React Interview Questions #

1. What is React? #

React is an open-source JavaScript library used for building user interfaces, especially single-page applications and mobile applications. It allows developers to create reusable UI components and manage their state efficiently.

2. What is JSX? #

JSX (JavaScript XML) is a syntax extension for JavaScript used in React. It allows you to write HTML-like code within your JavaScript code. JSX is transpiled to regular JavaScript using tools like Babel.

3. Explain the concept of Virtual DOM. #

The Virtual DOM is an in-memory representation of the actual DOM elements. When there are changes to a React component’s state, React creates a new Virtual DOM representation, compares it with the previous one, and then updates the actual DOM with only the changes that are needed. This helps optimize rendering performance.

4. What are React Props? #

Props (short for properties) are a way to pass data from a parent component to a child component in React. They are read-only and help you achieve a unidirectional data flow.

5. What is the difference between state and props? #

• Props: Props are passed down from parent to child components and are immutable. They are used to pass data from a parent component to its child components.

• State: State is a way to manage a component’s internal data. It’s mutable and controlled by the component itself. Changes to state trigger component re-rendering.

6. Explain React component lifecycle methods. #

React components have lifecycle methods that allow you to hook into different phases of a component’s existence. Common lifecycle methods include componentDidMount, componentDidUpdate, and componentWillUnmount.

7. What is the significance of setState()? #

setState() is a method provided by React to update a component’s state. When you use setState(), React will schedule an update to the component and then re-render it. It’s important to note that setState() is asynchronous.

8. What are controlled components and uncontrolled components? #

• Controlled Components: In controlled components, form elements like input fields get their values and updates from state. The component’s state is the “single source of truth.”

• Uncontrolled Components: In uncontrolled components, form elements store their own state internally, rather than relying on React state.

9. What is the purpose of the key prop when rendering lists of components? #

The key prop is used to help React identify which items have changed, been added, or been removed in a list of components. It improves performance by enabling efficient updates and minimizing re-rendering of unchanged items.

10. What is Redux? #

Redux is a state management library often used with React applications. It provides a predictable way to manage application state and enables components to access and update state without the need to pass props through multiple levels.

11. What is the role of the connect function in React Redux? #

The connect function is used in React Redux to connect a React component to the Redux store. It allows the component to access the state and dispatch actions without having to directly interact with the store.

12. Explain the concept of React Hooks. #

React Hooks are functions that let you “hook into” React state and lifecycle features from functional components. They allow you to use state and other React features without writing a class component.

13. What is the useEffect hook used for? #

The useEffect hook is used to perform side effects in functional components. It replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount.

14. How can you optimize the performance of React components? #

• Memoization: Use the React.memo() higher-order component to memoize and prevent unnecessary re-renders.
• Use the Virtual DOM: Let React handle DOM updates efficiently using the Virtual DOM.
• ShouldComponentUpdate: Implement shouldComponentUpdate to control when a component should re-render.
• Use PureComponent: Use PureComponent to automatically implement a shallow prop and state comparison in shouldComponentUpdate.

15. What are React Router and its main components? #

React Router is a library that provides navigation and routing capabilities for single-page applications. Its main components are BrowserRouter, Route, Link, and Switch.