Top 50 React JS mostly asked interview question answer in detail with example

1. What is React?

Answer: React is a JavaScript library developed by Facebook for building user interfaces, particularly single-page applications. It allows developers to create reusable UI components and manage the state of those components efficiently.

Example: A simple React component:

import React from 'react';

class HelloWorld extends React.Component {
  render() {
    return <h1>Hello, World!</h1>;
  }
}

export default HelloWorld;

2. What are components in React?

Answer: Components are the building blocks of a React application. They are JavaScript functions or classes that optionally accept inputs (called “props”) and return React elements describing what should appear on the screen.

Example:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

export default Welcome;

3. What is the difference between state and props?

Answer:

Props: Short for properties, they are read-only and passed from parent to child components. They are used to configure a component.
State: A component’s local state is managed within the component and can be changed using setState. It is mutable and used for managing data that changes over time.

Example:

// Props Example
function Greeting(props) {
  return <h1>Hello, {props.name}</h1>;
}

// State Example
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>
    );
  }
}

4. What is JSX?

Answer: JSX stands for JavaScript XML. It is a syntax extension for JavaScript that looks similar to HTML. JSX allows developers to write elements and components in a more readable and concise manner.

Example:

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

5. How do you handle events in React?

Answer: Events in React are handled similarly to native HTML but with some syntactic differences. In React, you use camelCase syntax for event names and pass a function as the event handler.

Example:

class MyComponent extends React.Component {
  handleClick = () => {
    alert('Button clicked!');
  };

  render() {
    return <button onClick={this.handleClick}>Click Me</button>;
  }
}

6. What is the virtual DOM?

Answer: The virtual DOM is a lightweight representation of the real DOM. React maintains a virtual DOM to improve performance by minimizing direct manipulations of the real DOM. When a component’s state or props change, React updates the virtual DOM first, compares it with the previous version, and then updates only the necessary parts of the real DOM.

Example:

const oldVdom = <div><p>Old Text</p></div>;
const newVdom = <div><p>New Text</p></div>;

// React updates only the <p> element in the real DOM

7. What are React hooks?

Answer: React hooks are functions that let you use state and other React features without writing a class. Common hooks include useState, useEffect, and useContext.

Example:

import React, { useState, useEffect } from 'react';

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

  useEffect(() => {
    document.title = `You clicked ${count} times`;
  }, [count]);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
}

8. What is `useState`?

Answer: useState is a hook that lets you add state to functional components. It returns an array with two elements: the current state and a function to update it.

Example:

import React, { useState } from 'react';

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

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

9. What is `useEffect`?

Answer: useEffect is a hook that lets you perform side effects in functional components. It is used for operations like data fetching, subscriptions, or manually changing the DOM.

Example:

import React, { useEffect, useState } from 'react';

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

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

  return <div>{data ? data.message : 'Loading...'}</div>;
}

10. What is the context API?

Answer: The Context API is a feature in React that allows you to share state between components without having to pass props down manually through every level of the component tree.

Example:

import React, { createContext, useState, useContext } from 'react';

const ThemeContext = createContext();

function ThemedComponent() {
  const theme = useContext(ThemeContext);
  return <div style={{ background: theme.background, color: theme.color }}>Themed Component</div>;
}

function App() {
  const [theme, setTheme] = useState({ background: 'black', color: 'white' });

  return (
    <ThemeContext.Provider value={theme}>
      <ThemedComponent />
    </ThemeContext.Provider>
  );
}

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

Answer: A higher-order component (HOC) is a function that takes a component and returns a new component with additional props or behavior. HOCs are used for code reuse and composition.

Example:

function withLoadingIndicator(WrappedComponent) {
  return function WithLoadingIndicator(props) {
    return props.isLoading ? <p>Loading...</p> : <WrappedComponent {...props} />;
  };
}

12. How do you optimize performance in React?

Answer: Performance can be optimized in React by:

Using React.memo for functional components.
Using shouldComponentUpdate or PureComponent for class components.
Lazy loading components with React.lazy and Suspense.
Memoizing expensive calculations with useMemo.
Debouncing or throttling expensive operations.

Example:

import React, { memo } from 'react';

const ExpensiveComponent = memo(function ExpensiveComponent(props) {
  // component logic
});

13. What is React Router?

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

Example:

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

function App() {
  return (
    <Router>
      <Switch>
        <Route path="/" exact component={Home} />
        <Route path="/about" component={About} />
      </Switch>
    </Router>
  );
}

14. What is Redux?

Answer: Redux is a state management library for JavaScript applications. It helps manage application state in a predictable manner using a single store and actions to update the state.

Example:

import { createStore } from 'redux';

const initialState = { count: 0 };

function reducer(state = initialState, action) {
  switch (action.type) {
    case 'INCREMENT':
      return { count: state.count + 1 };
    default:
      return state;
  }
}

const store = createStore(reducer);

15. How does `ReactDOM.render` work?

Answer: ReactDOM.render is a method that renders a React element into a DOM node. It takes two arguments: the React element and the DOM node where the element should be rendered.

Example:

import React from 'react';
import ReactDOM from 'react-dom';

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

ReactDOM.render(element, document.getElementById('root'));

16. What is `useReducer`?

Answer: useReducer is a hook for managing complex state logic in functional components. It is similar to useState but provides more control for state updates.

Example:

import React, { useReducer } from 'react';

const initialState = { count: 0 };

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

();
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);

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

17. What are controlled components?

Answer: Controlled components are form elements whose value is controlled by the React component’s state. The state of the component drives the value of the form element.

Example:

import React, { useState } from 'react';

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

  const handleChange = (e) => setValue(e.target.value);

  return (
    <form>
      <input type="text" value={value} onChange={handleChange} />
    </form>
  );
}

18. What are uncontrolled components?

Answer: Uncontrolled components are form elements that manage their own state internally, and you access their values using refs instead of state.

Example:

import React, { useRef } from 'react';

function UncontrolledForm() {
  const inputRef = useRef(null);

  const handleSubmit = (e) => {
    e.preventDefault();
    alert('A name was submitted: ' + inputRef.current.value);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input type="text" ref={inputRef} />
      <button type="submit">Submit</button>
    </form>
  );
}

19. What is `React.StrictMode`?

Answer: React.StrictMode is a wrapper component that helps find problems in an application during development. It activates additional checks and warnings for its descendants.

Example:

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root')
);

20. What is `React.Fragment`?

Answer: React.Fragment is a component that lets you group a list of children without adding extra nodes to the DOM.

Example:

import React from 'react';

function List() {
  return (
    <React.Fragment>
      <li>Item 1</li>
      <li>Item 2</li>
    </React.Fragment>
  );
}

21. What is `create-react-app`?

Answer: create-react-app is a command-line tool that sets up a new React project with a sensible default configuration, including build tools, a development server, and more.

Example:

npx create-react-app my-app
cd my-app
npm start

22. What are the lifecycle methods in React?

Answer: Lifecycle methods are special methods in class components that are called at different stages of a component’s lifecycle. They include componentDidMount, componentDidUpdate, componentWillUnmount, etc.

Example:

class MyComponent extends React.Component {
  componentDidMount() {
    console.log('Component mounted');
  }

  componentDidUpdate(prevProps, prevState) {
    console.log('Component updated');
  }

  componentWillUnmount() {
    console.log('Component will unmount');
  }

  render() {
    return <div>My Component</div>;
  }
}

23. What is `React.lazy` and `Suspense`?

Answer: React.lazy is a function that lets you dynamically import components, which can be used with Suspense to handle loading states. This is useful for code splitting and lazy loading components.

Example:

import React, { Suspense, lazy } from 'react';

const LazyComponent = lazy(() => import('./LazyComponent'));

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <LazyComponent />
    </Suspense>
  );
}

24. What is `shouldComponentUpdate`?

Answer: shouldComponentUpdate is a lifecycle method in class components that determines whether a component should re-render or not. It helps optimize performance by preventing unnecessary renders.

Example:

class MyComponent extends React.Component {
  shouldComponentUpdate(nextProps, nextState) {
    return nextProps.someValue !== this.props.someValue;
  }

  render() {
    return <div>{this.props.someValue}</div>;
  }
}

25. What is `React.memo`?

Answer: React.memo is a higher-order component that memoizes a functional component, preventing unnecessary re-renders if the props have not changed.

Example:

import React, { memo } from 'react';

const MyComponent = memo((props) => {
  return <div>{props.value}</div>;
});

26. How do you pass data between components?

Answer: Data is usually passed between components using props. For more complex scenarios, you can use context, state management libraries like Redux, or callback functions.

Example:

// Parent component
function Parent() {
  const data = 'Hello, World!';
  return <Child data={data} />;
}

// Child component
function Child(props) {
  return <div>{props.data}</div>;
}

27. What is `useCallback`?

Answer: useCallback is a hook that returns a memoized callback function. It is used to optimize performance by preventing functions from being recreated on every render.

Example:

import React, { useCallback, useState } from 'react';

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

  const handleClick = useCallback(() => {
    setCount(count + 1);
  }, [count]);

  return <button onClick={handleClick}>Click me</button>;
}

28. What is `useMemo`?

Answer: useMemo is a hook that returns a memoized value. It is used to optimize performance by preventing expensive calculations from being re-computed on every render.

Example:

import React, { useMemo, useState } from 'react';

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

  const computedValue = useMemo(() => {
    // Expensive calculation
    return count * 2;
  }, [count]);

  return <div>{computedValue}</div>;
}

29. What is `useContext`?

Answer: useContext is a hook that lets you access the context value in a functional component. It is used to consume context created with React.createContext.

Example:

import React, { createContext, useContext } from 'react';

const ThemeContext = createContext('light');

function ThemedComponent() {
  const theme = useContext(ThemeContext);
  return <div>Current theme: {theme}</div>;
}

function App() {
  return (
    <ThemeContext.Provider value="dark">
      <ThemedComponent />
    </ThemeContext.Provider>
  );
}

30. What are error boundaries?

Answer: Error boundaries are components that catch JavaScript errors anywhere in their child component tree and log those errors. They also display a fallback UI instead of crashing the entire app.

Example:

import React from 'react';

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError() {
    return { hasError: true };
  }

  componentDidCatch(error, info) {
    console.error('Error caught:', error, info);
  }

  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }

    return this.props.children;
  }
}

31. What is `forwardRef`?

Answer: forwardRef is a React function that allows you to forward refs to child components. It’s useful when you need to access the DOM node or component instance in a parent component.

Example:

import React, { forwardRef } from 'react';

const FancyInput = forwardRef((props, ref) => (
  <input ref={ref} {...props} />
));

function App() {
  const inputRef = React.createRef();

  return (
    <div>
      <FancyInput ref={inputRef} />
    </div>
  );
}

32. What is the purpose of `componentDidCatch`?

Answer: componentDidCatch is a lifecycle method used in error boundaries to catch errors that occur during rendering, in lifecycle methods, or in constructors of the whole tree beneath the component.

Example:

componentDidCatch(error, info) {
  // Handle the error
}

33. What are portals?

Answer: Portals provide a way to render children into a DOM node that exists outside the hierarchy of the parent component.

Example:

import React from 'react';
import ReactDOM from 'react-dom';

function Modal({ children }) {
  return ReactDOM.createPortal(
    <div className="modal">{children}</div>,
    document.getElementById('modal-root')
  );
}

34.

How does React handle form submissions?

Answer: React handles form submissions by defining an onSubmit event handler on the form element and controlling the form’s state.

Example:

import React, { useState } from 'react';

function MyForm() {
  const [inputValue, setInputValue] = useState('');

  const handleSubmit = (event) => {
    event.preventDefault();
    alert('Form submitted with value: ' + inputValue);
  };

  return (
    <form onSubmit={handleSubmit}>
      <input
        type="text"
        value={inputValue}
        onChange={(e) => setInputValue(e.target.value)}
      />
      <button type="submit">Submit</button>
    </form>
  );
}

35. What is the `key` prop in React?

Answer: The key prop is used to uniquely identify elements in a list. It helps React optimize rendering by identifying which items have changed, been added, or removed.

Example:

function List({ items }) {
  return (
    <ul>
      {items.map((item) => (
        <li key={item.id}>{item.value}</li>
      ))}
    </ul>
  );
}

36. How do you handle dynamic imports in React?

Answer: Dynamic imports in React can be handled using React.lazy and Suspense for code splitting.

Example:

const LazyComponent = React.lazy(() => import('./LazyComponent'));

function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <LazyComponent />
    </Suspense>
  );
}

37. What is `useImperativeHandle`?

Answer: useImperativeHandle is a hook that allows you to customize the instance value that is exposed when using ref with forwardRef.

Example:

import React, { forwardRef, useImperativeHandle, useRef } from 'react';

const FancyInput = forwardRef((props, ref) => {
  const inputRef = useRef();

  useImperativeHandle(ref, () => ({
    focus: () => {
      inputRef.current.focus();
    }
  }));

  return <input ref={inputRef} {...props} />;
});

function App() {
  const inputRef = useRef();

  return (
    <div>
      <FancyInput ref={inputRef} />
      <button onClick={() => inputRef.current.focus()}>Focus Input</button>
    </div>
  );
}

38. What is `React.StrictMode` and how does it differ from `React.Fragment`?

Answer: React.StrictMode is used to identify potential problems in an application by activating additional checks and warnings during development. It does not render any extra DOM elements. React.Fragment, on the other hand, is used to group elements without adding extra nodes to the DOM.

Example:

// React.StrictMode
<React.StrictMode>
  <App />
</React.StrictMode>

// React.Fragment
<React.Fragment>
  <Child1 />
  <Child2 />
</React.Fragment>

39. What is `useLayoutEffect`?

Answer: useLayoutEffect is similar to useEffect, but it fires synchronously after all DOM mutations. It’s useful for reading layout from the DOM and synchronously re-rendering.

Example:

import React, { useLayoutEffect, useRef } from 'react';

function LayoutEffectExample() {
  const divRef = useRef();

  useLayoutEffect(() => {
    console.log(divRef.current.getBoundingClientRect());
  }, []);

  return <div ref={divRef}>Hello, world!</div>;
}

40. What is `useDebugValue`?

Answer: useDebugValue is a hook used to display a label for custom hooks in React DevTools. It helps with debugging by showing additional information.

Example:

import { useDebugValue } from 'react';

function useCustomHook(value) {
  useDebugValue(value ? 'Value is true' : 'Value is false');
  return value;
}

41. What is `ReactDOMServer`?

Answer: ReactDOMServer is a package for server-side rendering of React components. It allows you to render React components to static HTML on the server.

Example:

import React from 'react';
import ReactDOMServer from 'react-dom/server';

const element = <h1>Hello, world!</h1>;
const html = ReactDOMServer.renderToString(element);

console.log(html); // Outputs: <h1>Hello, world!</h1>

42. What is `useTransition`?

Answer: useTransition is a hook that lets you manage state transitions in a way that keeps the interface responsive. It is part of the experimental features in React.

Example:

import React, { useState, useTransition } from 'react';

function App() {
  const [isPending, startTransition] = useTransition();
  const [inputValue, setInputValue] = useState('');

  const handleChange = (event) => {
    startTransition(() => {
      setInputValue(event.target.value);
    });
  };

  return (
    <div>
      <input type="text" value={inputValue} onChange={handleChange} />
      {isPending && <p>Loading...</p>}
    </div>
  );
}

43. What is `React.StrictMode` used for?

Answer: React.StrictMode is a tool for identifying components with unsafe lifecycle methods, deprecated APIs, and other issues. It helps with detecting potential problems in the application.

Example:

<React.StrictMode>
  <App />
</React.StrictMode>

44. How do you handle authentication in React?

Answer: Authentication can be handled using context for managing user state, or by integrating with authentication libraries and services. Routes can be protected based on authentication status.

Example:

import React, { createContext, useState, useContext } from 'react';
import { BrowserRouter as Router, Route, Redirect } from 'react-router-dom';

const AuthContext = createContext();

function AuthProvider({ children }) {
  const [isAuthenticated, setIsAuthenticated] = useState(false);

  const login = () => setIsAuthenticated(true);
  const logout = () => setIsAuthenticated(false);

  return (
    <AuthContext.Provider value={{ isAuthenticated, login, logout }}>
      {children}
    </AuthContext.Provider>
  );
}

function PrivateRoute({ component: Component, ...rest }) {
  const { isAuthenticated } = useContext(AuthContext);

  return (
    <Route
      {...rest}
      render={(props) =>
        isAuthenticated ? <Component {...props} /> : <Redirect to="/login" />
      }
    />
  );
}

function App() {
  const { login, logout, isAuthenticated } = useContext(AuthContext);

  return (
    <Router>
      <Route path="/login" render={() => <button onClick={login}>Login</button>} />
      <PrivateRoute path="/protected" component={() => <button onClick={logout}>Logout</button>} />
    </Router>
  );
}

45. What are some best practices for writing React components?

Answer:

Keep components small and focused.
Use functional components and hooks when possible.
Use prop types or TypeScript for type checking.
Use useEffect and useCallback wisely to manage side effects and memoize functions.
Use React.memo to prevent unnecessary re-renders.
Write reusable components and avoid duplicating code.

46. How do you manage side effects in functional components?

Answer: Side effects in functional components are managed using the useEffect hook. It can be used for data fetching, subscriptions, or manual DOM manipulations.

Example:

import React, { 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

  return <div>{data ? data.message : 'Loading...'}</div>;
}

47. What are custom hooks?

Answer: Custom hooks are JavaScript functions that start with use and allow you to reuse stateful logic between components. They encapsulate and share logic without changing the component hierarchy.

Example:

import { useState, useEffect } from 'react';

function useWindowWidth() {
  const [windowWidth, setWindowWidth] = useState(window.innerWidth);

  useEffect(() => {
    const handleResize = () => setWindowWidth(window.innerWidth);
    window.addEventListener('resize', handleResize);

    return () => window.removeEventListener('resize', handleResize);
  }, []);

  return windowWidth;
}

48. How do you handle asynchronous operations in React?

Answer: Asynchronous operations are often handled using the useEffect hook for side effects, async/await syntax for promises, and useReducer for complex state management.

Example:

import React, { useState, useEffect } from 'react';

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

  useEffect

(() => {
    const fetchData = async () => {
      const response = await fetch('https://api.example.com/data');
      const result = await response.json();
      setData(result);
    };

    fetchData();
  }, []);

  return <div>{data ? data.message : 'Loading...'}</div>;
}

49. What is the purpose of the `useEffect` hook?

Answer: The useEffect hook is used for managing side effects in functional components. It can handle tasks like data fetching, subscriptions, and manual DOM manipulations.

Example:

import React, { useEffect, useState } from 'react';

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

  useEffect(() => {
    document.title = `You clicked ${count} times`;
  }, [count]); // Dependency array

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
}

50. How do you optimize React performance?

Answer: Performance optimization in React can be achieved by:

Using React.memo to prevent unnecessary re-renders.
Using useCallback and useMemo to memoize functions and values.
Code splitting with React.lazy and Suspense.
Using PureComponent for class components.
Profiling with React DevTools to identify performance bottlenecks.

Example:

import React, { useCallback, useState } from 'react';

const MemoizedComponent = React.memo(({ onClick }) => {
  console.log('Rendering MemoizedComponent');
  return <button onClick={onClick}>Click me</button>;
});

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

  const handleClick = useCallback(() => {
    setCount(count + 1);
  }, [count]);

  return (
    <div>
      <MemoizedComponent onClick={handleClick} />
      <p>Count: {count}</p>
    </div>
  );
}

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Top 50 React JS mostly asked interview question answer in detail with example

1. What is React?

2. What are components in React?

3. What is the difference between state and props?

4. What is JSX?

5. How do you handle events in React?

6. What is the virtual DOM?

7. What are React hooks?

8. What is useState?

9. What is useEffect?

10. What is the context API?

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

12. How do you optimize performance in React?

13. What is React Router?

14. What is Redux?

15. How does ReactDOM.render work?

16. What is useReducer?

17. What are controlled components?

18. What are uncontrolled components?

19. What is React.StrictMode?

20. What is React.Fragment?

21. What is create-react-app?

22. What are the lifecycle methods in React?

23. What is React.lazy and Suspense?

24. What is shouldComponentUpdate?

25. What is React.memo?

26. How do you pass data between components?

27. What is useCallback?

28. What is useMemo?

29. What is useContext?

30. What are error boundaries?

31. What is forwardRef?

32. What is the purpose of componentDidCatch?

33. What are portals?

34.

35. What is the key prop in React?

36. How do you handle dynamic imports in React?

37. What is useImperativeHandle?

38. What is React.StrictMode and how does it differ from React.Fragment?

39. What is useLayoutEffect?

40. What is useDebugValue?

41. What is ReactDOMServer?

42. What is useTransition?

43. What is React.StrictMode used for?

44. How do you handle authentication in React?

45. What are some best practices for writing React components?

46. How do you manage side effects in functional components?

47. What are custom hooks?

48. How do you handle asynchronous operations in React?

49. What is the purpose of the useEffect hook?

50. How do you optimize React performance?

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