Top 10 React.js Interview Questions You Must Know

React.js has become one of the most popular libraries for building user interfaces, making it a crucial skill for front-end developers. In this post, we’ll cover the top 10 React.js interview questions you must know to ace your interview.

1. What is React.js?

Question: What is React, and why is it popular?

Answer:

React.js is an open-source JavaScript library for building user interfaces, particularly for single-page applications (SPAs). Developed by Facebook, React allows developers to create reusable UI components and manage application state efficiently.

Key Features:

• Virtual DOM: Improves performance by updating only changed elements in the real DOM.
• Component-Based Architecture: Encourages reusability and modularity.
• Unidirectional Data Flow: Simplifies state management.

2. What are React Hooks?

Question: What are React Hooks, and why are they useful?

Answer:

React Hooks are functions that let you use state and other React features in functional components. Introduced in React 16.8, they eliminate the need for class components in many cases.

Common Hooks:

• useState: Manages state in functional components.
• useEffect: Handles side effects (e.g., API calls, subscriptions).
• useContext: Accesses React context directly.
• useRef: Accesses DOM elements or persists values across renders.

3. What is the Virtual DOM?

Question: Explain the Virtual DOM and how it works.

Answer:

The Virtual DOM is a lightweight, in-memory representation of the real DOM. React uses it to optimize UI rendering.

How It Works:

1. React updates the Virtual DOM when the state or props of a component change.
2. It compares the new Virtual DOM with the previous version (a process called "diffing").
3. React calculates the minimal set of changes required and updates the real DOM efficiently.

4. What is the Difference Between Props and State?

Question: Compare props and state in React.

Answer:

5. What is the Purpose of key in React?

Question: Why do we use the key attribute in React?

Answer:

The key attribute helps React identify which items have changed, been added, or removed in a list. It improves the efficiency of rendering by enabling React to perform minimal updates to the DOM.

Example:

const items = ['Item 1', 'Item 2', 'Item 3']

<ul>
  {items.map((item, index) => (
    <li key={index}>{item}</li>
  ))}
</ul>

6. What is Context API?

Question: What is the Context API, and how is it used?

Answer:

The Context API allows components to share state globally without passing props down manually through every level of the component tree.

Example:

const ThemeContext = React.createContext()

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

function Toolbar() {
  const theme = React.useContext(ThemeContext)
  return <div>The theme is {theme}</div>
}

7. What is the Difference Between Functional and Class Components?

Question: Compare functional and class components.

Answer:

8. What are React Lifecycle Methods?

Question: What are React lifecycle methods?

Answer:

Lifecycle methods are special methods in class components that allow you to run code at specific points in a component’s lifecycle.

Common Methods:

• Mounting: componentDidMount()
• Updating: componentDidUpdate()
• Unmounting: componentWillUnmount()

In Functional Components:

Use the useEffect Hook to mimic lifecycle behavior.

9. What is React Router?

Question: How does React Router work?

Answer:

React Router is a library for routing in React applications. It enables navigation between different views of a single-page app without refreshing the page.

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>
  )
}

10. What is Redux?

Question: What is Redux, and how is it used with React?

Answer:

Redux is a state management library commonly used with React. It provides a centralized store for managing application state, making it easier to manage and debug.

Key Concepts:

• Store: Holds the application state.
• Actions: Describe changes to the state.
• Reducers: Pure functions that update the state based on actions.

Example:

import { createStore } from 'redux'

const initialState = { count: 0 }

function counterReducer(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(counterReducer)

Conclusion

These top 10 React.js interview questions cover essential concepts like state, props, hooks, and routing, ensuring you’re well-prepared for your next interview. Master these topics and showcase your React skills confidently!

Happy coding, and good luck with your interviews!