React Interview Questions & Answers
Beginner Level Questions
Explore the fundamentals of React with our collection of essential beginner-level questions.
Advance Level Questions
Deepen your React knowledge with our advanced questions that cover state management and cross-platform compatibility.
Expert Level Questions
Master React with our expert-level questions that delve into performance optimization and the inner workings of the React bridge.
Question 1:- What is ReactJS, and what are its main features?
Answer: ReactJS is a JavaScript library for building user interfaces, especially for single-page applications.
It allows developers to create reusable UI components and manage the application's state efficiently.
Main features include:
- Virtual DOM for fast rendering
- Component-based architecture
- Unidirectional data flow
- Support for server-side renderingQuestion 2:- What are components in React, and how are they different from elements?
Answer: Components are the building blocks of a React application. They are JavaScript functions or classes that return React elements, which describe what you see on the screen.
Differences:
- Components can have state and lifecycle methods.
- Elements are immutable descriptions of what you want to render.
Think of components as blueprints and elements as the actual objects created from those blueprints.Question 3:- What is JSX, and why is it used in React?
Answer: JSX is a syntax extension for JavaScript that looks similar to HTML.
It allows you to write HTML-like code within JavaScript, making it easier to create and visualize UI components.
JSX is used in React because:
- It combines JavaScript logic with markup.
- It improves readability and maintainability by allowing UI elements and logic to be written in a single file.
Example:
```jsx
const element = <h1>Hello, world!</h1>;
```Question 4:- What is the virtual DOM, and how does React use it?
Answer: The virtual DOM is an in-memory representation of the actual DOM.
React uses the virtual DOM to efficiently update the UI. When the state of a component changes:
1. React creates a new virtual DOM.
2. It compares it with the previous version.
3. React calculates the minimal set of changes required to update the real DOM.
This process is called reconciliation and improves performance by reducing direct DOM manipulations.Question 5:- Explain the difference between a class component and a functional component.
Answer: Class Components:
- Are ES6 classes that extend React.Component.
- Can have lifecycle methods and manage state.
- Use 'this' to refer to component properties.
Functional Components:
- Are simple JavaScript functions.
- Can use hooks like useState and useEffect for state management.
- Are preferred for their simplicity and ability to use hooks.
Example of a Functional Component:
```jsx
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
```
Example of a Class Component:
```jsx
class Welcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}
```Question 6:- What are props in React, and how are they different from state?
Answer: Props (short for properties) are used to pass data from parent to child components in React. They are read-only and cannot be modified by the child component.
State is used to manage data that changes over time within a component. It is mutable and can be updated using the setState method in class components or the useState hook in functional components.
Differences:
- Props: Used to pass data between components.
- State: Used to manage data within a component.Question 7:- What is the purpose of render() in React?
Answer: The render() method is used in class components to return the React elements that represent the UI.
It is a required method in class components and is called whenever the component's state or props change, triggering a re-render of the component.
The render method should be pure, meaning it should not modify the component's state or interact with external systems.Question 8:- How does the component lifecycle work in React?
Answer: The component lifecycle in React consists of three main phases:
1. **Mounting**: When a component is created and inserted into the DOM.
- Lifecycle methods: componentDidMount
2. **Updating**: When a component's props or state change.
- Lifecycle methods: componentDidUpdate
3. **Unmounting**: When a component is removed from the DOM.
- Lifecycle methods: componentWillUnmountQuestion 9:- What is the use of keys in React, and why are they important?
Answer: Keys are unique identifiers used by React to track changes in a list of elements. They help React identify which items have changed, been added, or been removed, optimizing the rendering process.
Using keys improves performance by allowing React to update only the necessary elements in the DOM.
Keys should be stable, predictable, and unique to each element in the list.
Example:
```jsx
const listItems = numbers.map((number) =>
<li key={number.toString()}>{number}</li>
);
```Question 10:- What is the purpose of using useState in a functional component?
Answer: useState is a hook in React that allows functional components to have state.
It returns an array with two elements: the current state value and a function to update the state.
By using useState, functional components can manage local state, making them more powerful and similar in capability to class components, but with simpler syntax.
Example:
```jsx
const [count, setCount] = useState(0);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
```Question 11:- What is useEffect in React, and how does it work?
Answer: useEffect is a hook in React that allows you to perform side effects in functional components.
It is similar to lifecycle methods in class components, such as componentDidMount, componentDidUpdate, and componentWillUnmount.
useEffect runs after the render and can be used for:
- Fetching data
- Directly updating the DOM
- Setting up subscriptions or timers
It accepts two arguments: a function containing the side-effect logic and an optional array of dependencies that determine when the effect should run.
Example:
```jsx
useEffect(() => {
// Run this effect when the component mounts
console.log('Component mounted');
return () => {
// Cleanup when the component unmounts
console.log('Component unmounted');
};
}, []); // Empty array means this effect runs once when mounted
```Question 12:- How do you conditionally render components in React?
Answer: In React, you can conditionally render components using:
1. **Ternary Operators**: Use a ternary expression to render a component based on a condition.
```jsx
{isLoggedIn ? <Dashboard /> : <Login />}
```
2. **Logical && Operator**: Use the && operator to render a component only if a condition is true.
```jsx
{isLoggedIn && <Dashboard />}
```
3. **if-else Statements**: Use traditional if-else logic to return different components.
```jsx
if (isLoggedIn) {
return <Dashboard />;
} else {
return <Login />;
}
```Question 13:- What is React Router, and why is it used?
Answer: React Router is a library for routing in React applications.
It enables navigation between different components and pages without reloading the entire application.
React Router provides:
- Declarative routing using components like <Route> and <Link>.
- Nested routing to build complex interfaces.
- Dynamic routing based on URL parameters.
Example usage:
```jsx
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>
);
}
```Question 14:- What are higher-order components (HOCs) in React?
Answer: Higher-order components (HOCs) are functions that take a component and return a new component.
HOCs are used to add additional functionality to existing components without modifying them directly.
They allow for code reuse, logic sharing, and abstraction.
Example of a HOC that adds logging functionality:
```jsx
function withLogging(WrappedComponent) {
return class extends React.Component {
componentDidMount() {
console.log('Component mounted');
}
render() {
return <WrappedComponent {...this.props} />;
}
};
}
const EnhancedComponent = withLogging(MyComponent);
```Question 15:- What is the purpose of useContext in React?
Answer: useContext is a hook in React that allows functional components to access context values.
It provides a way to share data between components without passing props through every level of the component tree.
useContext simplifies accessing context data and makes components easier to read and maintain.
Example:
```jsx
const ThemeContext = React.createContext('light');
function ThemeButton() {
const theme = useContext(ThemeContext);
return <button className={theme}>Click me</button>;
}
```Question 16:- How do you handle forms in React?
Answer: Handling forms in React involves managing input values, handling user input, and submitting form data.
Steps to handle forms:
1. Set up state to store input values using useState.
2. Create input fields and bind them to the state.
3. Handle input changes using onChange events.
4. Submit the form using an onSubmit handler.
Example:
```jsx
function MyForm() {
const [name, setName] = useState('');
const handleSubmit = (event) => {
event.preventDefault();
console.log('Submitted name:', name);
};
return (
<form onSubmit={handleSubmit}>
<input
type="text"
value={name}
onChange={(e) => setName(e.target.value)}
/>
<button type="submit">Submit</button>
</form>
);
}
```Question 17:- What are fragments in React, and how are they used?
Answer: Fragments are used to group a list of children elements without adding extra nodes to the DOM.
They are useful for returning multiple elements from a component without adding an unnecessary wrapper element.
Fragments can be used with the <React.Fragment> tag or the shorthand <> syntax.
Example:
```jsx
function MyComponent() {
return (
<>
<h1>Title</h1>
<p>This is a paragraph.</p>
</>
);
}
```Question 18:- What is the purpose of shouldComponentUpdate, and when should it be used?
Answer: shouldComponentUpdate is a lifecycle method in class components that allows you to control when a component should re-render.
It is used to improve performance by preventing unnecessary re-renders.
This method is called before rendering when new props or state are received.
You can return false if you want to prevent the re-render, and true if you want to allow it.
Example:
```jsx
class MyComponent extends React.Component {
shouldComponentUpdate(nextProps, nextState) {
// Only re-render if the new props or state are different
return nextProps.value !== this.props.value;
}
render() {
return <div>{this.props.value}</div>;
}
}
```Question 19:- How do you optimize React application performance?
Answer: Optimizing React application performance can be achieved through various techniques:
- **Use Production Build**: Make sure to use the production build for deployment, which includes optimizations.
- **Code Splitting**: Use React.lazy and Suspense to load components lazily, reducing the initial bundle size.
- **Memoization**: Use React.memo and useMemo to prevent unnecessary re-renders of components and calculations.
- **Optimize State Management**: Keep the component state local where necessary and avoid redundant state updates.
- **Use Pure Components**: Use PureComponent or React.memo to automatically prevent re-renders when props or state have not changed.
- **Avoid Inline Functions**: Declare functions outside of render methods to avoid creating new functions on each render.Question 20:- What is prop drilling, and how can it be avoided?
Answer: Prop drilling is the process of passing data through multiple layers of components to reach a deeply nested component.
It can make components difficult to manage and understand.
Ways to avoid prop drilling:
- **Context API**: Use React's Context API to share data globally without passing props through every component.
- **State Management Libraries**: Use libraries like Redux or MobX to manage global state.
Example using Context API:
```jsx
const MyContext = React.createContext();
function Parent() {
return (
<MyContext.Provider value="shared data">
<Child />
</MyContext.Provider>
);
}
function Child() {
const value = useContext(MyContext);
return <div>{value}</div>;
}
```Contact Us
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