Mastering React: Top 50 Interview Questions You Need to Know
React has become one of the most popular JavaScript libraries for building user interfaces. As companies increasingly seek skilled React developers, preparing for interviews becomes crucial. This guide will explore the top 50 React interview questions, providing insights and code examples to help you succeed.
1. What is React?
React is an open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications. It allows developers to create reusable UI components.
2. What are components in React?
Components are the building blocks of a React application. They can be functional or class-based and encapsulate their own structure, behavior, and styling.
Example of a Functional Component:
function Greeting(props) {
return <h1>Hello, {props.name}!</h1>;
}Example of a Class Component:
class Greeting extends React.Component {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}3. What is JSX?
JSX (JavaScript XML) is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript. It makes it easier to create React elements.
Example of JSX:
const element = <h1>Hello, world!</h1>;4. What is the Virtual DOM?
The Virtual DOM is a lightweight copy of the actual DOM. React uses it to optimize rendering by updating only the parts of the DOM that have changed, improving performance.
5. Explain the lifecycle methods of a React component.
Lifecycle methods are hooks that allow you to run code at specific points in a component's life. Key lifecycle methods include:
- componentDidMount: Invoked immediately after a component is mounted.
- componentDidUpdate: Invoked immediately after updating occurs.
- componentWillUnmount: Invoked immediately before a component is unmounted.
6. What are props in React?
Props (short for properties) are read-only attributes passed from a parent component to a child component. They allow data to flow through the component tree.
Example of Props:
<Greeting name="Alice" />7. What is state in React?
State is a built-in object that stores property values that belong to a component. Unlike props, state is mutable and can be changed within the component.
Example of State:
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>
);
}
}8. What are hooks in React?
Hooks are functions that let you use state and other React features in functional components. The most common hooks are useState and useEffect.
Example of useState Hook:
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 the useEffect hook?
useEffect is a hook that allows you to perform side effects in functional components, such as fetching data or subscribing to events.
Example of useEffect Hook:
import React, { useEffect, useState } from 'react';
function DataFetcher() {
const [data, setData] = useState([]);
useEffect(() => {
fetch('https://api.example.com/data')
.then(response => response.json())
.then(data => setData(data));
}, []);
return <div>{JSON.stringify(data)}</div>;
}10. What is Redux?
Redux is a state management library for JavaScript applications, often used with React. It provides a centralized store for application state and ensures predictable state updates.
11. What is the difference between controlled and uncontrolled components?
Controlled components are managed by React state, while uncontrolled components store their own state internally. Controlled components provide better control over form data.
Example of a Controlled Component:
class NameInput extends React.Component {
constructor(props) {
super(props);
this.state = { name: '' };
}
handleChange = (event) => {
this.setState({ name: event.target.value });
};
render() {
return <input value={this.state.name} onChange={this.handleChange} />;
}
}12. What are higher-order components (HOCs)?
Higher-order components are functions that take a component and return a new component, allowing you to reuse component logic.
Example of a Higher-Order Component:
function withLoading(Component) {
return function LoadingComponent({ isLoading, ...props }) {
return isLoading ? <div>Loading...</div> : <Component {...props} />;
};
}13. What is the context API?
The Context API is a way to pass data through the component tree without having to pass props down manually at every level. It is useful for global state management.
14. How do you optimize performance in a React application?
Performance can be optimized by using techniques such as:
- Memoization with React.memo and useMemo
- Code splitting with React.lazy
- Using the production build of React
15. What is PropTypes?
PropTypes is a library for type-checking props in React components. It helps catch bugs by ensuring that components receive the correct data types.
Example of PropTypes:
import PropTypes from 'prop-types';
function Greeting({ name }) {
return <h1>Hello, {name}!</h1>;
}
Greeting.propTypes = {
name: PropTypes.string.isRequired,
};16. What is the difference between useEffect and useLayoutEffect?
useEffect runs asynchronously after the DOM has been painted, while useLayoutEffect runs synchronously after all DOM mutations. Use useLayoutEffect for reading layout from the DOM and synchronously re-rendering.
17. What are fragments in React?
Fragments allow you to group a list of children without adding extra nodes to the DOM. They can be used to return multiple elements from a component.
Example of Fragments:
function List() {
return (
<React.Fragment>
<li>Item 1</li>
<li>Item 2</li>
</React.Fragment>
);
}18. What is the purpose of keys in React lists?
Keys help React identify which items have changed, are added, or are removed. They should be unique among siblings to ensure efficient updates.
19. How do you handle forms in React?
Forms can be handled using controlled components or uncontrolled components. Controlled components use state to manage form data, while uncontrolled components use refs.
Example of Handling Forms:
class MyForm extends React.Component {
constructor(props) {
super(props);
this.state = { input: '' };
}
handleSubmit = (event) => {
event.preventDefault();
console.log(this.state.input);
};
handleChange = (event) => {
this.setState({ input: event.target.value });
};
render() {
return (
<form onSubmit={this.handleSubmit}>
<input value={this.state.input} onChange={this.handleChange} />
<button type="submit">Submit</button>
</form>
);
}
}20. What is the difference between class components and functional components?
Class components can hold and manage their own state and lifecycle methods, while functional components are simpler and primarily used for rendering UI. With the introduction of hooks, functional components can also manage state and side effects.
21. What is the use of the shouldComponentUpdate lifecycle method?
shouldComponentUpdate allows you to optimize performance by preventing unnecessary re-renders. It returns a boolean indicating whether the component should update.
22. How do you implement routing in a React application?
Routing can be implemented using the React Router library, which allows you to define routes and navigate between different components.
Example of Routing:
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>
);
}23. What is the purpose of the useRef hook?
useRef is a hook that returns a mutable ref object, which can be used to access DOM elements or store mutable values that do not trigger re-renders.
Example of useRef:
import React, { useRef } from 'react';
function FocusInput() {
const inputRef = useRef(null);
const focusInput = () => {
inputRef.current.focus();
};
return (
<div>
<input ref={inputRef} />
<button onClick={focusInput}>Focus Input</button>
</div>
);
}24. How do you handle errors in React?
Errors can be handled using error boundaries, which are React components that catch JavaScript errors in their child component tree and display a fallback UI.
25. What is the difference between useMemo and useCallback?
useMemo is used to memoize expensive calculations, while useCallback is used to memoize functions. Both help optimize performance by preventing unnecessary re-computations.
26. What are the advantages of using React?
- Component-based architecture for reusability
- Virtual DOM for improved performance
- Strong community support and ecosystem
27. What is the purpose of the key prop in React?
The key prop is used to identify which items in a list have changed, allowing React to optimize rendering and improve performance.
28. How do you manage side effects in React?
Side effects can be managed using the useEffect hook, which allows you to perform operations like data fetching, subscriptions, or manual DOM manipulations.
29. What is the purpose of the React.StrictMode?
React.StrictMode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants.
30. How do you implement lazy loading in React?
Lazy loading can be implemented using React.lazy and Suspense, allowing you to load components only when they are needed.
Example of Lazy Loading:
const LazyComponent = React.lazy(() => import('./LazyComponent'));
function App() {
return (
<React.Suspense fallback="Loading...">
<LazyComponent />
</React.Suspense>
);
}31. What is the purpose of the React.Fragment?
React.Fragment allows you to group multiple elements without adding extra nodes to the DOM, which can help keep the DOM tree clean.
32. How do you create a custom hook in React?
Custom hooks are JavaScript functions that start with 'use' and can call other hooks. They allow you to encapsulate reusable logic.
Example of a Custom Hook:
function useCounter(initialValue = 0) {
const [count, setCount] = useState(initialValue);
const increment = () => setCount(count + 1);
return [count, increment];
}33. What is the purpose of the useLayoutEffect hook?
useLayoutEffect is similar to useEffect but runs synchronously after all DOM mutations. It is useful for reading layout from the DOM and synchronously re-rendering.
34. How do you handle authentication in a React application?
Authentication can be handled using context, hooks, or libraries like Firebase or Auth0. You can store tokens in local storage or cookies.
35. What is the purpose of the React.memo function?
React.memo is a higher-order component that memoizes a functional component, preventing unnecessary re-renders when the props have not changed.
36. How do you implement pagination in React?
Pagination can be implemented by managing the current page state and slicing the data array to display only the items for the current page.
37. What is the purpose of the useImperativeHandle hook?
useImperativeHandle customizes the instance value that is exposed when using ref. It is useful for exposing imperative methods to parent components.
38. How do you implement a modal in React?
A modal can be implemented using a state variable to control its visibility and rendering a component conditionally based on that state.
Example of a Modal:
function Modal({ isOpen, onClose }) {
if (!isOpen) return null;
return (
<div className="modal">
<div className="modal-content">
<span className="close" onClick={onClose}>×</span>
<p>Modal Content</p>
</div>
</div>
);
}39. What is the purpose of the useDebugValue hook?
useDebugValue is used to display a label for custom hooks in React DevTools, helping developers understand the state of the hook.
40. How do you implement a search feature in React?
A search feature can be implemented by managing the search query in state and filtering the displayed items based on that query.
41. What is the purpose of the React.StrictMode?
React.StrictMode is a tool for highlighting potential problems in an application. It activates additional checks and warnings for its descendants.
42. How do you handle API requests in React?
API requests can be handled using the fetch API or libraries like Axios, often within the useEffect hook to manage side effects.
43. What is the purpose of the useContext hook?
useContext allows you to access the context value directly in a functional component, making it easier to consume context without needing a Consumer component.
44. How do you implement a responsive design in React?
Responsive design can be implemented using CSS media queries, CSS-in-JS libraries, or responsive design frameworks like Bootstrap.
45. What is the purpose of the useTransition hook?
useTransition is a hook that allows you to mark updates as non-urgent, enabling smoother transitions in UI updates.
46. How do you implement drag-and-drop functionality in React?
Drag-and-drop functionality can be implemented using libraries like react-beautiful-dnd or react-dnd, which provide higher-level abstractions for drag-and-drop interactions.
47. What is the purpose of the useDeferredValue hook?
useDeferredValue allows you to defer updating a value until the browser is idle, improving performance during complex updates.
48. How do you implement internationalization in React?
Internationalization can be implemented using libraries like react-i18next or react-intl, which provide tools for translating and formatting content based on locale.
49. What is the purpose of the useMemo hook?
useMemo is used to memoize expensive calculations, preventing unnecessary re-computations when the dependencies have not changed.
50. What are the best practices for writing React components?
- Keep components small and focused
- Use functional components and hooks when possible
- Manage state and side effects properly
- Use PropTypes or TypeScript for type-checking
In conclusion, mastering these top 50 React interview questions will not only prepare you for interviews but also deepen your understanding of React. As you continue to learn and grow as a developer, remember that practice and real-world application of these concepts will solidify your knowledge and skills.
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