
Top 10 React Js Interview Questions

1. What are the key features of React.js?
One of the key features of React.js is its component-based architecture. This allows me to develop small, reusable pieces of code that manage their own state. For example, I can create a Button
component that can be used throughout my application without rewriting the same code.
Here’s a simple example:
function Button({ label, onClick }) {
return <button onClick={onClick}>{label}</button>;
}
Another significant feature is JSX, a syntax extension that looks similar to HTML. JSX is great because it makes writing and adding HTML in React more intuitive. This makes it easier to visualize the UI in the code, which enhances the development experience.
React’s one-way data binding is also essential. It ensures that the flow of data throughout the application is easier to understand and manage, reducing side effects and making debugging easier. When data flows from a parent component to a child component through props, it becomes predictable and straightforward to trace issues in large applications.
2. How does React.js differ from other JavaScript frameworks like Angular or Vue.js?
React.js primarily differs from frameworks like Angular or Vue.js in its simplicity and flexibility. React is fundamentally a library for building user interfaces, not a full-blown framework. This means that while I have to integrate React with other libraries for routing or state management, I have the freedom to choose the libraries that best fit my project’s needs.
For instance, Angular comes as a complete solution with its own opinionated ways of doing routing, state management, and dependency injection. It prescribes specific frameworks and patterns, whereas React allows me to tailor the tech stack. Here’s how I might integrate React with a routing library like React Router:
import React from 'react';
import { BrowserRouter as Router, Route, Link } from 'react-router-dom';
function App() {
return (
<Router>
<div>
<Link to="/">Home</Link>
<Link to="/about">About</Link>
<Route path="/" exact component={Home} />
<Route path="/about" component={About} />
</div>
</Router>
);
}
Vue.js, on the other hand, also offers a more integrated setup but remains less opinionated than Angular. It provides a balance between the flexibility of React and the full-framework approach of Angular.
3. Can you explain the concept of Virtual DOM in React.js?
The Virtual DOM is a core concept in React.js that provides a high-performance user experience. Essentially, it’s a lightweight copy of the actual DOM (Document Object Model). Here’s why it’s useful: React creates an in-memory cache of a DOM tree, consisting of nodes and elements, which React updates based on changes in the state of the application.
Whenever the state of a component changes, React first updates this Virtual DOM. Then, it compares the updated Virtual DOM with a snapshot of the Virtual DOM taken right before the update. By doing this, React identifies exactly which DOM elements have changed. This process is called “diffing.”
After the differences are determined, React updates only those changed elements in the real DOM. This selective rendering avoids the performance cost of updating the entire DOM every time, thus making the application faster and more efficient. Here’s a simple representation:
class Counter extends React.Component {
state = { count: 0 };
increment = () => {
this.setState({ count: this.state.count + 1 });
};
render() {
return (
<div>
<p>{this.state.count}</p>
<button onClick={this.increment}>Increment</button>
</div>
);
}
}
In this example, clicking the increment button triggers an update to the component’s state, causing React to compare and update the Virtual DOM efficiently, rather than re-rendering the entire DOM.
4. What are Props and State in React.js and how do they differ?
In React.js, props
(short for properties) and state
are both crucial for controlling data in components, but they serve different purposes and manage data differently. Props
are passed to a component from a parent component and are immutable within the component that receives them. This means that I cannot change the props inside the component. They are useful for configuring components and passing data down the component tree. Here’s an example:
function Greeting(props) {
return <h1>Hello, {props.name}</h1>;
}
In contrast, state
is local and mutable data that is specific to a component and determines the component’s behavior. It can be changed within the component. Changes in state typically trigger the component to re-render.
For example:
class Counter extends React.Component {
constructor(props) {
super(props);
this.state = { count: 0 };
}
incrementCount = () => {
this.setState({ count: this.state.count + 1 });
}
render() {
return (
<div>
<p>Count: {this.state.count}</p>
<button onClick={this.incrementCount}>Increment</button>
</div>
);
}
}
5. How do you handle events in React.js?
Handling events in React.js is quite similar to handling events on DOM elements. However, there are some syntactic differences: React events are named using camelCase rather than lowercase, and with JSX you pass a function as the event handler rather than a string. Here’s how I handle events:
class ToggleButton extends React.Component {
state = { isOn: true };
toggle = () => {
this.setState(state => ({
isOn: !state.isOn
}));
}
render() {
return (
<button onClick={this.toggle}>
{this.state.isOn ? 'ON' : 'OFF'}
</button>
);
}
}
In this example, onClick
is the event handler attached to the button element, which triggers the toggle
method when the button is clicked.
6. What are React Hooks and can you give an example of a commonly used Hook?
React Hooks are functions that let you use state and other React features without writing a class. One of the most commonly used hooks is the useState
Hook, which lets you add React state to function components. Here is how I use it:
import React, { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
In this example, useState
is used to keep track of the count
state in a function component.
7. How does React.js implement forms and handle form submission?
In React.js, forms are handled similarly to other DOM elements, but the form data is typically managed using the component’s state. Here’s how I handle form submission:
class LoginForm extends React.Component {
state = { username: '' };
handleInputChange = (event) => {
this.setState({ username: event.target.value });
}
handleSubmit = (event) => {
event.preventDefault();
console.log('User submitted:', this.state.username);
}
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
Username:
<input type="text" value={this.state.username} onChange={this.handleInputChange} />
</label>
<button type="submit">Submit</button>
</form>
);
}
}
In this code, handleInputChange
updates the state as the user types in the input field, and handleSubmit
logs the state to the console when the form is submitted, preventing the default form submission behavior with event.preventDefault()
. This approach ensures that the form data is managed by React’s state, making the form easier to handle and more integrated with the component’s lifecycle.
8. Can you describe the lifecycle of a React component?
The lifecycle of a React component is a series of events that occur from the moment the component is first mounted onto the DOM to the time it is destroyed. This lifecycle can be broken into three phases: mounting, updating, and unmounting.
Mounting is the phase where the component is created and inserted into the DOM. The main methods in this phase include constructor()
, static getDerivedStateFromProps()
, render()
, and componentDidMount()
. For example:
class App extends React.Component {
constructor(props) {
super(props);
console.log('Constructor: Component is being constructed');
}
componentDidMount() {
console.log('ComponentDidMount: Component has been mounted');
}
render() {
return <h1>Hello, React!</h1>;
}
}
Updating occurs when a component’s state or props change. This phase involves static getDerivedStateFromProps()
, shouldComponentUpdate()
, render()
, and componentDidUpdate()
. Here’s how I might use the update lifecycle:
componentDidUpdate(prevProps, prevState) {
console.log('ComponentDidUpdate: Component has updated');
}
Unmounting is the final phase when the component is removed from the DOM. The method used here is componentWillUnmount()
, which is a great place to clean up resources to prevent memory leaks.
componentWillUnmount() {
console.log('ComponentWillUnmount: Component is being removed from the DOM');
}
9. How do you handle state management in React.js applications?
State management in React.js applications can vary in complexity depending on the application’s size and needs. For simple state management, React’s built-in useState
hook is sufficient. This allows me to keep track of state in functional components without using classes:
import { useState } from 'react';
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<button onClick={() => setCount(count + 1)}>Increment</button>
<p>Count: {count}</p>
</div>
);
}
For more complex applications, I often use Redux or Context API to manage state across multiple components or large applications. Redux provides a central store for state that can be accessed throughout the application, making it easier to manage complex state interactions and data flow.
10. What is Redux and how does it relate to React.js?
Redux is a predictable state container for JavaScript applications, commonly used with React but also usable with other frameworks. It helps manage the state of an application in a single centralized store, making it easier to track state changes and manage data flow across the application.
Redux works by having a single store that holds the application state, actions that describe the changes in the state, and reducers that actually carry out state modifications based on those actions. Here’s a basic example of how Redux might be set up with React:
import { createStore } from 'redux';
function reducer(state = { count: 0 }, action) {
switch (action.type) {
case 'INCREMENT':
return { count: state.count + 1 };
default:
return state;
}
}
const store = createStore(reducer);
store.subscribe(() => console.log(store.getState()));
store.dispatch({ type: 'INCREMENT' });
In a React application, I would use the react-redux
library to connect my components to the Redux store, allowing components to dispatch actions and subscribe to store updates. This integration makes managing complex interactions and data states across the application more structured and predictable.