Top 45 ReactJS Interview Questions and Answers || ReactJS

1.What is ReactJS?

ReactJS is a JavaScript library that is used to build user interfaces.

2.What is JSX?

JSX is an extension to JavaScript that allows developers to write HTML-like code in their JavaScript files.

Example:

const element = <h1>Hello, world!</h1>;

3.What is the difference between props and state in ReactJS?

Props are used to pass data from one component to another, whereas state is used to manage data within a component.

Example:

class MyComponent extends React.Component {

constructor(props) {

super(props);

this.state = { count: 0 };

}

render() {

return (

<div>

<p>Count: {this.state.count}</p>

<button onClick={() => this.setState({ count: this.state.count + 1 })}>Increment</button>

</div>

);

}

In the example above, count is managed using state.

4.What is a React component?

A React component is a reusable piece of code that renders a part of a user interface.

Example:

class MyComponent extends React.Component {

render() {

return <h1>Hello, world!</h1>;

}

5.What is a React lifecycle method?

React lifecycle methods are methods that get called at different stages of a component's lifecycle, such as when it is first mounted or when it is about to be removed from the DOM.

Example:

class MyComponent extends React.Component {

componentDidMount() {

console.log('Component mounted');

}

componentWillUnmount() {

console.log('Component will unmount');

}

render() {

return <h1>Hello, world!</h1>;

}

In the example above, componentDidMount and componentWillUnmount are lifecycle methods.

6.What is a React hook?

React hooks are functions that allow you to use state and other React features in functional components.

Example:

import React, { useState } from 'react';

function MyComponent() {

const [count, setCount] = useState(0);

return (

<div>

<p>Count: {count}</p>

<button onClick={() => setCount(count + 1)}>Increment</button>

</div>

);

}

In the example above, useState is a React hook that is used to manage state in a functional component.

7.What is Redux?

Redux is a state management library for JavaScript applications, and it is often used with ReactJS.

8.What is a higher-order component (HOC)?

A higher-order component is a function that takes a component and returns a new component with additional functionality.

Example:

function withLogging(Component) {

return function(props) {

console.log('Component rendered');

return <Component {...props} />;

};

}

class MyComponent extends React.Component {

render() {

return <h1>Hello, world!</h1>;

}

export default withLogging(MyComponent);

In the example above, withLogging is a higher-order component that adds logging functionality to MyComponent.

9.What is the difference between a controlled component and an uncontrolled component in ReactJS?

In a controlled component, the value of an input element is controlled by ReactJS through its state, whereas in an uncontrolled component, the value is managed by the DOM.

Example of a controlled component:

class MyComponent extends React.Component {

constructor(props) {

super(props);

this.state = { value: '' };

this.handleChange = this.handleChange.bind(this);

}

handleChange(event) {

this.setState({ value: event.target.value });

}

render() {

return (

<div>

<p>Value: {this.state.value}</p>

</div>

);

}

In the example above, the value of the input element is controlled by ReactJS through its state.

Example of an uncontrolled component:

function MyComponent() {

const inputRef = React.useRef(null);

function handleClick() {

console.log('Value:', inputRef.current.value);

}

return (

<div>

<button onClick={handleClick}>Log Value</button>

</div>

);

}

In the example above, the value of the input element is managed by the DOM.

10.What is the virtual DOM?

The virtual DOM is a concept in ReactJS that represents the state of the actual DOM in memory. When a component's state changes, ReactJS updates the virtual DOM, and then calculates the most efficient way to update the actual DOM based on the changes.

11.What is the purpose of the key prop in ReactJS?

The key prop is used to give each child element in an array a unique identity. This helps ReactJS to efficiently update the DOM when the array changes.

Example:

function MyList(props) {

return (

<ul>

{props.items.map(item => (

))}

</ul>

);

}

In the example above, the key prop is used to give each li element a unique identity based on its id.

12.What is server-side rendering?

Server-side rendering is a technique in which a web server returns a fully rendered HTML page to the client instead of just an empty shell HTML file. This can improve performance and SEO for a web application.

Example:

import ReactDOMServer from 'react-dom/server';

import App from './App';

const html = ReactDOMServer.renderToString(<App />);

console.log(html);

In the example above, ReactDOMServer.renderToString is used to render the App component to a string of HTML, which can be sent to the client by a web server.

These are just a few examples of the types of questions you might encounter in a ReactJS interview. It's important to have a good understanding of the fundamentals of ReactJS and to be able to apply that knowledge to real-world scenarios.

13.What is a higher-order component?

A higher-order component (HOC) is a function that takes a component and returns a new component with additional functionality.

Example:

function withLog(Component) {

return function WithLog(props) {

console.log('Rendering:', Component.name);

return <Component {...props} />;

};

}

const MyComponent = withLog(props => <div>Hello, {props.name}!</div>);

In the example above, withLog is a higher-order component that takes a component and returns a new component that logs when it is rendered.

14.What is the context API?

The context API is a feature in ReactJS that allows data to be passed down through the component tree without having to pass props manually at every level. It is often used for data that is global to an application, such as a user's authentication status.

Example:const MyContext = React.createContext();

function App() {

const [loggedIn, setLoggedIn] = useState(false);

return (

<MyContext.Provider value={{ loggedIn, setLoggedIn }}>

</MyContext.Provider>

);

}

function MyComponent() {

const { loggedIn, setLoggedIn } = useContext(MyContext);

function handleClick() {

setLoggedIn(!loggedIn);

}

return (

<div>

<p>Logged in: {loggedIn ? 'Yes' : 'No'}</p>

<button onClick={handleClick}>Toggle Login</button>

</div>

);

}

In the example above, the MyContext.Provider component is used to provide the loggedIn state and setLoggedIn function to all child components that use the useContext hook.

15.What is Redux?

Redux is a state management library for JavaScript applications, including ReactJS. It provides a predictable and centralized way to manage the state of an application, which can improve scalability and maintainability.

Example:

import { createStore } from 'redux';

const initialState = { count: 0 };

function reducer(state = initialState, action) {

switch (action.type) {

case 'INCREMENT':

return { count: state.count + 1 };

case 'DECREMENT':

return { count: state.count - 1 };

default:

return state;

}

const store = createStore(reducer);

function MyComponent() {

const { count } = store.getState();

function handleIncrement() {

store.dispatch({ type: 'INCREMENT' });

}

function handleDecrement() {

store.dispatch({ type: 'DECREMENT' });

}

return (

<div>

<p>Count: {count}</p>

<button onClick={handleIncrement}>Increment</button>

<button onClick={handleDecrement}>Decrement</button>

</div>

);

}

In the example above, a Redux store is created with an initial state of { count: 0 }. The MyComponent component uses the store.getState function to access the current state, and the store.dispatch function to update the state in response to user interactions.

16.What is React Router?

React Router is a library for declaratively routing URLs in a ReactJS application. It allows developers to define the routes of an application as a set of components, making it easy to manage complex routing logic.

Example:

import { BrowserRouter as Router, Route, Link } from 'react-router-dom';

function Home() {

return <h1>Home</h1>;

}

function About() {

return <h1>About</h1>;

}

function App() {

return (

</Router>

);

}

In the example above, BrowserRouter is used as the router implementation, and Route components are used to define the routes of the application. The exact prop on the Home route ensures that it only matches the exact URL of '/', while the About route matches the URL of '/about'.

17.What is React.memo?

React.memo is a higher-order component that memoizes a component, preventing it from being re-rendered unnecessarily. It is similar to React.PureComponent, but can be used with functional components instead of class components.

Example:

const MyComponent = React.memo(props => {

console.log('Rendering MyComponent');

return <div>Hello, {props.name}!</div>;

});

In the example above, MyComponent is memoized using React.memo, which means that it will only re-render if its props have changed.

18.What is the virtual DOM?

The virtual DOM is an in-memory representation of the actual DOM in a web browser. It is used by ReactJS to optimize the rendering of components by reducing the number of actual DOM manipulations.

Example:

function MyComponent(props) {

return (

<div>

<p>Hello, {props.name}!</p>

</div>

);

}

In the example above, when MyComponent is rendered, ReactJS creates a virtual DOM representation of the <div> and <p> elements. It then calculates the difference between the current virtual DOM and the previous virtual DOM, and only updates the actual DOM with the necessary changes.

19.What is JSX?

JSX is a syntax extension for JavaScript that allows developers to write HTML-like code in JavaScript files. It is used by ReactJS to define the structure and appearance of components.

Example:

function MyComponent(props) {

return <div>Hello, {props.name}!</div>;

}

In the example above, the MyComponent function returns a JSX expression that creates a <div> element containing the text "Hello, " followed by the value of the name prop, followed by "!".

20.What is the difference between props and state?

Props and state are both used to store data in a ReactJS application, but they serve different purposes. Props are used to pass data from a parent component to a child component, while state is used to store data that is internal to a component and can change over time.

Example:

function Parent() {

return <Child name="Alice" />;

}

function Child(props) {

const [count, setCount] = useState(0);

function handleClick() {

setCount(count + 1);

}

return (

<div>

<p>Hello, {props.name}!</p>

<p>Count: {count}</p>

<button onClick={handleClick}>Increment</button>

</div>

);

}

In the example above, the name prop is passed from the Parent component to the Child component, while the count state is internal to the Child component and can be changed by the handleClick function.

21.What is the role of setState() in ReactJS?

setState() is a method provided by ReactJS that is used to update the state of a component. When setState() is called, ReactJS updates the component's state and triggers a re-render of the component.

Example:

function MyComponent() {

const [count, setCount] = useState(0);

function handleClick() {

setCount(count + 1);

}

return (

<div>

<p>Count: {count}</p>

<button onClick={handleClick}>Increment</button>

</div>

);

}

In the example above, the handleClick function calls setCount with a new value for the count state, which triggers a re-render of the component.

22.What is the difference between controlled and uncontrolled components in ReactJS?

Controlled components are ReactJS components that manage their state using props, while uncontrolled components manage their state internally using refs.

Example of a controlled component:

function ControlledInput(props) {

const [value, setValue] = useState('');

function handleChange(event) {

setValue(event.target.value);

}

return (

);

}

In the example above, the ControlledInput component is a controlled component because it manages its state using the value prop passed to it. The handleChange function updates the value state whenever the input is changed, which triggers a re-render of the component.

Example of an uncontrolled component:

function UncontrolledInput(props) {

const inputRef = useRef(null);

function handleClick() {

console.log(inputRef.current.value);

}

return (

<div>

<button onClick={handleClick}>Submit</button>

</div>

);

}

In the example above, the UncontrolledInput component is an uncontrolled component because it manages its state internally using the inputRef ref. The handleClick function reads the value of the input using inputRef.current.value, without triggering a re-render of the component.

23.What is the significance of keys in ReactJS?

Keys are used by ReactJS to identify individual elements in a list and to optimize the rendering of the list when changes occur. Keys should be unique among sibling elements, but don't need to be globally unique.

Example:

function MyList(props) {

const items = props.items.map(item => (

));

return <ul>{items}</ul>;

}

In the example above, each li element is assigned a unique key based on its id property, which allows ReactJS to optimize the rendering of the list when changes occur.

24.What is the role of refs in ReactJS?

Refs are used in ReactJS to access the DOM nodes or components rendered by a component. They can be used to set focus, trigger animations, measure the size of elements, and more.

Example:

function MyComponent() {

const inputRef = useRef(null);

function handleClick() {

inputRef.current.focus();

}

return (

<div>

<button onClick={handleClick}>Focus</button>

</div>

);

}

In the example above, the inputRef ref is used to access the input element rendered by MyComponent. When the button is clicked, the `handleClick function sets the focus on the input element using inputRef.current.focus().

25.What are higher-order components in ReactJS?

Higher-order components (HOCs) are functions that take a component as input and return a new component with additional functionality. HOCs are used to implement cross-cutting concerns such as logging, authentication, and error handling.

Example:

function withLogging(Component) {

function LoggedComponent(props) {

console.log(`Rendering ${Component.name}`);

return <Component {...props} />;

}

return LoggedComponent;

}

function MyComponent(props) {

return <div>Hello, {props.name}!</div>;

}

const LoggedMyComponent = withLogging(MyComponent);

ReactDOM.render(

<LoggedMyComponent name="Alice" />,

document.getElementById('root')

);

In the example above, the withLogging function is a HOC that takes a component as input and returns a new component that logs when it is rendered. The MyComponent component is wrapped by the withLogging function to create the LoggedMyComponent component, which is then rendered by ReactJS.

26.What is Redux?

Redux is a state management library for JavaScript applications. It provides a predictable state container that can be used to manage the state of an application across multiple components and interactions.

Example:

import { createStore } from 'redux';

const initialState = { count: 0 };

function reducer(state = initialState, action) {

switch (action.type) {

case 'INCREMENT':

return { ...state, count: state.count + 1 };

case 'DECREMENT':

return { ...state, count: state.count - 1 };

default:

return state;

}

const store = createStore(reducer);

store.subscribe(() => {

console.log(store.getState());

});

store.dispatch({ type: 'INCREMENT' });

store.dispatch({ type: 'DECREMENT' });

In the example above, a Redux store is created using the createStore function and a reducer function. The reducer function defines how the state of the store should be updated in response to actions dispatched to the store. The store is subscribed to using the subscribe method, which logs the current state of the store whenever it changes. Actions are dispatched to the store using the dispatch method, which triggers the execution of the reducer function and updates the state of the store.

27.What are the benefits of using Redux in ReactJS?

Redux provides several benefits when used in ReactJS applications:

Centralized state management: Redux provides a single source of truth for the state of an application, which makes it easier to manage and reason about the state of an application.

Predictable state changes: Redux enforces a strict pattern for updating the state of an application, which makes it easier to reason about how the state of an application will change in response to user interactions and other events.
Time-travel debugging: Redux stores the entire history of state changes, which makes it possible to debug and reproduce issues that occurred in the past.
Testability: Redux makes it easier to write unit tests for components and application logic by providing a clear separation between the state of an application and the logic that updates the state.

28.What is React Router?

React Router is a library for routing in ReactJS applications. It provides a declarative way to define the routes of an application and to handle navigation between those routes.

Example:

import { BrowserRouter, Route, Link } from 'react-router-dom';

function Home(props) {

return <div>Welcome to the home page!</div>;

}

function About(props) {

return <div>Welcome to the about page!</

}

function App(props) {

return (

<div>

<nav>

<ul>

<li><Link to="/about">About</Link></li>

</ul>

</nav>

</div>

</BrowserRouter>

);

}

ReactDOM.render(

<App />,

document.getElementById('root')

);

In the example above, the `BrowserRouter` component is used to define the routes of the application. The `Route` components define the paths and components that should be rendered when the user navigates to those paths. The `Link` components are used to create links to those paths.

29. What is server-side rendering in ReactJS?

Server-side rendering (SSR) is the process of rendering ReactJS components on the server and sending the rendered HTML to the client. This approach is used to improve the performance and SEO of ReactJS applications by reducing the time to first render and making the content of the application visible to search engines.

Example:

import express from 'express';

import React from 'react';

import { renderToString } from 'react-dom/server';

import App from './App';

const app = express();

app.get('/', (req, res) => {

const html = renderToString(<App />);

res.send(`

<!doctype html>

<html>

<head>

<title>My React App</title>

</head>

<body>

</body>

</html>

`);

});

app.listen(3000, () => {

console.log('Server started on port 3000');

});

In the example above, an express server is used to render the App component to HTML using the renderToString function from react-dom/server. The rendered HTML is then sent to the client as a response to a GET request to the root path of the server. The server also serves a JavaScript bundle that contains the client-side code for the application.

30.What is React Native?

React Native is a framework for building native mobile applications using ReactJS. It allows developers to build mobile applications using the same principles and patterns as ReactJS web applications, but with native performance and user experience.

Example:

import React from 'react';

import { StyleSheet, Text, View } from 'react-native';

export default function App() {

return (

<Text>Hello, React Native!</Text>

</View>

);

}

const styles = StyleSheet.create({

container: {

flex: 1,

backgroundColor: '#fff',

alignItems: 'center',

justifyContent: 'center',

});

In the example above, a simple React Native application is defined using the View and Text components from the react-native library. The styles for the application are defined using the StyleSheet API, which provides a way to define styles using a JavaScript object.

31.What is the difference between ReactJS and React Native?

ReactJS and React Native are both based on the same principles and patterns, but they are used to build different types of applications:

ReactJS is used to build web applications that run in a browser, using HTML, CSS, and JavaScript.
React Native is used to build native mobile applications that run on iOS and Android, using native UI components and APIs.
While the core principles and patterns of ReactJS and React Native are similar, there are some differences in the way they are used and the libraries and APIs they provide. For example:
ReactJS provides a set of components for building web applications, such as div, span, input, and button, while React Native provides a different set of components for building mobile applications, such as View, Text, Image, and Touchable.
ReactJS uses HTML and CSS for styling, while React Native uses a subset of CSS called StyleSheet that is optimized for mobile devices.
ReactJS uses the browser's DOM API to manipulate the document, while React Native uses native APIs provided by the platform to manipulate the UI.

32.What is Redux in ReactJS?

Redux is a state management library for ReactJS applications. It provides a centralized store for managing the state of the application, and a set of rules for updating the state in a predictable way.

Example:

import { createStore } from 'redux';

const initialState = {

counter: 0,

};

function reducer(state = initialState, action) {

switch (action.type) {

case 'INCREMENT':

return { counter: state.counter + 1 };

case 'DECREMENT':

return { counter: state.counter - 1 };

default:

return state;

}

const store = createStore(reducer);

store.dispatch({ type: 'INCREMENT' });

console.log(store.getState()); // { counter: 1 }

store.dispatch({ type: 'DECREMENT' });

console.log(store.getState()); // { counter: 0 }

In the example above, a Redux store is created using the createStore function from the redux library. The store is initialized with an initial state object that contains a counter property with a value of 0. A reducer function is defined that specifies how the state should be updated in response to different types of actions. The store.dispatch function is used to dispatch an INCREMENT action, which causes the counter property of the state to be incremented by 1. The store.getState function is used to retrieve the current state of the store.

33.What is the difference between controlled and uncontrolled components in ReactJS?

Controlled components are ReactJS components that manage their state using props. This means that the value of a controlled component is always controlled by the parent component, and the parent component can update the value of the component by passing a new value through props.

Example:

import React, { useState } from 'react';

function ControlledInput(props) {

const [value, setValue] = useState(props.value);

function handleChange(event) {

setValue(event.target.value);

}

return (

);

}

function App() {

const [value, setValue] = useState('');

function handleSubmit(event) {

event.preventDefault();

console.log(value);

}

return (

<button type="submit">Submit</button>

</form>

);

}

In the example above, the ControlledInput component is a controlled component because its value is controlled by the parent App component through the value prop. When the input is changed, the handleChange function updates the local state of the ControlledInput component, which causes the input to be re-rendered with the new value.

Uncontrolled components are ReactJS components that manage their own state using the browser's DOM API. This means that the value of an uncontrolled component is determined by the user input, and the component does not provide a way to update its value programmatically.

Example:

function UncontrolledInput() {

const inputRef = useRef(null);

function handleSubmit(event) {

event.preventDefault();

console.log(inputRef.current.value);

}

return (

<button type="submit">Submit</button>

</form>

);

}

In the example above, the UncontrolledInput component is an uncontrolled component because its value is managed by the browser's DOM API. When the form is submitted, the handleSubmit function retrieves the value of the input element using the inputRef.current.value expression.

34.What is the difference between state and props in ReactJS?

State and props are two different ways of managing data in ReactJS components.

State refers to the local state of a component, which can be updated using the useState hook or the this.setState method. State is used to manage data that can change over time, such as user input or network requests.

Example:

import React, { useState } from 'react';

function Counter() {

const [count, setCount] = useState(0);

function handleClick() {

setCount(count + 1);

}

return (

<div>

<p>Count: {count}</p>

<button onClick={handleClick}>Increment</button>

</div>

);

}

In the example above, the Counter component manages a local state using the useState hook. The state consists of a single count variable that is initialized to 0. When the handleClick function is called, the state is updated by setting the count variable to its current value plus 1.

Props, on the other hand, are properties that are passed from a parent component to a child component. Props are used to pass data and behavior from one component to another, and they are read-only, meaning that the child component cannot modify the props that it receives.

Example:

import React from 'react';

function Greeting(props) {

return <p>Hello, {props.name}!</p>;

}

function App() {

return (

<div>

</div>

);

}

In the example above, the Greeting component receives a name prop from its parent App component, and it uses the prop to render a personalized greeting. The name prop is read-only, meaning that the Greeting component cannot modify it directly.

35.What is the difference between ReactJS and Angular?

ReactJS and Angular are two popular front-end frameworks for building web applications. They have some similarities, such as their use of components and their focus on declarative programming, but they also have some differences in their design and functionality.
One key difference between ReactJS and Angular is their approach to templating. ReactJS uses a JSX syntax that allows developers to write HTML-like markup directly in their JavaScript code. Angular, on the other hand, uses a separate template language that is more closely tied to HTML and provides features such as data binding and event handling.
Another difference between ReactJS and Angular is their approach to state management. ReactJS provides a simple state management system based on the useState and useReducer hooks, as well as third-party libraries such as Redux. Angular, on the other hand, provides a more complex state management system based on a combination of services, observables, and reactive programming techniques.
Finally, ReactJS and Angular have different approaches to component styling. ReactJS uses a traditional CSS approach that allows developers to write stylesheets in a separate file and apply them to components using CSS classes. Angular, on the other hand, uses a component-based styling system called Angular Material that provides a set of pre-built UI components and a flexible theming system.

Overall, the choice between ReactJS and Angular depends on the specific needs of the project and the preferences of the development team. ReactJS may be a better choice for small to medium-sized projects that require a simple, flexible, and lightweight framework, while Angular may be a better choice for larger and more complex projects that require a more robust and structured framework.

36.What is the difference between a functional component and a class component in ReactJS?

Functional components and class components are two different ways of defining components in ReactJS.

Functional components are simple JavaScript functions that receive props as input and return a React element as output. Functional components are easy to read, test, and reason about, and they are the preferred way of defining components in ReactJS.

Example:

function Greeting(props) {

return <p>Hello, {props.name}!</p>;

}

Class components, on the other hand, are ES6 classes that extend the React.Component class. Class components have a more complex syntax than functional components, and they require the use of the this keyword to access props and state.

Example:

import React, { Component } from 'react';

class Greeting extends Component {

render() {

return <p>Hello, {this.props.name}!</p>;

}

The main difference between functional components and class components is their implementation of lifecycle methods. Functional components do not have lifecycle methods, and they are not capable of managing their own state. Class components, on the other hand, have lifecycle methods such as componentDidMount and componentDidUpdate, and they can manage their own state using the this.setState method.

In general, functional components are preferred over class components in ReactJS because they are simpler, more concise, and easier to optimize. However, class components may still be useful in some cases, such as when managing complex state or integrating with third-party libraries that require class-based components.

37.What is the Virtual DOM in ReactJS?

The Virtual DOM is a programming concept used by ReactJS to optimize the rendering of components. The Virtual DOM is a lightweight representation of the actual DOM (Document Object Model) that ReactJS uses to keep track of changes to the component tree.

When a component's state or props change, ReactJS creates a new Virtual DOM tree and compares it to the previous Virtual DOM tree. ReactJS then calculates the minimum number of changes required to update the actual DOM, and applies those changes efficiently.

The Virtual DOM allows ReactJS to avoid expensive DOM operations such as reflow and repaint, and to update the UI more efficiently. This makes ReactJS faster and more responsive than traditional UI frameworks that manipulate the DOM directly.

38.What are controlled and uncontrolled components in ReactJS?

Controlled and uncontrolled components are two different ways of managing form data in ReactJS.

A controlled component is a component that receives its current value from its parent component and notifies its parent component when its value changes. In a controlled component, the parent component controls the value of the component, and the component itself does not have its own state.

Example:

class ControlledInput extends React.Component {

render() {

return (

<input

type="text"

value={this.props.value}

onChange={this.props.onChange}

);

}

class ParentComponent extends React.Component {

constructor(props) {

super(props);

this.state = { value: '' };

this.handleChange = this.handleChange.bind(this);

}

handleChange(event) {

this.setState({ value: event.target.value });

}

render() {

return (

<ControlledInput

value={this.state.value}

onChange={this.handleChange}

);

}

In this example, ParentComponent controls the value of ControlledInput by passing it a value prop and an onChange prop. When the user types into ControlledInput, ParentComponent updates its state and re-renders ControlledInput with the new value.

An uncontrolled component, on the other hand, is a component that manages its own state internally using the ref attribute. In an uncontrolled component, the component itself controls the value, and the parent component does not need to know the current value.

Example:

class UncontrolledInput extends React.Component {

constructor(props) {

super(props);

this.inputRef = React.createRef();

}

render() {

return <input type="text" ref={this.inputRef} />;

}

componentDidMount() {

this.inputRef.current.value = this.props.value;

}

componentDidUpdate() {

this.inputRef.current.value = this.props.value;

}

class ParentComponent extends React.Component {

constructor(props) {

super(props);

this.state = { value: '' };

this.handleChange = this.handleChange.bind(this);

}

handleChange(event) {

this.setState({ value: event.target.value });

}

render() {

return (

<input

type="text"

value={this.state.value}

onChange={this.handleChange}

</>

);

}

Example:

class UncontrolledInput extends React.Component {

constructor(props) {

super(props);

this.inputRef = React.createRef();

}

render() {

return <input type="text" ref={this.inputRef} />;

}

componentDidMount() {

this.inputRef.current.value = this.props.value;

}

componentDidUpdate() {

this.inputRef.current.value = this.props.value;

}

class ParentComponent extends React.Component {

constructor(props) {

super(props);

this.state = { value: '' };

this.handleChange = this.handleChange.bind(this);

}

handleChange(event) {

this.setState({ value: event.target.value });

}

render() {

return (

<input

type="text"

value={this.state.value}

onChange={this.handleChange}

</>

);

}

In this example, UncontrolledInput manages its own state using a ref to the input element. When the component is mounted or updated, it updates the value of the input element to match the current value prop. The parent component, ParentComponent, also controls the value of the input element using a controlled component.

In general, controlled components are recommended over uncontrolled components because they provide more control and predictability over the state of the application.

39.What is Redux, and how does it work with ReactJS?

Redux is a state management library for JavaScript applications. It is designed to help manage the state of complex applications by providing a predictable and consistent way to manage and update application state.

In a ReactJS application, Redux can be used to manage the state of the entire application, or just a subset of the application. Redux works by maintaining a single "store" object that represents the entire state of the application. The store object is immutable and can only be updated through "actions" that describe the changes that need to be made.
When an action is dispatched, Redux uses a "reducer" function to update the store object. The reducer function takes the current state of the store and the action object as input, and returns a new state object that represents the updated state of the store.
Redux provides a way to connect React components to the Redux store using the "connect" function. The connect function is a higher-order function that takes a component as input and returns a new component that is connected to the Redux store. The connected component can then access the state of the store and dispatch actions to update the store.

Example:

import { createStore } from 'redux';

import { connect } from 'react-redux';

// Define the initial state of the store

const initialState = { count: 0 };

// Define the reducer function

function reducer(state = initialState, action) {

switch (action.type) {

case 'INCREMENT':

return { count: state.count + 1 };

case 'DECREMENT':

return { count: state.count - 1 };

default:

return state;

}

// Create the Redux store

const store = createStore(reducer);

// Define the Counter component

function Counter(props) {

return (

<h1>Count: {props.count}</h1>

</>

);

}

// Define the mapStateToProps function

function mapStateToProps(state) {

return { count: state.count };

}

// Define the mapDispatchToProps function

const mapDispatchToProps = {

increment: () => ({ type: 'INCREMENT' }),

decrement: () => ({ type: 'DECREMENT' }),

};

// Connect the Counter component to the Redux store

const ConnectedCounter = connect(mapStateToProps, mapDispatchToProps)(Counter);

// Render the ConnectedCounter component

ReactDOM.render(

</Provider>,

document.getElementById('root')

);

In this example, we define a simple counter component and connect it to the Redux store using the connect function. The mapStateToProps function maps the state of the store to the props of the component, and the mapDispatchToProps object maps action creators to props of the component. The Provider component is used to provide the Redux store to the connected component hierarchy.

40.What is the difference between componentWillMount() and componentDidMount() lifecycle methods in ReactJS?

Both componentWillMount() and componentDidMount() are lifecycle methods in ReactJS that are called during the lifecycle of a component. The main difference between them is when they are called in relation to the rendering of the component.
The componentWillMount() method is called right before the component is mounted to the DOM. This means that the method is called only once during the lifecycle of the component, and it is called before the component is rendered. Any state changes made inside this method will not trigger a re-render of the component.
The componentDidMount() method, on the other hand, is called immediately after the component is mounted to the DOM. This means that the method is called once during the lifecycle of the component, and it is called after the component is rendered. Any state changes made inside this method will trigger a re-render of the component.
In general, componentDidMount() is preferred over componentWillMount() because it allows you to perform any initialization that requires access to the DOM. For example, if you need to initialize a third-party library that requires access to a DOM element, you would use componentDidMount() to ensure that the element exists before initializing the library.

Example:

class MyComponent extends React.Component {

constructor(props) {

super(props);

this.state = { count: 0 };

}

componentWillMount() {

console.log('componentWillMount() called');

}

componentDidMount() {

console.log('componentDidMount() called');

setInterval(() => {

this.setState({ count: this.state.count + 1 });

}, 1000);

}

render() {

console.log('render() called');

return (

<div>

<h1>{this.state.count}</h1>

</div>

);

}

ReactDOM.render(<MyComponent />, document.getElementById('root'));

In this example, we define a simple component that logs messages to the console during the different lifecycle phases. We also use the setInterval() method inside componentDidMount() to update the component state every second. When the component is rendered, we can see that the messages are logged in the following order:

componentWillMount() called

render() called

componentDidMount() called

render() called

...

As we can see, componentWillMount() is called before the component is rendered, and componentDidMount() is called after the component is rendered. The setInterval() method inside componentDidMount() updates the state of the component every second, triggering a re-render of the component.

41.What is Redux and how does it work in ReactJS?

Redux is a predictable state container for JavaScript applications. It helps manage the state of an application in a predictable and scalable way. Redux is often used in conjunction with ReactJS, but it can be used with other libraries or frameworks as well.

At its core, Redux consists of three main components: the store, actions, and reducers. The store is where the entire state of the application is stored. Actions are plain JavaScript objects that describe what happened in the application. Reducers are pure functions that take the current state of the application and an action as input, and return a new state as output.

When an action is dispatched in the application, it is sent to the reducer along with the current state of the application. The reducer then calculates the new state of the application based on the action, and returns the new state to the store. The store then updates its state to the new state returned by the reducer.

In ReactJS, Redux can be integrated with the help of the react-redux library, which provides bindings between the two libraries. With react-redux, we can define a Provider component that wraps our entire application and provides access to the Redux store. We can also define connect() functions that connect our React components to the Redux store, allowing them to read and update the state of the application.

Example:

import { createStore } from 'redux';

import { Provider, connect } from 'react-redux';

// Define the initial state of the application

const initialState = {

count: 0

};

// Define the reducer function

function reducer(state = initialState, action) {

switch (action.type) {

case 'INCREMENT':

return { count: state.count + 1 };

case 'DECREMENT':

return { count: state.count - 1 };

default:

return state;

}

// Create a Redux store with the reducer function and initial state

const store = createStore(reducer);

// Define a React component that reads from and updates the state of the Redux store

function Counter(props) {

return (

<div>

<h1>{props.count}</h1>

</div>

);

}

// Define the functions that connect the Counter component to the Redux store

const mapStateToProps = state => ({ count: state.count });

const mapDispatchToProps = dispatch => ({

increment: () => dispatch({ type: 'INCREMENT' }),

decrement: () => dispatch({ type: 'DECREMENT' })

});

// Connect the Counter component to the Redux store using the connect() function

const ConnectedCounter = connect(mapStateToProps, mapDispatchToProps)(Counter);

// Render the Counter component wrapped in the Provider component

ReactDOM.render(

</Provider>,

document.getElementById('root')

);

In this example, we define a simple counter application using Redux and ReactJS. We start by defining the initial state of the application, and a reducer function that updates the state based on actions. We then create a Redux store with the reducer function and initial state.
We define a React component called Counter that reads from and updates the state of the Redux store. We use the connect() function to connect this component to the Redux store. We then render the Counter component wrapped in a Provider component, which provides access to the Redux store for the entire application.
When the + or - button is clicked, an action is dispatched to the Redux store, which updates the state of the application. The Counter component reads the updated state from the store and re-rend

42.What is the purpose of React Router and how does it work?

React Router is a library that allows us to implement client-side routing in a ReactJS application. Client-side routing is the process of updating the URL and the rendered content of a web page without requiring a full server refresh. This can result in a faster and more seamless user experience, as well as better search engine optimization (SEO) for web applications.
React Router provides a set of components and functions that allow us to define and manage routes in our application. The main components are:
BrowserRouter: a component that provides the routing functionality for our application. It uses the HTML5 history API to update the URL of the page without causing a full refresh.
Switch: a component that renders the first child Route that matches the current URL.
Route: a component that defines a route in our application. It specifies the path that should match the current URL, and the component that should be rendered when the path is matched.
Link: a component that provides a declarative way to navigate between routes in our application. It generates an <a> tag with an href attribute that points to the specified route.

Example:

import { BrowserRouter, Switch, Route, Link } from 'react-router-dom';

function App() {

return (

<div>

<nav>

<ul>

<li>

</li>

<li>

<Link to="/about">About</Link>

</li>

<li>

<Link to="/contact">Contact</Link>

</li>

</ul>

</nav>

</Route>

</Route>

</Route>

</Switch>

</div>

</BrowserRouter>

);

}

function Home() {

return <h1>Home</h1>;

}

function About() {

return <h1>About</h1>;

}

function Contact() {

return <h1>Contact</h1>;

}

In this example, we define a simple application with three routes: /, /about, and /contact. We use the BrowserRouter component to provide the routing functionality for our application. We define a Switch component with three Route components, each with a different path and component to render.
We also define a Link component for each route in our application, which generates an <a> tag with an href attribute that points to the specified route. When the user clicks on a link, React Router updates the URL of the page and renders the appropriate component based on the current path.
When the path matches one of the routes defined in our application, React Router renders the component specified in the Route component. In this example, we define three components for each route: Home, About, and Contact. These components could contain any valid JSX, including other components, state, and props.

43.How do you handle forms in React?

In React, handling forms is similar to handling forms in traditional HTML. However, there are some key differences, such as the fact that React components are typically stateful, and that updates to form fields need to be explicitly handled through state updates.

Here are the steps to handle forms in React:

Create a component that contains the form fields you need, along with any relevant state for the form. This can include input fields, radio buttons, checkboxes, and more.

Set up a handleChange function to update the state of your form fields whenever they change. This function will typically use the setState function to update the state with the new value of the field.

import { useState } from 'react';

function MyForm() {

const [username, setUsername] = useState('');

const [password, setPassword] = useState('');

const handleUsernameChange = (event) => {

setUsername(event.target.value);

};

const handlePasswordChange = (event) => {

setPassword(event.target.value);

};

const handleSubmit = (event) => {

event.preventDefault();

console.log('username:', username);

console.log('password:', password);

};

return (

<label>

Username:

</label>

<label>

Password:

</label>

<button type="submit">Submit</button>

</form>

);

}

In this example, we define a form with two input fields for username and password. We use the useState hook to define state for each field, and two separate handleChange functions to update the state of each field whenever it changes. We also define a handleSubmit function to handle form submissions, which logs the username and password values to the console.

Render the form component in your application, and handle form submissions using the onSubmit event.

function App() {

return (

<div>

</div>

);

}

In this example, we render the MyForm component inside the App component. We pass the form data up to the parent component using a callback function passed in as a prop to the form.

Overall, handling forms in React is similar to handling forms in traditional HTML, but requires some additional state management and event handling. By using state and event handling correctly, you can create powerful and dynamic forms that respond to user input and update your application state in real time.

44.What are React Hooks?

React Hooks are a set of functions that were introduced in React 16.8 that allow you to use state and other React features in function components. Prior to the introduction of Hooks, state could only be used in class components using the this.state syntax.

Hooks are functions that allow you to "hook into" React state and lifecycle methods from functional components. There are several built-in Hooks provided by React, including:
useState: This Hook allows you to use state in a functional component. You can define one or multiple pieces of state, and update them using the setState function returned by the Hook.

useEffect: This Hook allows you to run side effects in your functional components. You can use this Hook to fetch data, set up event listeners, or perform other operations that require access to the DOM or other external resources.

useContext: This Hook allows you to access context from a functional component. You can use this Hook to access data or functions that are passed down through a context provider.

useRef: This Hook allows you to create a mutable reference that persists across component renders. You can use this Hook to store a reference to a DOM element or other mutable value.

useReducer: This Hook allows you to use the Reducer pattern to manage state in a functional component. You can define a reducer function and an initial state, and use the dispatch function returned by the Hook to update the state based on actions.

useCallback: This Hook allows you to memoize functions in your functional components. You can use this Hook to prevent unnecessary re-renders when a function is passed down through props.

useMemo: This Hook allows you to memoize values in your functional components. You can use this Hook to prevent unnecessary re-calculations of expensive values.

useLayoutEffect: This Hook allows you to run a side effect synchronously after the DOM has been updated. This Hook is similar to useEffect, but runs synchronously instead of asynchronously.

useImperativeHandle: This Hook allows you to expose an imperative API for a component. You can use this Hook to create a ref that exposes certain methods or properties to parent components.

Overall, Hooks provide a powerful way to use state and lifecycle methods in functional components, and allow you to write cleaner, more concise code that is easier to reason about and test.

45.What are Higher-Order Components (HOCs)?

Higher-Order Components (HOCs) are a pattern in React that allow you to reuse component logic across multiple components. HOCs are functions that take in a component and return a new component with additional props or behavior.

Here's an example of a Higher-Order Component that adds a "hover" prop to a component when the mouse is over it:

function withHover(Component) {

return function WithHover(props) {

const [hovering, setHovering] = useState(false);

function handleMouseOver() {

setHovering(true);

}

function handleMouseOut() {

setHovering(false);

}

return (

</div>

);

};

}

In this example, we define a Higher-Order Component called withHover that takes in a component as an argument. We then return a new component called WithHover that wraps the original component in a div and adds event handlers to detect when the mouse is over the component. We also define state for hovering using the useState hook, and pass it down as a prop to the original component using the spread operator.

You can use this Higher-Order Component by passing in a component as an argument, like this:

function MyComponent({ hovering }) {

return <div>{hovering ? "Hovering!" : "Not hovering"}</div>;

}

const MyComponentWithHover = withHover(MyComponent);

In this example, we define a component called MyComponent that takes in a hovering prop. We then use the withHover Higher-Order Component to create a new component called MyComponentWithHover that has the same props as MyComponent, plus an additional hovering prop that is added by the Higher-Order Component.

By using Higher-Order Components, you can create reusable pieces of logic that can be applied to multiple components, without having to repeat the same code in each component. This can help to make your code more modular and easier to maintain over time.

Recents in Beach

Top 45 ReactJS Interview Questions and Answers || ReactJS

1.What is ReactJS?

2.What is JSX?

3.What is the difference between props and state in ReactJS?

4.What is a React component?

5.What is a React lifecycle method?

6.What is a React hook?

7.What is Redux?

8.What is a higher-order component (HOC)?

9.What is the difference between a controlled component and an uncontrolled component in ReactJS?

10.What is the virtual DOM?

11.What is the purpose of the key prop in ReactJS?

12.What is server-side rendering?

13.What is a higher-order component?

14.What is the context API?

15.What is Redux?

16.What is React Router?

17.What is React.memo?

18.What is the virtual DOM?

19.What is JSX?

20.What is the difference between props and state?

21.What is the role of setState() in ReactJS?

22.What is the difference between controlled and uncontrolled components in ReactJS?

23.What is the significance of keys in ReactJS?

24.What is the role of refs in ReactJS?

25.What are higher-order components in ReactJS?

26.What is Redux?

27.What are the benefits of using Redux in ReactJS?

28.What is React Router?

29. What is server-side rendering in ReactJS?

30.What is React Native?

31.What is the difference between ReactJS and React Native?

32.What is Redux in ReactJS?

33.What is the difference between controlled and uncontrolled components in ReactJS?

34.What is the difference between state and props in ReactJS?

35.What is the difference between ReactJS and Angular?

36.What is the difference between a functional component and a class component in ReactJS?

37.What is the Virtual DOM in ReactJS?

38.What are controlled and uncontrolled components in ReactJS?

39.What is Redux, and how does it work with ReactJS?

40.What is the difference between componentWillMount() and componentDidMount() lifecycle methods in ReactJS?

41.What is Redux and how does it work in ReactJS?

42.What is the purpose of React Router and how does it work?

43.How do you handle forms in React?

44.What are React Hooks?

45.What are Higher-Order Components (HOCs)?

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