Top 10 React JS Interview Questions: A Comprehensive Guide
As React.js continues to gain popularity among developers, interviews focusing on this powerful JavaScript library have become increasingly prevalent. In this article, we’ll explore the top 10 React JS interview questions that every developer should be prepared to answer. Whether you’re a seasoned developer aspiring to land a job at a leading tech firm or a beginner looking to enhance your skills, this guide will provide you with the insights you need.
1. What is React.js?
Answer: React.js is an open-source JavaScript library used for building user interfaces, particularly single-page applications where a seamless user experience is essential. Developed by Facebook, React allows developers to create large web applications that can change data without reloading the page.
React is component-based, meaning applications are built using reusable components, which makes code easier to manage and scale.
2. What is the Virtual DOM, and how does it work?
Answer: The Virtual DOM is a concept used in React to improve performance and provide an efficient way to update the browser’s Document Object Model (DOM). Instead of manipulating the real DOM directly, React creates a lightweight copy known as the Virtual DOM.
When the state of a component changes, the Virtual DOM is updated first. React then compares this updated Virtual DOM with a previous version using a process called “reconciliation.” Based on the differences, React updates only the necessary parts of the real DOM, thus optimizing performance and enhancing user experience.
Example:
const element = <h1>Hello, World!</h1>
ReactDOM.render(element, document.getElementById('root'));
3. What are components in React? Explain the difference between class components and functional components.
Answer: Components are the building blocks of a React application. A component can be a class, a function, or even a user-defined construct that returns a piece of UI.
Class Components: These are ES6 classes that extend from React.Component. They can hold and manage their own state and lifecycle.
class MyComponent extends React.Component {
render() {
return <div>Hello from Class Component</div>;
}
}
Functional Components: These are simpler components defined as JavaScript functions. Commonly used with React Hooks for managing state and side effects, functional components are preferred for their simplicity and readability.
function MyComponent() {
return <div>Hello from Functional Component</div>;
}
4. What are props in React?
Answer: Props, short for properties, are a way to pass data from one component to another in React. Props are immutable within the component receiving them, which means that a component can receive data but cannot modify it.
Using props promotes reusability across components, thereby enhancing modularity.
Example:
function Greeting(props) {
return <p>Hello, {props.name}!</p>;
}
// Usage
5. Explain the concept of state in React.
Answer: State in React is an object that holds some information that may change over time. Each component can maintain its own state, which allows it to react to user input or API responses more dynamically.
When the state of a component changes, React automatically re-renders the UI to reflect the new 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>
);
}
}
6. What are React Hooks?
Answer: React Hooks are functions that allow you to use state and other React features in functional components. Introduced in React 16.8, hooks simplify state management and side effects in functional components.
Commonly used hooks include useState for managing state and useEffect for handling side effects like data fetching.
Example:
import React, { useState, useEffect } from 'react';
function FetchData() {
const [data, setData] = useState(null);
useEffect(() => {
fetch('https://api.example.com/data')
.then(response => response.json())
.then(data => setData(data));
}, []);
return <div>{data ? JSON.stringify(data) : 'Loading...' }</div>;
}
7. How does the `useEffect` hook work?
Answer: The useEffect hook is used to perform side effects in functional components. It runs after every render by default, and you can control when the effect should run by passing a dependency array as a second argument.
Common use cases for useEffect include data fetching, subscriptions, and manually changing the DOM.
Example:
useEffect(() => {
// Your code here
}, [dependency]);
8. What are Controlled and Uncontrolled components?
Answer: Controlled components are components whose state is controlled by React. In the case of input elements, the value is set by the state of the component. Any changes to the input are handled via an event handler that updates the state.
function ControlledInput() {
const [value, setValue] = useState('');
return <input value={value} onChange={e => setValue(e.target.value)} />;
}
Uncontrolled components, on the other hand, manage their own state, and React has no control. This can be achieved using a ref.
function UncontrolledInput() {
const inputRef = useRef(null);
const handleSubmit = () => {
alert(inputRef.current.value);
};
return (
<form onSubmit={handleSubmit}>
<input ref={inputRef} />
<button type="submit">Submit</button>
</form>
);
}
9. What are higher-order components (HOCs) in React?
Answer: A higher-order component is a function that takes a component as an argument and returns a new component. HOCs are used for code reuse, logic abstraction, and manipulation of the component’s props or rendering behavior.
Common use cases for HOCs include conditional rendering, fetching data, and adding common styling.
Example:
function withLogging(WrappedComponent) {
return function EnhancedComponent(props) {
console.log('Rendering:', WrappedComponent.name);
return <WrappedComponent {...props} />;
};
}
10. What is the purpose of keys in React?
Answer: Keys are unique identifiers used by React to determine which items in a list have changed, been added, or been removed. They help React optimize the reconciliation process and enhance performance during rendering.
When rendering lists of components, each component should have a unique key prop.
Example:
const list = ['Apple', 'Banana', 'Cherry'];
function FruitList() {
return (
<ul>
{list.map((fruit, index) => (
<li key={index}>{fruit}</li>
))}</ul>
);
}
Conclusion
Preparing for a React JS interview involves a deep understanding of its core concepts, including components, state, props, and hooks. By familiarizing yourself with the above questions and practicing your responses, you’ll be well-equipped for your next interview.
Staying updated with the latest features and best practices in React is essential for long-term success as a developer. Make sure to keep building projects and exploring community resources to keep your React skills sharp!
