessibility of a React application?
There are two main categories of automated tools that can be used to identify accessibility issues:
1. Static Analysis Tools:
Linting tools like ESLint can be used with plugins such as eslint-plugin-jsx-a11y to analyse React projects at a component level.
Static analysis tools run very quickly, so they bring a good benefit at a low cost.
2. Browser Tools:
Browser accessibility tools such as aXe and Google Lighthouse perform automated accessibility at the app level. This can discover
more real-world issues, because a browser is used to simulate the way that a user interacts with a website.
↥
# 1. REDUX OVERVIEW
Q. What is Redux?
Redux is a state management tool. While it is mostly used with React, it can be used with any other JavaScript framework or library.
With Redux, the state of your application is kept in a store, and each component can access any state that it needs from this store.
Architecture
In Redux architecture, application event is denoted as an Action, which is dispatched to the reducer, the pure function. Then reducer
updates the centralized store with new data based on the kind of action it receives. Store creates a new state and sends an update to
view. At that time, the view was recreated to reflect the update.
There is a central store that holds the entire state of the application. Each component can access the stored state without having to
send down props from one component to another. There are three building parts: actions, store, and reducers.
Q. What are the benefits of using Redux?
1. State transfer
State is stored together in a single place called the ‘store.’ While you do not need to store all the state variables in the ‘store,’ it is
especially important to when state is being shared by multiple components or in a more complex architecture. It also allows you to
call state data from any component easily.
2. Predictability
Redux is “a predictable state container for Javascript apps.” Because reducers are pure functions, the same result will always be
produced when a state and action are passed in.
3. Maintainability
Redux provides a strict structure for how the code and state should be managed, which makes the architecture easy to replicate and
scale for somebody who has previous experience with Redux.
4. Ease of testing and debugging
Redux makes it easy to test and debug your code since it offers powerful tools such as Redux DevTools in which you can time travel
to debug, track your changes, and much more to streamline your development process.
↥
Q. What are redux core concepts?
1. Actions in Redux
Action is static information about the event that initiates a state change. When you update your state with Redux, you always start
with an action. Actions are in the form of Javascript objects, containing a type and an optional payload. Actions are sent using
the store.dispatch() method. Actions are created via an action creator.
Action creators are simple functions that help to create actions. They are functions that return action objects, and then, the returned
object is sent to various reducers in the application.
2. Reducers in Redux
Reducers are pure functions that take the current state of an application, perform an action, and return a new state. These states are
stored as objects, and they specify how the state of an application changes in response to an action sent to the store.
It is based on the reduce function in JavaScript, where a single value is calculated from multiple values after a callback function has
been carried out.
combine multiple reducers: The combineReducers() helper function turns an object whose values are different reducing functions
into a single reducing function you can pass to createStore.
Syntax:
constrootReducers = combineReducer(reducer1, reducer2)
3. Store in Redux
A Store is an object that holds the whole state tree of your application. The Redux store is the application state stored as objects.
Whenever the store is updated, it will update the React components subscribed to it. The store has the responsibility of storing,
reading, and updating state.
When using Redux with React, states will no longer need to be lifted up; thus, it makes it easier to trace which action causes any
change.
4. Dispatch
Dispatch is a method that triggers an action with type and payload to Reducer.
store.dispatch()
5. Subscribe
Subscribe is a method that is used to subscribe data/state from the Store.
store.subscribe()
6. Provider
The Provider is a component that has a reference to the Store and provides the data from the Store to the component it wraps.
7. Connect
Connect is a function that communicates with the Provider.
8. Middleware
Middleware is the suggested way to extend Redux with custom functionality. Middlewares are used to dispatch async functions. We
configure Middleware's while creating a store.
Syntax:
conststore = createStore(reducers, initialState, middleware);
Example:
/**
* React Redux Simple Example
*/
importReact from "react";
import"./styles.css";
import { signIn, signOut } from "./actions";
import { useSelector, useDispatch } from "react-redux";
exportdefault function App() {
constisLogged = useSelector((state) => state.isLogged);
constdispatch = useDispatch();
return (
<div className="App">
<h1>React Redux Example</h1>
<button onClick={() => dispatch(signIn())}>SignIn</button>
<button onClick={() => dispatch(signOut())}>SignOut</button>
{isLogged ? <h2>You are now logged in...</h2> : ""}
</div>
);
}
/**
* Actions
*/
exportconst signIn = () => {
return {
type: "SIGN_IN"
};
};
exportconstsignOut = () => {
return {
type: "SIGN_OUT"
};
};
/**
* Reducers
*/
import { combineReducers } from "redux";
constloggedReducer = (state = false, action) => {
switch (action.type) {
case "SIGN_IN":
return true;
case "SIGN_OUT":
return false;
default:
return state;
}
};
constallReducers = combineReducers({
isLogged: loggedReducer
});
exportdefault allReducers;
⚝
↥
Q. What is difference between presentational
component and container component in react redux?
1. Container Components
Container components are primarily concerned with how things work
They rarely have any HTML tags of their own, aside from a wrapping <div>
They are often stateful
They are responsible for providing data and behavior to their children (usually presentational components)
Container is an informal term for a React component that is connect-ed to a redux store. Containers receive Redux state updates
and dispatch actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
Example:
classCollage extends Component {
constructor(props) {
super(props);
this.state = {
images: []
};
}
componentDidMount() {
fetch('/api/current_user/image_list')
.then(response => response.json())
.then(images => this.setState({images}));
}
render() {
return (
<div className="image-list">
{this.state.images.map(image => {
<divclassName="image">
<img src={book.image_url} />
</div>
})}
</div>
)
}
}
2. Presentational Components
Presentational Components are primarily concerned with how things look
Probably only contain a render method and little else logic
They do not know how to load or alter the data that they render
They are best written as stateless functional components
Example:
//defining the component as a React Component
classImage extendsComponent {
render() {
return <img src={this.props.image} />;
}
}
exportdefaultImage
//defining the component as a constant
constImage = props=> (
<imgsrc={props.image} />
)
exportdefaultImage
↥
# 2. REDUX SETUP
Q. How to add redux into create react app?
Redux is the most popular State container library for frontend apps. It helps you manage your state in a predictable and easy way.
Installation:
React Redux 8.x requires React 16.8.3 or later / React Native 0.59 or later, in order to make use of React Hooks.
# Redux + Plain JS template
npxcreate-react-app my-app--template redux
# Redux + TypeScripttemplate
npxcreate-react-app my-app--template redux-typescript
An Existing React App:
To use React Redux with your React app, install it as a dependency:
# If you use npm:
npminstall redux react-redux redux-thunk--save
# Or ifyou use Yarn:
yarnadd redux react-redux redux-thunk--save
Folder structure:
The most of the applications has several top-level directories as below:
Components - Contains all 'dumb' or presentational components, consisting only of HTML and styling.
Containers - Contains all corresponding components with logic in them. Each container will have one or more component
depending on the view represented by the container.
Actions - All Redux actions
Reducers - All Redux reducers
API - API connectivity related code. Handler usually involves setting up an API connector centrally with authentication and
other necessary headers.
Utils - Other logical codes that are not React specific. For example, authUtils would have functions to process the JWT
token from the API to determine the user scopes.
Store - Used for redux store initialization.
Example:
└──src
├──actions
│├──articleActions.js
│├──categoryActions.js
│└──userActions.js
├──api
│├──apiHandler.js
│├──articleApi.js
│├──categoryApi.js
│└── userApi.js
├──components
│├──ArticleComponent.jsx
│├──ArticleListComponent.jsx
│├──CategoryComponent.jsx
│├──CategoryPageComponent.jsx
│└──HomePageComponent.jsx
├──containers
│├──ArticleContainer.js
│├──CategoryPageContainer.js
│└──HomePageContainer.js
├──index.js
├──reducers
│├──articleReducer.js
│├──categoryReducer.js
│└──userReducer.js
├──routes.js
├──store.js
└──utils
└──authUtils.js
↥
# 3. REDUX DATA FLOW
Q. How to set the dataflow using react with redux?
Redux offers this data sharing of components possible by maintaining one single state in the store. A single source of truth. All the
components which want to get state data at some point are subscribed to the store and they will receive the state each time it gets
updated.
Redux has five main entities. Action Creators, Dispatching Function, Reducers, State and Store.
An action is dispatched when a user interacts with the application.
The root reducer function is called with the current state and the dispatched action. The root reducer may divide the task
among smaller reducer functions, which ultimately returns a new state.
The store notifies the view by executing their callback functions.
The view can retrieve updated state and re-render again.
↥
Q. What are the three principles that Redux follows?
Redux can be described in three fundamental principles:
1. Single source of truth
The state of your whole application is stored in an object tree inside a single store.
This makes it easy to create universal apps, as the state from your server can be serialized and hydrated into the client with no extra
coding effort. A single state tree also makes it easier to debug or inspect an application; it also enables you to persist your app's
state in development, for a faster development cycle.
Example:
console.log(store.getState())
/* Prints
{
visibilityFilter: 'SHOW_ALL',
todos: [
{
text: 'Consider using Redux',
completed: true,
},
{
text: 'Keep all state in a single tree',
completed: false
}
]
}
*/
2. State is read-only
The only way to change the state is to emit an action, an object describing what happened.
This ensures that neither the views nor the network callbacks will ever write directly to the state. Instead, they express an intent to
transform the state. Because all changes are centralized and happen one by one in a strict order, there are no subtle race conditions
to watch out for.
Example:
store.dispatch({
type: 'COMPLETE_TODO',
index: 1
})
store.dispatch({
type: 'SET_VISIBILITY_FILTER',
filter: 'SHOW_COMPLETED'
})
3. Changes are made with pure functions
To specify how the state tree is transformed by actions, you write pure reducers.
Reducers are just pure functions that take the previous state and an action, and return the next state. Remember to return new state
objects, instead of mutating the previous state. You can start with a single reducer, and as your app grows, split it off into smaller
reducers that manage specific parts of the state tree.
import { combineReducers, createStore } from 'redux'
functionvisibilityFilter(state = 'SHOW_ALL', action) {
switch (action.type) {
case 'SET_VISIBILITY_FILTER':
return action.filter
default:
return state
}
}
function todos(state = [], action) {
switch (action.type) {
case 'ADD_TODO':
return [
...state,
{
text: action.text,
completed: false
}
]
case 'COMPLETE_TODO':
return state.map((todo, index) => {
if (index === action.index) {
return Object.assign({}, todo, {
completed: true
})
}
return todo
})
default:
return state
}
}
constreducer = combineReducers({ visibilityFilter,todos })
conststore = createStore(reducer)
↥
Q. What do you understand by "Single source of truth"
in Redux?
The single source of truth is our state tree, that is not rewritten or reshaped. It gives us the availability to easily retrieve information
in constant time and maintain a clean structure for the state of our application.
In React-Redux applications, when your Redux is a single source of truth, it means that the only way to change your data in UI is to
dispatch redux action which will change state within redux reducer. And your React components will watch this reducer and if that
reducer changes, then UI will change itself too. But never other way around, because Redux state is single source of truth.
A practical example would be that you have Redux store which contains items you want to display. In order to change list of items to
be displayed, you don't change this data anywhere else other than store. And if that is changed, everything else related to it, should
change as well.
↥
Q. What are the features of Workflow in Redux?
When using Redux with React, states will no longer need to be lifted up. Everything is handled by Redux. Redux simplifies the app
and makes it easier to maintain.
Redux offers a solution for storing all your application state in one place, called a store.
Components then dispatch state changes to the store, not directly to other components.
The components that need to be aware of state changes can subscribe to the store.
The store can be thought of as a "middleman" for all state changes in the application.
With Redux involved, components don't communicate directly with each other. Rather, all state changes must go through
the single source of truth, the store.
Core Principal
Redux has three core principals:
1. Single Source of Truth: The state of your whole application is stored in an object tree within a single store.
2. State Is Read-Only: The only way to change the state is to dispatch an action, an object describing what happened.
3. Changes Are Made With Pure Functions: To specify how the state tree is transformed by actions, you write pure reducers.
Redux Workflow
Redux allows you to manage the state of the application using Store. A child component can directly access the state from the Store.
The following are details of how Redux works:
When UI Event triggers (OnClick, OnChange, etc) it can dispatch Actions based on the event.
Reducers process Actions and return a new state as an Object.
The new state of the whole application goes into a single Store.
Components can easily subscribe to the Store.
↥
# 4. REDUX STORE
Q. What is a store in Redux?
A store is an object that holds the whole state tree of your application. The Redux store is the application state stored as objects.
Whenever the store is updated, it will update the React components subscribed to it. The store has the responsibility of storing,
reading, and updating state.
Example:
/**
* store in Redux
*/
import { createRoot } from "react-dom/client";
import { Provider } from "react-redux";
import { createStore } from "redux";
importrootReducer from "./reducers";
importApp from "./components/App";
constrootElement = document.getElementById("root");
constroot = createRoot(rootElement);
// create store
conststore = createStore(rootReducer);
root.render(
<provider store="{store}">
<App/>
</provider>
);
When using Redux with React, states will no longer need to be lifted up; thus, it makes it easier to trace which action causes any
change.
↥
Q. What is the best way to access redux store outside a
react component?
To access redux store outside a react component, Redux connect function works great for regular React components.
In the examples below shows how to access a JWT token from the Redux store.
Option 1: Export the Store
import { createStore } from 'redux'
importreducer from'./reducer'
conststore = createStore(reducer)
exportdefaultstore
Here, we are creating the store and exporting it. This will make it available to other files. Here we'll see an api file making a call
where we need to pass a JWT token to the server:
importstore from './store'
exportfunction getProtectedThing() {
// grab current state
conststate = store.getState()
// get the JWT token out of it
// (obviously depends on how your store is structured)
constauthToken = state.currentUser.token
// Pass the token to the server
return fetch('/user/thing', {
method: 'GET',
headers: {
Authorization: `Bearer ${authToken}`
}
}).then(res => res.json())
}
Option 2: Pass the Value From a React Component
It's simple to get access to the store inside a React component –no need to pass the store as a prop or import it, just use
the connect() function from React Redux, and supply a mapStateToProps() function that pulls out the data.
importReact from 'react'
import { connect } from 'react-redux'
import * as api from'api'
constItemList = ({authToken, items }) => {
return (
<ul>
{items.map(item => (
<li key={item.id}>
{item.name}
<button
onClick={
() =>api.deleteItem(item, authToken)
}>
DELETE THIS ITEM
</button>
</li>
)}
</ul>
)
}
constmapStateToProps = state=> ({
authToken: state.currentUser&& state.currentUser.authToken,
items: state.items
})
exportconnect(mapStateToProps)(ItemList)
↥
Q. Should all component states be kept in Redux Store
There is no "right" answer for this. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and
controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as "is this dropdown currently
open", inside a component's internal state.
Some common rules for determining what kind of data should be put into Redux:
Do other parts of the application needs data to be shared.
Is the same data being used to drive multiple components.
Do you want to cache the data.
Do you want to keep this data consistent while hot-reloading UI components.
↥
Q. How to use connect from React Redux?
The connect() function connects a React component to a Redux store. It provides its connected component with the pieces of the
data it needs from the store, and the functions it can use to dispatch actions to the store.
It does not modify the component class passed to it; instead, it returns a new, connected component class that wraps the
component you passed in.
UsemapStateToProps(): It maps the state variables from your store to the props that you specify.
Connect props to container: The object returned by the mapStateToProps function is connected to the container.
Example:
importReact from 'react'
import { connect } from 'react-redux'
classApp extends React.Component {
render() {
return <div>{this.props.containerData}</div>
}
}
functionmapStateToProps(state) {
return { containerData: state.data }
}
exportdefault connect(mapStateToProps)(App)
↥
# 5. REDUX ACTIONS
Q. What is an action in Redux?
Actions are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only
source of information for the store. Actions must have a type property that indicates the type of action being performed.
An action is an object that contains two keys and their values. The state update that happens in the reducer is always dependent on
the value of action.type.
Example:
constaction = {
type: 'NEW_CONTACT',
name: 'Alex K',
location: 'Lagos Nigeria',
email: '[email protected]'
}
There is typically a payload value that contains what the user is sending and would be used to update the state of the application. It
is important to note that action.type is required, but action.payload is optional. Making use of payload brings a level of structure to
how the action object looks like.
↥
Q. How to create action creators react with redux?
1. Action Type:
An action type is a string that simply describes the type of an action. They're commonly stored as constants or collected in
enumerations to help reduce typos.
Example:
exportconstActions = {
GET_USER_DETAILS_REQUEST: 'GET_USER_DETAILS_REQUEST',
GET_USER_DETAILS_SUCCESS: 'GET_USER_DETAILS_SUCCESS',
GET_USER_DETAILS_FAILURE: 'GET_USER_DETAILS_FAILURE',
...
}
2. Action:
An action is like a message that we send (i.e. dispatch) to our central Redux store. It can literally be anything. But ideally we want to
stick to an agreed-upon pattern. And the standard pattern is as follows (this is a TypeScript type declaration):
Example:
typeAction = {
type: string; // Actions MUST have a type
payload?: any; // Actions MAY have a payload
meta?: any; // Actions MAY have meta information
error?: boolean; // Actions MAY have an error field
// when true, payload SHOULD contain an Error
}
An action to fetch the user named "Ram" might look something like this
{
type: 'GET_USER_DETAILS_REQUEST',
payload: 'Ram'
}
3. Action Creator:
When writing basic Redux, an action creator simply returns an action. You would typically dispatch the action to your store
immediately.
Example:
exportconst getUserDetailsRequest = id => ({
type: Actions.GET_USER_DETAILS_REQUEST,
payload: id,
})
store.dispatch(getUserDetailsRequest('Ram'))
Although, realistically, you'll be doing this via dispatch properties that are passed into a React component like this:
// ES6
exportconst mapDispatchToProps = dispatch => ({
onClick: () => dispatch(getUserDetailsRequest('Ram'))
})
↥
Q. How to dispatch an action on load?
You can dispatch an action in componentDidMount() method and in render() method you can verify the data.
Example:
/**
* Dispatch an action on load
*/
classApp extends Component {
componentDidMount() {
this.props.fetchData()
}
render() {
return this.props.isLoaded
? <div>{'Loaded'}</div>
: <div>{'Not Loaded'}</div>
}
}
constmapStateToProps = (state)=> ({
isLoaded: state.isLoaded
})
constmapDispatchToProps = { fetchData }
exportdefault connect(mapStateToProps, mapDispatchToProps)(App)
↥
# 6. REDUX REDUCERS
Q. What is reducers in redux?
Reducers are pure functions that take the current state of an application, perform an action, and return a new state. These states are
stored as objects, and they specify how the state of an application changes in response to an action sent to the store.
It is based on the reduce function in JavaScript, where a single value is calculated from multiple values after a callback function has
been carried out.
Example:
constLoginComponent = (state = initialState, action) => {
switch (action.type) {
// This reducer handles any action with type "LOGIN"
case "LOGIN":
return state.map(user => {
if (user.username !== action.username) {
return user
}
if (user.password == action.password) {
return {
...user,
login_status: "LOGGED IN"
}
}
});
default:
return state;
}
}
↥
Q. Explain the role of Reducer?
A reducer is a function that determines changes to an application's state. It uses the action it receives to determine this change.
Redux manage an application's state changes in a single store so that they behave consistently. Redux relies heavily on reducer
functions that take the previous state and an action in order to execute the next state.
1. State
State changes are based on a user's interaction, or even something like a network request. If the application's state is managed by
Redux, the changes happen inside a reducer function — this is the only place where state changes happen. The reducer function
makes use of the initial state of the application and something called action, to determine what the new state will look like.
Syntax:
constcontactReducer = (state = initialState, action) => {
// Do something
}
2. State Parameter
The state parameter that gets passed to the reducer function has to be the current state of the application. In this case, we're calling
that our initialState because it will be the first (and current) state and nothing will precede it.
contactReducer(initialState, action)
Example:
Let's say the initial state of our app is an empty list of contacts and our action is adding a new contact to the list.
constinitialState = {
contacts: []
}
3. Action Parameter
An action is an object that contains two keys and their values. The state update that happens in the reducer is always dependent on
the value of action.type.
constaction = {
type: 'NEW_CONTACT',
name: 'Alex K',
location: 'Lagos Nigeria',
email: '[email protected]'
}
There is typically a payload value that contains what the user is sending and would be used to update the state of the application. It
is important to note that action.type is required, but action.payload is optional. Making use of payload brings a level of structure to
how the action object looks like.
4. Updating State
The state is meant to be immutable, meaning it shouldn't be changed directly. To create an updated state, we can make use
ofObject.assign() or opt for the spread operator.
Example:
constcontactReducer = (state, action) => {
switch (action.type) {
case 'NEW_CONTACT':
return {
...state, contacts:
[...state.contacts, action.payload]
}
default:
return state
}
}
This ensures that the incoming state stays intact as we append the new item to the bottom.
constinitialState = {
contacts: [{
name: 'Alex K',
age: 26
}]
}
constcontactReducer = (state = initialState, action) => {
switch (action.type) {
case "NEW_CONTACT":
return Object.assign({},state, {
contacts: [...state.contacts, action.payload]
});
default:
return state
}
}
classApp extends React.Component {
constructor(props) {
super(props)
this.name = React.createRef()
this.age = React.createRef()
this.state = initialState
}
handleSubmit = e => {
e.preventDefault()
const action = {
type: "NEW_CONTACT",
payload: {
name: this.name.current.value,
age: this.age.current.value
}
}
const newState = contactReducer(this.state, action)
this.setState(newState)
}
render() {
const { contacts } = this.state
return (
<divclassName="box">
<div className="content">
<pre>{JSON.stringify(this.state, null, 2)}</pre>
</div>
<div className="field">
<form onSubmit={this.handleSubmit}>
<div className="control">
<inputclassName="input" placeholder="Full Name" type="text" ref={this.name} />
</div>
<div className="control">
<inputclassName="input"placeholder="Age"type="number" ref={this.age} />
</div>
<div>
<button type="submit" className="button">Submit</button>
</div>
</form>
</div>
</div>
)
}
}
ReactDOM.render(
<App />,
document.getElementById('root')
)
↥
Q. Why should the reducer be a "pure" function?
Redux takes a given state (object) and passes it to each reducer in a loop. And it expects a brand new object from the reducer if
there are any changes. And it also expects to get the old object back if there are no changes.
Redux simply checks whether the old object is the same as the new object by comparing the memory locations of the two objects.
So if you mutate the old object's property inside a reducer, the "new state" and the "old state" will both point to the same object.
Hence Redux thinks nothing has changed! So this won't work.
↥
Q. How to split the reducers?
Putting all your update logic into a single reducer function is quickly going to become unmaintainable. While there's no single rule
for how long a function should be, it's generally agreed that functions should be relatively short and ideally only do one specific
thing. It's good programming practice to take pieces of code that are very long or do many different things, and break them into
smaller pieces that are easier to understand.
In Redux reducer, we can split some of our reducer logic out into another function, and call that new function from the parent
function. These new functions would typically fall into one of three categories:
1.Small utility functions containing some reusable chunk of logic that is needed in multiple places (which may or may not be
actually related to the specific business logic)
2.Functions for handling a specific update case, which often need parameters other than the typical (state, action) pair
3.Functions which handle all updates for a given slice of state. These functions do generally have the typical (state, action)
parameter signature
These terms will be used to distinguish between different types of functions and different use cases:
reducer: any function with the signature (state, action) -> newState (ie, any function that could be used as an argument
toArray.prototype.reduce)
root reducer: the reducer function that is actually passed as the first argument to createStore. This is the only part of the
reducer logic that must have the (state, action) -> newState signature.
slice reducer: a reducer that is being used to handle updates to one specific slice of the state tree, usually done by
passing it to combineReducers
case function: a function that is being used to handle the update logic for a specific action. This may actually be a reducer
function, or it may require other parameters to do its work properly.
higher-order reducer: a function that takes a reducer function as an argument, and/or returns a new reducer function as
a result (such as combineReducers, or redux-undo).
Benefits
For fast page loads
Splitting reducers will have and advantage of loading only required part of web application which in turn makes it very efficient in
rendering time of main pages
Organization of code
Splitting reducers on page level or component level will give a better code organization instead of just putting all reducers at one
place. Since reducer is loaded only when page/component is loaded will ensure that there are standalone pages which are not
dependent on other parts of the application.
One page/component
One reducer design pattern. Things are better written, read and understood when they are modular. With dynamic reducers, it
becomes possible to achieve it.
SEO
With reducer level code-splitting, reducers can be code split on a split component level which will reduce the loading time of
website thereby increasing SEO rankings.
↥
# 7. REDUX MIDDLEWARE
Q. What is Redux Thunk used for?
Redux Thunk is a middleware that lets you call action creators that return a function instead of an action object. That function
receives the store's dispatch method, which is then used to dispatch regular synchronous actions inside the body of the function
once the asynchronous operations have completed. The inner function receives the store methods dispatch() and getState() as
parameters.
Setup
# install create react app
npm install -g create-react-app
# Create a React App
create-react-app my-simple-async-app
# Switch directory
cd my-simple-app
# install Redux-Thunk
npm install --save redux react-redux redux-thunk
Example:
We are going to use Redux Thunk to asynchronously fetch the most recently updated repos by username from Github using this
REST URL:
https://api.github.com/users/learning-zone/repos?sort=updated
import { applyMiddleware, combineReducers, createStore } from'redux'
importthunk from 'redux-thunk'
// actions.js
exportconst addRepos = repos=> ({
type: 'ADD_REPOS',
repos,
})
exportconst clearRepos = ()=> ({ type: 'CLEAR_REPOS' })
exportconst getRepos = username => async dispatch=> {
{
const url = `https://api.github.com/users/${username}/repos?sort=updated`
const response = await fetch(url)
const responseBody = awaitresponse.json()
dispatch(addRepos(responseBody))
} catch (error) {
console.error(error)
dispatch(clearRepos())
}
}
// reducers.js
exportconst repos = (state = [], action) => {
switch (action.type) {
case 'ADD_REPOS':
return action.repos
case 'CLEAR_REPOS':
return []
default:
return state
}
}
exportconst reducers = combineReducers({ repos })
// store.js
exportfunction configureStore(initialState = {}) {
conststore = createStore(reducers, initialState,applyMiddleware(thunk))
return store
}
exportconst store = configureStore()
applyMiddleware(thunk): This tells redux to accept and execute functions as return values. Redux usually only accepts objects like {
type: 'ADD_THINGS', things: ['list', 'of', 'things'] }.
The middleware checks if the action's return value is a function and if it is it will execute the function and inject a callback function
named dispatch. This way you can start an asynchronous task and then use the dispatch callback to return a regular redux object
action some time in the future.
// This is your typical redux sync action
function syncAction(listOfThings) {
return { type: 'ADD_THINGS', things: listOfThings }
}
// This would be the async version
// where we may need to go fetch the
// list of things from a server before
// adding them via the sync action
functionasyncAction() {
return function(dispatch) {
setTimeout(function() {
dispatch(syncAction(['list', 'of', 'things']))
}, 1000)
}
}
App.js
importReact, { Component } from 'react'
import { connect } from 'react-redux'
import { getRepos } from './redux'
// App.js
exportclass App extends Component {
state = { username: 'learning-zone' }
componentDidMount() {
this.updateRepoList(this.state.username)
}
updateRepoList = username =>this.props.getRepos(username)
render() {
return (
<div>
<h1>I AM AN ASYNC APP!!!</h1>
<strong>Github username: </strong>
<input
type="text"
value={this.state.username}
onChange={ev => this.setState({ username: ev.target.value})}
placeholder="Github username..."
/>
<button onClick={()=>this.updateRepoList(this.state.username)}>
Get Lastest Repos
</button>
<ul>
{this.props.repos.map((repo, index) => (
<li key={index}>
<a href={repo.html_url} target="_blank">
{repo.name}
</a>
</li>
))}
</ul>
</div>
)
}
}
// AppContainer.js
constmapStateToProps = (state,ownProps) => ({ repos: state.repos })
constmapDispatchToProps = { getRepos }
constAppContainer = connect(mapStateToProps, mapDispatchToProps)(App)
exportdefault AppContainer
index.js
importReact from 'react'
importReactDOMfrom 'react-dom'
importAppContainerfrom './App'
import'./index.css'
// Add these imports - Step 1
import { Provider } from 'react-redux'
import { store } from './redux'
// Wrap existing app in Provider - Step 2
ReactDOM.render(
<Provider store={store}>
<AppContainer/>
</Provider>,
document.getElementById('root')
)
↥
Q. What are typical middleware choices for handling
asynchronous calls in Redux?
By default, Redux's actions are dispatched synchronously, which is a problem for any non-trivial app that needs to communicate with
an external API or perform side effects. Redux also allows for middleware that sits between an action being dispatched and the
action reaching the reducers.
There are three very popular middleware libraries that allow for side effects and asynchronous actions: Redux ThunkRedux
Saga and Redux Promise.
↥
Q. How can I represent "side effects" such as AJAX
calls? Why do we need things like "action creators",
"thunks", and "middleware" to do async behavior?
Any meaningful web app needs to execute complex logic, usually including asynchronous work such as making AJAX requests. That
code is no longer purely a function of its inputs, and the interactions with the outside world are known as "side effects".
Redux is inspired by functional programming, and out of the box, has no place for side effects to be executed. In particular, reducer
functions must always be pure functions of (state, action) => newState. However, Redux's middleware (eg. Redux Thunk, Redux
Saga) makes it possible to intercept dispatched actions and add additional complex behavior around them, including side effects.
↥
Q. Are there any similarities between Redux and RxJS?
Redux:
Predictable state container for JavaScript apps. Redux helps you write applications that behave consistently, run in different
environments (client, server, and native), and are easy to test. On top of that, it provides a great developer experience, such as live
code editing combined with a time traveling debugger. However, Redux has one, but very significant problem - it doesn't handle
asynchronous operations very well by itself.
RxJS
The Reactive Extensions for JavaScript. RxJS is a library for reactive programming using Observables, to make it easier to compose
asynchronous or callback-based code.
Redux belongs to "State Management Library" category of the tech stack, while RxJS can be primarily classified under "Concurrency
Frameworks".
Redux
RxJS
Redux is a tool for managing state throughout the
application.
RxJS is a reactive programming library
It is usually used as an architecture for UIs.
It is usually used as a tool to accomplish asynchronous tasks in
JavaScript.
Redux uses the Reactive paradigm because the Store is
reactive. The Store observes actions from a distance, and
changes itself.
RxJS also uses the Reactive paradigm, but instead of being an
architecture, it gives you basic building blocks, Observables, to
accomplish this pattern.
Example: React, Redux and RxJS
importReact from 'react';
importReactDOMfrom 'react-dom';
import { Subject } from 'rxjs/Subject';
// create our stream as a subject so arbitrary data can be sent on the stream
constaction$ = newSubject();
// Initial State
constinitState = { name: 'Alex' };
// Redux reducer
constreducer = (state, action)=> {
switch(action.type) {
case 'NAME_CHANGED':
return {
...state,
name: action.payload
};
default:
return state;
}
}
// Reduxification
conststore$ = action$
.startWith(initState)
.scan(reducer);
// Higher order function to send actions to the stream
constactionDispatcher = (func)=> (...args) =>
action$.next(func(...args));
// Example action function
constchangeName = actionDispatcher((payload) => ({
type: 'NAME_CHANGED',
payload
}));
// React view component
constApp = (props) => {
const { name } = props;
return (
<div>
<h1>{ name }</h1>
<button onClick={() => changeName('Alex')} >Alex</button>
<button onClick={() => changeName('John')} >John</button>
</div>
);
}
// subscribe and render the view
constdom = document.getElementById('app');
store$.subscribe((state) =>
ReactDOM.render(<App {...state} />, dom));
Async actions
Let's say we want to do something asynchronous like fetch some information from a rest api all we need to do is send an ajax stream
in place of our action payload and then use one of the lodash style stream operators, flatMap() to squash the results of the
asynchronous operation back onto the action$ stream.
import { isObservable } from './utils';
// Action creator
constactionCreator = (func) => (...args) => {
constaction = func.call(null, ...args);
action$.next(action);
if (isObservable(action.payload))
action$.next(action.payload);
return action;
};
// method called from button click
constloadUsers = actionCreator(() => {
return {
type: 'USERS_LOADING',
payload: Observable.ajax('/api/users')
.map(({response}) => map(response, 'username'))
.map((users) =>({
type: 'USERS_LOADED',
payload: users
}))
};
});
// Reducer
exportdefault function reducer(state, action) {
switch (action.type) {
case 'USERS_LOADING':
return {
...state,
isLoading: true
};
case 'USERS_LOADED':
return {
...state,
isLoading: false,
users: action.payload,
};
//...
}
}
// rest of code...
// Wrap input to ensure we only have a stream of observables
constensureObservable = (action) =>
isObservable(action)
? action
: Observable.from([action]);
// Using flatMap to squash async streams
const action$
.flatMap(wrapActionToObservable)
.startWith(initState)
.scan(reducer);
The advantage of swapping the action payload for a stream is so we can send data updates at the start and the end of the async
operation
↥
Q. What are the differences between redux-saga and
redux-thunk?
1. Redux Thunk:
Redux Thunk is a middleware that lets you call action creators that return a function instead of an action object. That function
receives the store's dispatch method, which is then used to dispatch regular synchronous actions inside the body of the function
once the asynchronous operations have completed.
npm i --save react-redux redux redux-logger redux-saga redux-thunk
Thunk is a function which optionaly takes some parameters and returns another function, it takes dispatch and getState functions
and both of these are supplied by Redux Thunk middleware.
Here is the basic structure of Redux-thunk
exportconst thunkName = parameters => (dispatch, getState) => {
// You can write your application logic here
}
Example:
importaxios from "axios"
importGET_LIST_API_URLfrom "../config"
constfetchList = () => {
return (dispatch) => {
axios.get(GET_LIST_API_URL)
.then((responseData) => {
dispatch(getList(responseData.list))
})
.catch((error) => {
console.log(error.message)
})
}
}
constgetList = (payload)=> {
return {
type: "GET_LIST",
payload
}
}
export { fetchList }
2. Redux Saga:
Redux Saga leverages an ES6 feature called Generators, allowing us to write asynchronous code that looks synchronous, and is very
easy to test. In the saga, we can test our asynchronous flows easily and our actions stay pure. It organized complicated asynchronous
actions easily and make then very readable and the saga has many useful tools to deal with asynchronous actions.
Example:
importaxios from "axios"
importGET_LIST_API_URLfrom "../config"
import {call, put} from "redux-saga/effects"
constfetchList = () => {
return axios.get(GET_LIST_API_URL)
}
function *fetchList() {
{
const responseData = yieldcall(getCharacters)
yield put({type: "GET_LIST", payload: responseData.list})
} catch (error) {
console.log(error.message)
}
}
export { fetchList }
Both Redux Thunk and Redux Saga take care of dealing with side effects. In very simple terms, applied to the most common scenario
(async functions, specifically AJAX calls) Thunk allows Promises" to deal with them, Saga uses Generators. Thunk is simple to use and
Promises are familiar to many developers, Saga/Generators are more powerful but you will need to learn them. When Promises are
just good enough, so is Thunk, when you deal with more complex cases on a regular basis, Saga gives you better tools.
↥
Q. How to make Ajax request in Redux?
There are three most widely used and stable Redux Ajax middleware are:
Redux Promise Middleware
Redux Thunk Middleware
Redux Saga Middleware
1. Redux Promise Middleware
This is the most simple way of doing Ajax calls with Redux. When using Redux Promise, your action creator can return a Promise
inside the Action.
functiongetUserName(userId) {
return {
type: "SET_USERNAME",
payload: fetch(`/api/personalDetails/${userId}`)
.then(response=> response.json())
.then(json => json.userName)
}
}
This middleware automatically dispatches two events when the Ajax call succeeds: SETUSERNAMEPENDING and SETUSERNAMEFULFILLED. If
something fails it dispatches SETUSERNAMEREJECTED.
When to use
You want the simplest thing with minimum overhead
You prefer convention over configuration
You have simple Ajax requirements
2. Redux Thunk Middleware
This is the standard way of doing Ajax with Redux. When using Redux Thunk, your action creators returns a function that takes one
argument dispatch:
functiongetUserName(userId) {
return dispatch=> {
return fetch(`/api/personalDetails/${userId}`)
.then(response => response.json())
.then(json =>dispatch({ type: "SET_USERNAME", userName: json.userName })
}
}
The action creator can call dispatch inside .then to execute it asynchronously. The action creator can call dispatch as many time as it
wants.
When to use
You make many Ajax calls in one action, and need to dispatch many actions
You require full control of the format of your actions
3. Redux Saga Middleware
This is the most advanced way of doing Ajax with Redux. It uses an ES6 feature called generators. When using Redux Saga you do
your Ajax calls in a saga instead of an action creator. This is how a saga looks like:
import { call, put, takeEvery } from 'redux-saga/effects'
// call getUserName when action SET_USERNAME is dispatched
function* mySaga() {
yieldtakeEvery("SET_USERNAME", getUserName);
}
function* getUserName(action) {
{
const user = yield call(fetch, `/api/personalDetails/${userId}`);
yield put({type: "SET_USERNAME_SUCCEEDED", user: user});
} catch (e) {
yield put({type: "SET_USERNAME_FAILED", message: e.message});
}
}
exportdefault mySaga
Here, sagas listen to actions which you dispatch as regular synchronous actions. In this case, the saga getUserName is executed when
the action SET_USERNAME is dispatched. The * next to the function means it's a generator and yield is a generator keyword.
When to use
You need to be able to test the asynchronous flow easily
You are comfortable working with ES6 Generators
You value pure functions
↥
Q. What are the differences between call and put in
redux-saga?
Both call() and put()are effect creator functions. call() function is used to create effect description, which instructs middleware to
call the promise. put() function creates an effect, which instructs middleware to dispatch an action to the store.
Let's take example of how these effects work for fetching particular user data.
function* fetchUserSaga(action) {
// `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
// Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
constuserData = yield call(api.fetchUser, action.userId)
// Instructing middleware to dispatch corresponding action.
yieldput({
type: 'FETCH_USER_SUCCESS',
userData
})
}
↥
Q. What is the mental model of redux-saga?
Saga is like a separate thread in your application, that is solely responsible for side effects. redux-saga is a redux middleware, which
means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the
full Redux application state and it can dispatch Redux actions as well.
Example:
npm install --save redux-saga
Suppose we have a UI to fetch some user data from a remote server when a button is clicked.
classUserComponentextends React.Component {
...
onSomeButtonClicked() {
const { userId, dispatch } = this.props
dispatch({type: 'USER_FETCH_REQUESTED', payload: {userId}})
}
...
}
The Component dispatches a plain Object action to the Store. We'll create a Saga that watches for all USER_FETCH_REQUESTED actions
and triggers an API call to fetch the user data.
// sagas.js
import { call, put, takeEvery, takeLatest } from 'redux-saga/effects'
importApi from '...'
// worker Saga: will be fired on USER_FETCH_REQUESTED actions
function* fetchUser(action) {
{
const user = yield call(Api.fetchUser, action.payload.userId);
yield put({type: "USER_FETCH_SUCCEEDED", user: user});
} catch (e) {
yield put({type: "USER_FETCH_FAILED", message: e.message});
}
}
/*
Starts fetchUser on each dispatched `USER_FETCH_REQUESTED` action.
Allows concurrent fetches of user.
*/
function* mySaga() {
yieldtakeEvery("USER_FETCH_REQUESTED", fetchUser);
}
/*
Alternatively you may use takeLatest.
Does not allow concurrent fetches of user. If "USER_FETCH_REQUESTED" gets
dispatched while a fetch is already pending, that pending fetch is cancelled
and only the latest one will be run.
*/
function* mySaga() {
yieldtakeLatest("USER_FETCH_REQUESTED", fetchUser);
}
exportdefault mySaga;
To run our Saga, we'll have to connect it to the Redux Store using the redux-saga middleware.
// main.js
import { createStore, applyMiddleware } from 'redux'
importcreateSagaMiddleware from 'redux-saga'
importreducer from'./reducers'
importmySaga from './sagas'
// create the saga middleware
constsagaMiddleware = createSagaMiddleware()
// mount it on the Store
conststore = createStore(
reducer,
applyMiddleware(sagaMiddleware)
)
// then run the saga
sagaMiddleware.run(mySaga)
// render the application
↥
# 8. REDUX FORMS
Q. Explain Redux form with an example?
This is a simple demonstration of how to connect all the standard HTML form elements to redux-form.
For the most part, it is a matter of wrapping each form control in a <Field> component, specifying which type
ofReact.DOM component you wish to be rendered.
The Field component will provide your input with onChange, onBlur, onFocus, onDrag, and onDrop props to listen to the events, as well
as a value prop to make each input a controlled component. Notice that the SimpleForm component has no state; in fact, it uses
the functional stateless component syntax.
Example:
// SimpleForm.js
importReact from 'react'
import { Field, reduxForm } from 'redux-form'
constSimpleForm = (props) => {
const { handleSubmit, pristine, reset, submitting } = props
return (
<form onSubmit={handleSubmit}>
<div>
<label>Name</label>
<div>
<Field name="name" component="input" type="text" placeholder="Name"/>
</div>
</div>
<div>
<label>Sex</label>
<div>
<label><Field name="sex" component="input"type="radio" value="male"/> Male</label>
<label><Field name="sex" component="input"type="radio" value="female"/>Female</label>
</div>
</div>
<div>
<label>Favorite Color</label>
<div>
<Field name="favoriteColor" component="select">
<option></option>
<optionvalue="ff0000">Red</option>
<optionvalue="00ff00">Green</option>
<optionvalue="0000ff">Blue</option>
</Field>
</div>
</div>
<div>
<button type="submit" disabled={pristine ||submitting}>Submit</button>
<button type="button" disabled={pristine ||submitting} onClick={reset}>Clear</button>
</div>
</form>
)
}
exportdefault reduxForm({
form: 'simple' // a unique identifier for this form
})(SimpleForm)
Reference:
https://redux-form.com/6.5.0/examples/syncvalidation/
↥
Q. How Redux Form initialValues get updated from
state?
Add enableReinitialize : true When set to true, the formwill reinitialize every time the initialValues prop changes
...
exportdefault connect(mapStateToProps)(reduxForm({
form: 'contactForm',
enableReinitialize: true
})(ContactForm))
↥
# 9. REDUX MISCELLANEOUS
Q. What is the at symbol in the Redux connect
decorator?
Decorators make it possible to annotate and modify classes and properties at design time.
Here's an example of setting up Redux without and with a decorator:
Without a decorator
importReact from 'react'
import * as actionCreatorsfrom'./actionCreators'
import { bindActionCreators } from 'redux'
import { connect } from 'react-redux'
functionmapStateToProps(state) {
return { todos: state.todos }
}
functionmapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) }
}
classMyApp extendsReact.Component {
// ...define your main app here
}
exportdefault connect(mapStateToProps, mapDispatchToProps)(MyApp)
Using a decorator
importReact from 'react'
import * as actionCreatorsfrom'./actionCreators'
import { bindActionCreators } from 'redux'
import { connect } from 'react-redux'
functionmapStateToProps(state) {
return { todos: state.todos }
}
functionmapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) }
}
@connect(mapStateToProps, mapDispatchToProps)
exportdefaultclass MyApp extends React.Component {
// ...define your main app here
}
↥
Q. What is the difference between React State vs
Redux State?
React state is stored locally within a component. When it needs to be shared with other components, it is passed down through
props. In practice, this means that the top-most component in your app needing access to a mutable value will hold that value in its
state. If it can be mutated by subcomponents, you must pass a callback to handle the change into subcomponents.
When using Redux, state is stored globally in the Redux store. Any component that needs access to a value may subscribe to the
store and gain access to that value. Typically, this is done using container components. This centralizes all data but makes it very
easy for a component to get the state it needs, without surrounding components knowing of its needs.
↥
Q. What is Flux?
Flux is an application design paradigm used as a replacement for the more traditional MVC pattern. Flux is a Javascript architecture
or pattern for UI which runs on a unidirectional data flow and has a centralized dispatcher. It is useful when your project has dynamic
data and you need to keep the data updated in an effective manner. It was created by Facebook, and complements React as view.
Architecture
The Flux architecture is based on the following components:
Store/ Stores: Serves as a container for the app state & logic
Action: Enables data passing to the dispatcher
View: Same as the view in MVC architecture, but in the context of React components
Dispatcher: Coordinates actions & updates to stores
In the Flux architecture, when a user clicks on something, the view creates actions. Action can create new data and send it to the
dispatcher. The dispatcher then dispatches the action result to the appropriate store. The store updates the state based on the result
and sends an update to the view.
↥
Q. What are the drawbacks of Redux contrasted with
Flux?
Flux vs Redux
Flux
Redux
Follows the unidirectional flow
Follows the unidirectional flow
Includes multiple stores
Includes single store
Flux
Redux
Store handles all logic
Reducer handles all logic
Ensures simple debugging with the dispatcher
Single store makes debugging lot easier
↥
Q. Describe Flux vs MVC?
1. MVC
MVC stands for Model View Controller. It is an architectural pattern used for developing the user interface. It divides the application
into three different logical components: the Model, the View, and the Controller.
Model: It is responsible for maintaining the behavior and data of an application.
View: It is used to display the model in the user interface.
Controller: It acts as an interface between the Model and the View components. It takes user input, manipulates the
data(model) and causes the view to update.
MVC can be interpreted or modified in many ways to fit a particular framework or library. The core ideas of MVC can be formulated
as:
Separating the presentation from the model: enables implementation of different UIs and better testability
Separating the controller from the view: most useful with web interfaces and not commonly used in most GUI frameworks
In general, MVC makes no assumptions about whether data flow within an application should be unidirectional or bidirectional. In
server Side, MVC is good, but in Client side most of the JS frameworks provide data binding support which let the view can talk with
model directly, It shoudn't be, Many times it become hard to debug something as there are scope for a property being changed by
many ways.
2. Flux
Flux places unidirectional data flow front and center, by making it a requirement. Here are the four major roles that make up the Flux
architecture:
Actions, which are helper methods that relay information to the dispatcher
Stores are similar to the models in MVC, except they act as containers for application state and logic for a particular
domain within the application
The Dispatcher receives Actions and acts as the sole regis of callbacks to all stores within an application. It also manages
the dependencies between stores
Views are the same as the view in MVC, except in the context of React and Flux, and also include Controller-Views for
change events and retrieve application state from stores as required.
1.All data in the application flow through a central hub called the Dispatcher
2.This data is tracked as actions, which are provided to the dispatcher in an action creator method, often as a result of a user
interacting with the view
3.The dispatcher invokes the registered callback, effectively dispatching the action to all stores that have registered with that
callback
4.The stores in turn relay that change event to the controller-views to alert them of the change
5.The controller-views listen for events, retrieve data from the appropriate stores as required and re-render themselves and
all their children in the component tree accordingly.
MVC Vs. Flux
MVC
Flux
Bidirectional data flow
Unidirectional data flow
MVC
Flux
Data binding is the key
Events or actions are the main players
Controllers handle the business logic
Store does all calculations
Somewhat synchronous
Can be implemented as completely asynchronous
It is hard to debug.
It is easy to debug because it has common initiating point:
Dispatcher.
Its maintainability is difficult as the project scope goes
huge.
Its maintainability is easy and reduces runtime errors.
↥
Q. How to add multiple middleware to redux?
The most common use case for middleware is to support asynchronous actions without much boilerplate code or a dependency on
a library like RxJS. It does so by letting you dispatch async actions in addition to normal actions.
applyMiddleware takes each piece of middleware as a new argument (not an array). It provides a third-party extension point between
dispatching an action, and the moment it reaches the reducer. It can be use for logging, crash reporting, talking to an asynchronous
API, routing, and more.
constcreateStoreWithMiddleware = applyMiddleware(ReduxThunk, logger)(createStore);
Example: Custom Logger Middleware
import { createStore, applyMiddleware } from 'redux'
importtodos from './reducers'
function logger({ getState }) {
return next => action => {
console.log('will dispatch', action)
// Call the next dispatch method in the middleware chain.
const returnValue = next(action)
console.log('state after dispatch', getState())
// This will likely be the action itself, unless
// a middleware further in chain changed it.
return returnValue
}
}
conststore = createStore(todos, ['Use Redux'], applyMiddleware(logger))
store.dispatch({
type: 'ADD_TODO',
text: 'Understand the middleware'
})
// (These lines will be logged by the middleware:)
// will dispatch: { type: 'ADD_TODO', text: 'Understand the middleware' }
// state after dispatch: [ 'Use Redux', 'Understand the middleware' ]
↥
Q. How to set initial state in Redux?
1. Initializing State
In Redux, all application state is held in the store; which is an object that holds the complete state tree of your app. There is only one
way to change its state and that is by dispatching actions.
Actions are objects that consist of a type and a payload property. They are created and dispatched by special functions called action
creators.
Example: First creating the Redux store
import { createStore } from 'redux'
functiontodosReducer(state = [], action) {
switch (action.type) {
case 'ADD_TODO':
return state.concat([action.payload])
default:
return state
}
}
conststore = createStore(todosReducer)
Next updating the store
constADD_TODO = add_todo; // creates the action type
constnewTodo = ["blog on dev.to"];
functiontodoActionCreator (newTodo) {
constaction = {
type: ADD_TODO,
payload: newTodo
}
dispatch(action)
}
When a store is created, Redux dispatches a dummy action to your reducer to populate the store with the initial state.
2. createStore Pattern
The createStore method can accept an optional preloadedState value as its second argument. In our example, we
calledcreateStore() without passing this value. When a value is passed to the preloadedState it becomes the initial state.
constinitialState = ["eat", "code", "sleep"];
conststore = createStore(todosReducer, initialState)
3. Reducer Pattern
Reducers can also specify an initial state value by looking for an incoming state argument that is undefined, and returning the value
they'd like to use as a default.
functiontodosReducer(state = [], action) {
switch (action.type) {
case 'ADD_TODO':
return state.concat([action.payload])
default:
return state
}
}
/**
* sets initial state to []. But would only take effect if the initial state is undefined,
* which means it was not set using createStore().
**/
In general, preloadedState wins over the state specified by the reducer. This lets reducers specify initial data that makes sense to
them as default arguments, but also allows loading existing data (fully or partially) when you're hydrating the store from some
persistent storage or the server.
↥
Q. What is the purpose of the constants in Redux?
It helps keep the naming consistent because all action types are gathered in a single place.
Sometimes you want to see all existing actions before working on a new feature. It may be that the action you need was
already added by somebody on the team, but you didn't know.
The list of action types that were added, removed, and changed in a Pull Request helps everyone on the team keep track
of scope and implementation of new features.
If you make a typo when importing an action constant, you will get undefined. This is much easier to notice than a typo
when you wonder why nothing happens when the action is dispatched.
Example: Constants in Redux can be used into two places, reducers and during actions creation.
// actionTypes.js
exportconst ADD_TODO = 'ADD_TODO'
exportconst DELETE_TODO = 'DELETE_TODO'
exportconst EDIT_TODO = 'EDIT_TODO'
exportconst COMPLETE_TODO = 'COMPLETE_TODO'
exportconst COMPLETE_ALL = 'COMPLETE_ALL'
exportconst CLEAR_COMPLETED = 'CLEAR_COMPLETED'
And then require it in actions creator file
// actions.js
import { ADD_TODO } from './actionTypes'
exportfunction addTodo(text) {
return { type: ADD_TODO, text }
}
And in some reducer
import { ADD_TODO } from './actionTypes'
exportdefault (state = [],action) => {
switch (action.type) {
case ADD_TODO:
return [
...state,
{
text: action.text,
completed: false
}
]
default:
return state
}
}
It allows to easily find all usages of that constant across the project. It also prevents from introducing silly bugs caused by typos -- in
which case, you will get a ReferenceError immediately.
↥
Q. What is the difference between React context and
React Redux?
React Context:
Context provides a way to pass data through the component tree without having to pass props down manually at every level.
In a typical React application, data is passed top-down (parent to child) via props, but this can be cumbersome for certain types of
props (e.g. locale preference, UI theme) that are required by many components within an application. Context provides a way to
share values like these between components without having to explicitly pass a prop through every level of the tree.
Redux:
Redux is a pattern and library for managing and updating application state, using events called "actions". It serves as a centralized
store for state that needs to be used across your entire application, with rules ensuring that the state can only be updated in a
predictable fashion.
Redux helps you manage "global" state - state that is needed across many parts of your application.
The patterns and tools provided by Redux make it easier to understand when, where, why, and how the state in your application is
being updated, and how your application logic will behave when those changes occur.
Redux vs Context API
1.) Implementation
Context API is easy to is use as it has a short learning curve. It requires less code, and because there's no need of extra libraries,
bundle sizes are reduced. Redux on the other hand requires adding more libraries to the application bundle. The syntax is complex
and extensive creating unnecessary work and complexity. However, it's still a great alternative regarding prop drilling.
2.) Rendering
Context API prompts a re-render on each update of the state and re-renders all components regardless. Redux however, only re-
renders the updated components.
↥
Q. How to reset state in redux?
The root reducer would normally delegate handling the action to the reducer generated by combineReducers(). However, whenever it
receives USER_LOGOUT action, it returns the initial state all over again.
import { combineReducers } from 'redux';
importAppReducer from './AppReducer';
importUsersReducerfrom './UsersReducer';
importOrderReducerfrom './OrderReducer';
importNotificationReducerfrom'./NotificationReducer';
importCommentReducer from './CommentReducer';
/**
* In order to reset all reducers back to their initial states when user logout,
* rewrite rootReducer to assign 'undefined' to state when logout
*
* If state passed to reducer is 'undefined', then the next state reducer returns
* will be its initial state instead; since we have assigned it as the default value
* of reducer's state parameter
* ex: const Reducer = (state = InitialState, action) => { ... }
*
* See: https://goo.gl/GSJ98M and combineReducers() source codes for details
*/
constappReducer = combineReducers({
/* your app's top-level reducers */
users: UsersReducer,
orders: OrderReducer,
notifications: NotificationReducer,
comment: CommentReducer,
});
constrootReducer = (state, action) => {
// when a logout action is dispatched it will reset redux state
if (action.type === 'USER_LOGGED_OUT') {
state = undefined;
}
return appReducer(state, action);
}
exportdefault rootReducer
↥
Q. Why are Redux state functions called as reducers?
Redux state functions called a reducer because it's the type of function we pass to Array.prototype.reduce(reducer,
?initialValue). Reducers do not just return default values. They always return the accumulation of the state (based on all previous
and current actions).
Therefore, they act as a reducer of state. Each time a redux reducer is called, the state is passed in with the action (state, action).
This state is then reduced (or accumulated) based on the action, and then the next state is returned. This is one cycle of the
classic foldorreduce function.
↥
Q. What is Relay?
Relay is a JavaScript framework for fetching and managing GraphQL data in React applications that emphasizes maintainability, type
safety and runtime performance.
Relay achieves this by combining declarative data fetching and a static build step. With declarative data fetching, components
declare what data they need, and Relay figures out how to efficiently fetch it. During the static build step, Relay validates and
optimizes queries, and pre-computes artifacts to achieve faster runtime performance.
Reference:
https://relay.dev/docs/
↥
Q. How Relay is different from Redux?
Redux
Predictable state container for JavaScript apps. Redux helps you write applications that behave consistently, run in different
environments (client, server, and native). In redux the application state is located in a single store, each component can access the
state, and can also change the state by dispatching actions. Redux doesn't handle data fetching out of the box, though it can be
done manually: simply create an action that fetches the data from the server into the store.
Some of the features offered by Redux are:
Predictable state
Easy testing
Works with other view layers besides React
Relay
Created by facebook for react, and also used internally there. Relay is similar to redux in that they both use a single store. The main
difference is that relay only manages state originated from the server, and all access to the state is used via GraphQL querys (for
reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only
changed data and nothing more. Relay also supports optimistic updates, i.e. changing the state before the server's result arrives.
Relay provides the following key features:
Build data driven apps
Declarative style
Mutate data on the client and server
GraphQL is a web service framework and protocol using declarative and composable queries, and solves problem like over fetching
and under fetching, it is believed to be a valid candidate to replace REST.
↥
Q. When would bindActionCreators be used in
react/redux?
bindActionCreators(actionCreators, dispatch): Turns an object whose values are action creators, into an object with the same keys,
but with every action creator wrapped into a dispatch call so they may be invoked directly.
When we use Redux with React, react-redux will provide dispatch() function and we can call it directly. The only use case
for bindActionCreators() is when we want to pass some action creators down to a component that isn't aware of Redux, and we
don't want to pass dispatch or the Redux store to it.
Parameters
1.actionCreators (Function or Object): An action creator, or an object whose values are action creators.
2.dispatch (Function): A dispatch function available on the Store instance.
Returns
(Function or Object): An object mimicking the original object, but with each function immediately dispatching the action returned by
the corresponding action creator. If you passed a function as actionCreators, the return value will also be a single function.
Example:
// TodoActionCreators.js
exportfunction addTodo(text) {
return {
type: 'ADD_TODO',
text
}
}
exportfunction removeTodo(id) {
return {
type: 'REMOVE_TODO',
id
}
}
// TodoListContainer.js
import { Component } from 'react'
import { bindActionCreators } from 'redux'
import { connect } from 'react-redux'
import * as TodoActionCreatorsfrom'./TodoActionCreators'
console.log(TodoActionCreators)
// {
// addTodo: Function,
// removeTodo: Function
// }
classTodoListContainerextends Component {
constructor(props) {
super(props)
const { dispatch } = props
// Here's a good use case for bindActionCreators:
// You want a child component to be completely unaware of Redux.
// We create bound versions of these functions now so we can
// pass them down to our child later.
this.boundActionCreators = bindActionCreators(TodoActionCreators, dispatch)
console.log(this.boundActionCreators)
// {
// addTodo: Function,
// removeTodo: Function
// }
}
componentDidMount() {
// Injected by react-redux:
let { dispatch } = this.props
// Note: this won't work:
// TodoActionCreators.addTodo('Use Redux')
// You're just calling a function that creates an action.
// You must dispatch the action, too!
// This will work:
letaction = TodoActionCreators.addTodo('Use Redux')
dispatch(action)
}
render() {
// Injected by react-redux:
let { todos } = this.props
return <TodoList todos={todos} {...this.boundActionCreators} />
// An alternative to bindActionCreators is to pass
// just the dispatch function down, but then your child component
// needs to import action creators and know about them.
// return <TodoList todos={todos} dispatch={dispatch} />
}
}
exportdefault connect(state=> ({ todos: state.todos }))(TodoListContainer)
↥
Q. What is mapStateToProps and
mapDispatchToProps?
react-reduxpackage provides 3 functions Connect, mapStapteToProps and mapDispatchToProps. Connect is a higher order function that
takes in both mapStateToProps and mapDispatchToProps as parameters.
1. Using MapStateToProps
In React, MapStatetoProps pulls in the state of a specific reducer state object from global store and maps it to the props of
component. MapStateToProps is called everytime your store is updated. You pass in your state a retrieve that specific objects from
the reducer.
2. Using MapDisptachToProps
MapDispatchToProp takes the dispatch functions in component and executes them against the Redux Reducer when that function is
fired. MapDispatchToProps allows to dispatch state changes to your store.
In a simple term,
mapStateToProps: It connects redux state to props of react component.
mapDispatchToProps: It connects redux actions to react props.
Example:
const {createStore} = Redux
const {connect, Provider} = ReactRedux
constInitialState = {Collection: ["COW", "COW", "DUCK", "DUCK"]}
function reducer(state=InitialState, action) {
if (action.type=== "REVERSE") {
return Object.assign({},state, {
Collection: state.Collection.slice().reverse()
})
}
return state
}
varstore = createStore(reducer)
functionmapStateToProps(state) {
return state
}
varPresentationalComponent = React.createClass({
render: function() {
return (
<div>
<h2>Store State ( as Props) </h2>
<pre> {JSON.stringify(this.props.Collection)}</pre>
<StateChangerUI/>
</div>
)
}
})
// State changer UI
varStateChangerUI = React.createClass({
// Action Dispatch
handleClick: function() {
store.dispatch({
type: 'REVERSE'
})
},
render: function() {
return (
<button type="button" className="btn btn-success" onClick={this.handleClick}>REVERSE</button>
)
}
})
PresentationalComponent = connect(mapStateToProps)(PresentationalComponent)
ReactDOM.render(
<Providerstore={store}>
<PresentationalComponent />
</Provider>,
document.getElementById('App')
)
↥
Q. What are the different ways to write
mapDispatchToProps()?
mapDispatchToProps is the second argument that connect expects to receive. In the context of a react-redux application,
the mapDispatchToProps argument is responsible for enabling a component to dispatch actions.
In practical terms, mapDispatchToProps is where react events (and lifecycle events) are mapped to redux actions. There are a few ways
of binding action creators to dispatch()inmapDispatchToProps().
constmapDispatchToProps = (dispatch) =>({
action: () => dispatch(action())
})
// shorthand way
constmapDispatchToProps = { action }
constmapDispatchToProps = (dispatch) =>({
action: bindActionCreators(action, dispatch)
})
↥
Q. What is the use of the ownProps parameter in
mapStateToProps() and mapDispatchToProps()?
mapStateToProps:
functionmapStateToProps(state,ownProps?)
It should take a first argument called state, optionally a second argument called ownProps, and return a plain object containing the
data that the connected component needs.
This function should be passed as the first argument to connect, and will be called every time when the Redux store state changes. If
you do not wish to subscribe to the store, pass nullorundefined to connect in place of mapStateToProps.
Arguments
state
ownProps (optional)
State:
The first argument to a mapStateToProps function is the entire Redux store state.
Example:
// Employee.js
functionmapStateToProps(state) {
const { emp } = state
return { empList: emp.allIds}
}
exportdefault connect(mapStateToProps)(EmpList)
ownProps (optional):
If your component needs the data from its own props to retrieve data from the store. This argument will contain all of the props
given to the wrapper component that was generated by connect.
Example:
// Employee.js
functionmapStateToProps(state,ownProps) {
const { visibilityFilter } = state
// ownProps would look like { "id" : 100 }
const { id } = ownProps
constemp = getEmployeeById(state, id)
// component receives additionally:
return { emp, visibilityFilter }
}
// Later, in your application, a parent component renders:
;<ConnectedEmployeeid={100} />
// and your component receives props.id, props.emp, and props.visibilityFilter
↥
Q. What is reselect and how it works?
Reselect is a simple library for creating memoized, composable selector functions. Reselect selectors can be used to efficiently
compute derived data from the Redux store.
Selectors can compute derived data, allowing Redux to store the minimal possible state. Which can be considered as keep the store
as minimal as possible. A selector is not recomputed unless one of its arguments change. A memoized selector that recalculates only
when that part of the start tree changes which are input arguments to the selector. The value of selector doesn't change when there
is no change in other (unrelated) parts of the state tree.
selectors
In our context, selectors are nothing but functions which can compute or retrive data from the store. We usually fetch the state data
using mapStateToProps() function like this.
constmapStateToProps = (state)=> {
return {
activeData: getActiveData(state.someData, state.isActive)
}
}
Where getActiveData() will be a function which returns all the records from someData having the status as isActive. The drawback
with this function is, whenever any part of the state state updates, this function will recalculate this data.
When we use Reselect it caches the input arguments to the memoized function. So only when the arguments of the function
changes from the previous call, the selector recalculates.
Example:
// todo.reducer.js
// ...
import { createSelector } from'reselect';
consttodoSelector = state => state.todo.todos;
constsearchTermSelector = state => state.todo.searchTerm;
exportconst filteredTodos = createSelector(
[todoSelector, searchTermSelector],
(todos, searchTerm) => {
return todos.filter(todo=> todo.title.match(new RegExp(searchTerm, 'i')));
}
);
// ...
We can use the filteredTodos selectors to get all the todos if there's no searchTerm set in the state, or a filtered list otherwise.
↥
Q. What are the different ways to dispatch actions in
Redux?
Redux is a state container for Javascript apps, mostly used with React. It's based on actions that are dispatched and listened by
reducers which modify the state properly.
1. Passing dispatch method to component
The dispatch method is a method of the store object. An action is dispatched to trigger an update to the store.
// App.js
import { createStore } from 'redux';
import { MessageSender } from './MessageSender';
importreducer from'./reducer';
conststore = createStore(reducer);
classApp extends React.Component {
render() {
<MessageSender store={store} />
};
};
// MessageSender.js
import { sendMsg } from './actions';
// ...
this.props.store.dispatch(sendMsg(msg))
// ...
2. Using React-Redux to make dumb/smart components
The downside of the above approach is that our React component is aware of the app logic. It's best to separate the logic in our
smart component connected to the store from the user interface, i.e., from the dumb component.
From the official docs for connect(), we can describe mapDispatchToProps() this way: If an object is passed, each function inside it is
assumed to be a Redux action creator. An object with the same function names, but with every action creator wrapped into a
dispatch call so they may be invoked directly, will be merged into the component's props.
// MessageSender.container.js
import { connect } from 'react-redux';
import { sendMsg } from './actions';
importMessageSender from './MessageSender';
constmapDispatchToProps = {
sendMsg
};
exportdefault connect(null, mapDispatchToProps)(MessageSender);
// MessageSender.js
// ...
this.props.sendMsg(msg);
// ...
3. Using the bindActionCreators() method
The bindActionCreators() method allows us to dispatch actions from any React component that is not connected to the store
asmapDispatchToPros() in the connect function of react-redux.
// MsgSenderPage.js
import { bindActionCreators } from 'redux';
import { connect } from 'react-redux';
import * as actionsfrom './actions';
classMsgSenderPageextends React.Component {
constructor(props) {
super(props);
const { dispatch } = props;
this.boundedActions = bindActionCreators(actions, dispatch);
}
render() {
return <MsgSending {...this.boundedActions} />;
}
}
exportdefault connect()(MsgSenderPage);
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Q. What are Redux selectors and Why to use them?
A selector is simply a function that accepts Redux state as an argument and returns data that is derived from that state.
Selectors help to keep your Redux store state minimal and derive data from the state as needed. They can compute derived data,
allowing Redux to store the minimal possible state. Selectors are also very efficient. A selector is not recomputed unless one of its
arguments changes.
Example:
// Arrow function, direct lookup
constselectEntities = state=> state.entities
// Function declaration, mapping over an array to derive values
functionselectItemIds(state) {
return state.items.map(item=> item.id)
}
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Q. What is reselect and how it works?
Reselect is a selector library (for Redux) for building memoized selectors. Using memoization, we can prevent unnecessary rerenders
and recalculations of derived data which in turn will speed up our application.
Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the
the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.
Example: Let's look at a simple selector using Reselect
import { createselectetor } from 'reselect'
exportconst getItems = (state) => state.cart.get('items');
exportconst getItemsWithTotals = createSelector(
[getItems],
(items) => {
return items.map(i => {
return i.set('total', i.get('price', 0) * i.get('quantity'));
});
}
);
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Q. Can I dispatch an action in reducer?
Dispatching an action within a reducer is an anti-pattern. Your reducer should be without side effects, simply digesting the action
payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions
and other side effects.
↥
Q. What is Redux DevTools?
Redux-Devtools provide us debugging platform for Redux apps. It allows us to perform time-travel debugging and live editing with
hot reloading, action replay, and customizable UI.
Some of the features in official documentation are as follows −
It allows you inspect every state and action payload.
It allows you go back in time by "cancelling" actions.
If you change the reducer code, each "staged" action will be re-evaluated.
If the reducers throw, we can identify the error and also during which action this happened.
With persistState() store enhancer, you can persist debug sessions across page reloads.
Reference:
https://github.com/reduxjs/redux-devtools
Q. How to set conditional action on state in React-
redux
Actions are plain objects that send data or payloads of information from your component to the global store object.
Consider that the authentication is already in progress, in which case you do not want to dispatch the AUTHENTICATE action. So here,
you need to connect the component to the global store and retrieve the authentication status. You can do that by passing
the mapStateToProps argument to the connect() method. The condition will ensure that the AUTHENTICATE action is dispatched only
when the isAuthentication state is set to false.
importReact from "react";
import { connect } from "react-redux";
classLoginPage extends React.Component {
constructor(props) {
super(props);
this.handleFormSubmit = this.handleFormSubmit.bind(this);
}
handleFormSubmit(values) {
if (!this.props.isAuthenticating)
this.props.dispatch({
type: AUTHENTICATE,
payload: values,
});
}
render() {
return (
<div>
<LoginForm onSubmit={this.handleFormSubmit}/>
</div>
);
}
}
constmapStateToProps = (globalState) => {
const { isAuthenticating } = globalState;
return { isAuthenticating };
};
exportdefault connect(mapStateToProps)(LoginPage);
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