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Top 10 React JS interview questions and answers

Top React JS Interview Questions and Answers

React JS has become one of the most popular libraries for building user interfaces, especially single-page applications. Understanding React JS concepts deeply is crucial for anyone looking to excel in front-end development. In this article, we will explore some of the most common React JS interview questions and provide detailed answers to help you prepare for your next job interview.

1. What is React and why is it used?

Answer:

React is an open-source JavaScript library developed by Facebook for building user interfaces, primarily for single-page applications where you need a fast, interactive user experience. It allows developers to create large web applications that can update and render efficiently in response to data changes. React can be used for both web and mobile development (using React Native).

Key Features:

  • Component-Based Architecture: React is built around the concept of components, which are reusable and encapsulate their logic and presentation.
  • Declarative Syntax: It makes it easier to design interactive UIs by describing what the UI should look like for any given state.
  • Virtual DOM: React uses a virtual DOM to optimize updates to the real DOM, which improves performance.
  • Unidirectional Data Flow: React uses a one-way data binding mechanism, which makes the data flow predictable and easier to debug.

2. Explain the concept of Virtual DOM and its benefits.

Answer:

The Virtual DOM (VDOM) is a lightweight in-memory representation of the real DOM. When changes are made to the UI, React first updates the Virtual DOM rather than the real DOM directly. React then compares the updated Virtual DOM with a previous version using a process called “diffing” and calculates the minimal set of changes required to update the real DOM.

Benefits:

  • Performance Improvement: By minimizing direct manipulation of the real DOM, React improves the performance of updates and rendering.
  • Efficient Updates: The diffing algorithm ensures that only necessary changes are made, reducing the overhead of DOM manipulation.
  • Predictable Rendering: It simplifies the process of updating the UI by managing changes in a more controlled way.

3. What are React components and how are they used?

Answer:

React components are the building blocks of React applications. They are JavaScript functions or classes that return a React element (a description of what the UI should look like). Components can be either:

  • Functional Components: These are stateless components written as JavaScript functions. They receive props (input parameters) and return React elements. They are simple and easy to test.
  function Greeting(props) {
    return <h1>Hello, {props.name}</h1>;
  }
  • Class Components: These are stateful components written as ES6 classes. They have lifecycle methods and can maintain their own state.
  class Greeting extends React.Component {
    render() {
      return <h1>Hello, {this.props.name}</h1>;
    }
  }

Components can be nested inside one another to build complex UIs. They can also be reused throughout the application to maintain consistency and reduce code duplication.

4. What is JSX and why is it used in React?

Answer:

JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to HTML. It allows developers to write HTML-like code within JavaScript files. JSX is used in React to describe what the UI should look like.

Benefits of JSX:

  • Declarative Syntax: JSX provides a clear and concise way to define UI components and their structure.
  • Readability: It allows developers to write code that is more readable and easier to understand by blending HTML with JavaScript.
  • Tooling Support: JSX integrates well with various development tools and supports features like syntax highlighting and error checking.

Example:

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

Under the hood, JSX is transpiled to regular JavaScript using tools like Babel.

5. What are props and state in React?

Answer:

  • Props (Short for Properties): Props are read-only attributes passed from a parent component to a child component. They allow components to receive data and configuration from their parent. Props are immutable, meaning they cannot be changed by the child component. Example:
  function Welcome(props) {
    return <h1>Welcome, {props.name}</h1>;
  }

In this example, name is a prop that the Welcome component receives from its parent.

  • State: State is a mutable data structure that holds information about a component’s current situation. Unlike props, state is managed within the component itself and can be changed by the component using this.setState (in class components) or the useState hook (in functional components). Example using Class Component:
  class Counter extends React.Component {
    constructor(props) {
      super(props);
      this.state = { count: 0 };
    }

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

    render() {
      return (
        <div>
          <p>Count: {this.state.count}</p>
          <button onClick={this.increment}>Increment</button>
        </div>
      );
    }
  }

Example using Functional Component:

  import { useState } from 'react';

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

    return (
      <div>
        <p>Count: {count}</p>
        <button onClick={() => setCount(count + 1)}>Increment</button>
      </div>
    );
  }

6. Explain React Hooks and provide examples of some commonly used hooks.

Answer:

React Hooks are functions that let you use state and other React features in functional components. They were introduced in React 16.8 to enable functional components to have side effects, manage state, and interact with the lifecycle.

Commonly Used Hooks:

  • useState: Manages state in functional components.
  import { useState } from 'react';

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

    return (
      <div>
        <p>Count: {count}</p>
        <button onClick={() => setCount(count + 1)}>Increment</button>
      </div>
    );
  }
  • useEffect: Performs side effects in functional components, such as fetching data or subscribing to events.
  import { useEffect, useState } from 'react';

  function DataFetcher() {
    const [data, setData] = useState(null);

    useEffect(() => {
      fetch('https://api.example.com/data')
        .then(response => response.json())
        .then(data => setData(data));
    }, []); // Empty dependency array means this effect runs once on mount

    return (
      <div>
        {data ? <pre>{JSON.stringify(data, null, 2)}</pre> : 'Loading...'}
      </div>
    );
  }
  • useContext: Accesses the context values in functional components.
  import { createContext, useContext } from 'react';

  const ThemeContext = createContext('light');

  function ThemedComponent() {
    const theme = useContext(ThemeContext);

    return <div>Current theme: {theme}</div>;
  }
  • useReducer: An alternative to useState for managing complex state logic.
  import { useReducer } from 'react';

  function counterReducer(state, action) {
    switch (action.type) {
      case 'increment':
        return { count: state.count + 1 };
      case 'decrement':
        return { count: state.count - 1 };
      default:
        throw new Error('Unknown action type');
    }
  }

  function Counter() {
    const [state, dispatch] = useReducer(counterReducer, { count: 0 });

    return (
      <div>
        <p>Count: {state.count}</p>
        <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
        <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
      </div>
    );
  }

7. What is the purpose of shouldComponentUpdate and PureComponent?

Answer:

  • shouldComponentUpdate: This is a lifecycle method used in class components to optimize performance by preventing unnecessary re-renders. It takes nextProps and nextState as arguments and should return a boolean indicating whether the component should re-render or not. Example:
  class MyComponent extends React.Component {
    shouldComponentUpdate(nextProps, nextState) {
      // Only update if props or state have changed
      return nextProps.value !== this.props.value || nextState.count !== this.state.count;
    }

    render() {
      // Component render logic
    }
  }
  • PureComponent: PureComponent is a base class for class components that implements shouldComponentUpdate with a shallow comparison of props and state. It helps avoid unnecessary re-renders when the props and state have not changed. Example:
  class MyComponent extends React.PureComponent {
    render() {
      // Component render logic


 }
  }

By extending PureComponent, the component automatically implements a shallow comparison for props and state, which can lead to performance optimizations.

8. Explain the concept of React Context and how it can be used.

Answer:

React Context is a way to manage and share global state across a component tree without having to pass props down manually at every level. It is useful for sharing values like themes, user information, or localization settings.

Steps to Use React Context:

  1. Create Context:
   import { createContext } from 'react';

   const ThemeContext = createContext('light');
  1. Provide Context Value: Wrap your component tree with a Context.Provider and provide a value.
   import { ThemeContext } from './ThemeContext';

   function App() {
     return (
       <ThemeContext.Provider value="dark">
         <ThemedComponent />
       </ThemeContext.Provider>
     );
   }
  1. Consume Context Value: Use useContext hook in functional components or Context.Consumer in class components to access the context value. Functional Component:
   import { useContext } from 'react';
   import { ThemeContext } from './ThemeContext';

   function ThemedComponent() {
     const theme = useContext(ThemeContext);

     return <div>Current theme: {theme}</div>;
   }

Class Component:

   import { ThemeContext } from './ThemeContext';

   class ThemedComponent extends React.Component {
     render() {
       return (
         <ThemeContext.Consumer>
           {theme => <div>Current theme: {theme}</div>}
         </ThemeContext.Consumer>
       );
     }
   }

9. What are React lifecycle methods and how are they used?

Answer:

React lifecycle methods are special methods in class components that allow you to run code at specific points in a component’s life, such as when it is created, updated, or destroyed. The lifecycle is divided into three phases: Mounting, Updating, and Unmounting.

Mounting:

  • constructor: Initializes state and binds methods.
  • componentDidMount: Invoked immediately after the component is mounted. Used for initialization tasks like data fetching.
  componentDidMount() {
    fetchData().then(data => this.setState({ data }));
  }

Updating:

  • componentDidUpdate: Invoked after the component updates. Used to perform actions based on changes in props or state.
  componentDidUpdate(prevProps, prevState) {
    if (this.props.someValue !== prevProps.someValue) {
      // Perform some action
    }
  }

Unmounting:

  • componentWillUnmount: Invoked immediately before the component is unmounted and destroyed. Used for cleanup tasks like canceling subscriptions or timers.
  componentWillUnmount() {
    clearInterval(this.timerID);
  }

React Hooks provide alternatives to lifecycle methods in functional components, using useEffect for most lifecycle operations.

10. What is React Router and how is it used?

Answer:

React Router is a library for handling routing in React applications. It allows you to define routes and navigate between different views or pages in a single-page application.

Basic Usage:

  1. Install React Router:
   npm install react-router-dom
  1. Set Up Routes: Use BrowserRouter to wrap your application and Route to define routes.
   import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

   function App() {
     return (
       <Router>
         <Switch>
           <Route path="/home" component={Home} />
           <Route path="/about" component={About} />
           <Route path="/" component={LandingPage} />
         </Switch>
       </Router>
     );
   }
  1. Link Between Routes: Use Link or NavLink to navigate between routes.
   import { Link } from 'react-router-dom';

   function Navigation() {
     return (
       <nav>
         <Link to="/home">Home</Link>
         <Link to="/about">About</Link>
       </nav>
     );
   }
  1. Access Route Parameters: Use useParams to access route parameters in functional components.
   import { useParams } from 'react-router-dom';

   function UserProfile() {
     const { userId } = useParams();
     // Fetch user data based on userId
   }

React Router provides a powerful way to build complex navigation structures and manage routing in React applications.

This article covers fundamental React JS concepts and questions that are commonly asked in interviews. Understanding these topics will help you demonstrate your knowledge and skills effectively in your next React JS interview.

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