Here’s a brief description of the top 45 Reactjs interview questions that can help prepare for an interview:
1. What is Reactjs?
React (also known as React.js or ReactJS) is a JavaScript library developed by Facebook for building user interfaces, specifically single-page applications where a fast, interactive user experience is crucial. It allows developers to create large web applications that can update and render efficiently in response to data changes.
2. What are the features of Reactjs?
- JSX (JavaScript Syntax Extension): Allows you to write HTML-like code inside JavaScript.
- Virtual DOM: React uses a virtual representation of the DOM to efficiently update the UI.
- Components: Reusable UI elements in React.
- Unidirectional Data Flow: Data flows in one direction from parent to child components.
3. What is JSX?
JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows writing HTML elements in JavaScript. JSX is then compiled into regular JavaScript.
Example:
//test.jsx file
const element = <h1>Hello, World!</h1>;
4. What are components in React?
Components are the building blocks of a React application. A component is a JavaScript function or class that accepts input (called “props”) and returns a React element describing how the UI should appear.
Example:
// home.jsx
function Home(props) {
return Welcome, {props.name}
;
}
5. What is the difference between state and props in React?
Props (Properties): data passed from parent to child components.
State: Mutable data managed within the component. It can be changed within the component.
6. What is the Virtual DOM?
The Virtual DOM is a lightweight copy of the actual DOM. React creates a virtual DOM to improve performance by minimizing the number of direct DOM manipulations.
7. What is the purpose of render() in React?
The render() method is used to display the JSX or React elements to the browser. It is required in every class component.
class MyComponent extends React.Component {
render() {
return <h1>Hello World!</h1>;
}
}
8. What is a functional component?
A functional component is a stateless component that is defined as a JavaScript function.
function MyComponent() {
return <h1>Functional Component</h1>;
}
9. What is a class component?
A class component is a React component defined using ES6 class syntax. It can hold state and lifecycle methods.
class MyComponent extends React.Component {
render() {
return <h1>Class Component</h1>;
}
}
10. What is the use of key in React lists?
The key is a special string attribute used to identify elements in a list, helping React optimize rendering and efficiently update elements when the list changes.
const listItems = ['Apple', 'Banana', 'Orange'];
const list = listItems.map((item, index) =>
<li key={index}>{item}</li>
);
11. What are React Hooks?
React Hooks are functions that allow you to use state and other React features without writing a class component.
12. What is useState() in React?
useState() is a React hook that lets you add state to a functional component.
import { 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>
);
}
13. What is useEffect() in React?
useEffect() is a React hook and It is used for fetching data, updating the DOM, etc. in function components.
import { 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));
}, []); // Empty array means it runs once when the component mounts
return <div>{data ? JSON.stringify(data) : 'Loading...'}</div>;
}
14. What is the useContext() hook in React?
The useContext() hook allows you to access the value of a React Context directly in functional components.
15. What is React Context?
React Context is used to share state globally across the application without passing props manually to every level of the component tree.
16. What is the difference between useEffect() and componentDidMount()?
componentDidMount() is used in class components and is called after the component is rendered for the first time.
useEffect() is used in functional components and can be used for side effects. It can replicate componentDidMount(), componentDidUpdate(), and componentWillUnmount().
17. What is the difference between componentWillMount() and componentDidMount()?
componentWillMount() is called before the component is rendered.
componentDidMount() is called immediately after the component is rendered.
18. What is a controlled component?
A controlled component is one whose form data is controlled by the React state.
function NameForm() {
const [name, setName] = useState('');
const handleChange = (event) => setName(event.target.value);
return <input type="text" value={name} onChange={handleChange} />;
}
19. What is an uncontrolled component?
An uncontrolled component is one where the form data is handled by the DOM itself rather than React state.
20. What are React lifecycle methods?
React lifecycle methods are hooks in class components that allow you to run code at specific points in a component’s life, such as when it mounts, updates, or unmounts.
21. What is ReactDOM.render()?
ReactDOM.render() is used to render a React component into the DOM. It takes two arguments: the component to render and the DOM node to render it into.
22. What is shouldComponentUpdate()?
shouldComponentUpdate() is a lifecycle method that lets you optimize performance by preventing unnecessary re-renders of a component.
23. What is React.Fragment?
React.Fragment is a way to group multiple elements without adding an extra node to the DOM.
return (
<React.Fragment>
<p>Item 1</p>
<p>Item 2</p>
</React.Fragment>
);
24. What is dangerouslySetInnerHTML in React?
dangerouslySetInnerHTML is a property that allows you to set HTML content inside a component, though it should be used with caution to avoid XSS (Cross-Site Scripting) attacks.
25. What is the difference between == and === in JavaScript?
== compares values after type coercion.
=== compares values without type coercion (strict equality).
26. What is React.memo()?
React.memo() is a higher-order component that prevents re-renders of functional components when their props haven’t changed.
27. What is useRef() in React?
useRef() is a hook that returns a mutable object, which persists across renders. It is commonly used to access DOM elements or store a mutable value.
28. What is the useCallback() hook in React?
useCallback() is a hook that returns a memoized version of a callback function, preventing its recreation on every render.
29. What is useMemo() in React?
useMemo() is a hook that returns a memoized value, recomputing the value only when one of the dependencies changes.
30. What is Prop Drilling?
Prop drilling is the process of passing data from a parent component to a deeply nested child component through props.
31. What are higher-order components (HOCs) in React?
Higher-order components are functions that take a component and return a new component with added functionality.
32. What is Redux?
Redux is a state management library that helps manage application state in a predictable way using actions, reducers, and a store.
33. What is the purpose of mapStateToProps in Redux?
mapStateToProps is a function used to map the Redux store’s state to the component’s props.
34. What is a reducer in Redux?
A reducer is a pure function that takes the current state and an action, then returns a new state.
35. What is connect() in React-Redux?
connect() is a function from the React-Redux library that connects a React component to the Redux store.
36. What is the difference between componentDidUpdate() and getDerivedStateFromProps()?
componentDidUpdate() is called after a component’s updates, while getDerivedStateFromProps() is a static method that is called before every render.
37. What is server-side rendering (SSR) in React?
SSR is the process of rendering React components on the server and sending the fully rendered page to the client, improving SEO and performance.
38. What is React Router?
React Router is a library for handling routing in React applications, enabling navigation between different views.
39. How do you create a route in React Router?
import { BrowserRouter as Router, Route } from 'react-router-dom';
function App() {
return (
<Router>
<Route path="/home" component={Home} />
</Router>
);
}
40. What is React.lazy()?
React.lazy() is a function that allows you to dynamically import components, enabling code splitting and lazy loading.
41. What is the create-react-app tool?
create-react-app is a command-line tool used to quickly set up a React project with a predefined configuration.
42. What is React Native?
React Native is a framework for building native mobile applications for iOS and Android using React.
43. What are React Hooks and their advantages?
React Hooks allow you to use state and lifecycle features in functional components. They reduce the need for class components and lead to cleaner, more maintainable code.
44. What is the Context API in React?
The Context API is a React feature that allows you to pass data through the component tree without having to pass props manually.
45. What is useLayoutEffect()?
useLayoutEffect() is a hook that works similarly to useEffect(), but it is fired synchronously after the DOM is updated.