Latest Top interview questions and answers for a React JS developer for experienced and freshers, get ready to crack any interview.

Basics of React.js interview question:

1. What is React.js?
   • Answer: React.js is a JavaScript library developed by Facebook for building user interfaces. It allows developers to create reusable UI components and manage the state of an application efficiently.
2. Explain the concept of JSX in React.js.
   • Answer: JSX (JavaScript XML) is a syntax extension for JavaScript used with React. It allows you to write HTML elements and components in a syntax similar to XML or HTML within JavaScript code.
3. What is the significance of the virtual DOM in React.js?
   • Answer: The virtual DOM is a lightweight copy of the actual DOM. React uses it to perform efficient updates by comparing the virtual DOM with the actual DOM and updating only the changed parts, reducing the need for direct DOM manipulation.
4. Describe the one-way data flow in React.js.
   • Answer: In React, data flows in one direction—from parent components to child components. Parent components pass data down to child components via props, and changes in child components trigger events handled by parent components.
5. What are React components?
   • Answer: React components are reusable, self-contained building blocks for user interfaces. They can be class components or functional components and are the foundation of a React application.
6. Explain the concept of state in React.js.
   • Answer: State in React.js represents the current condition of a component. It is mutable and managed within the component. When the state changes, React re-renders the component.
7. What is the significance of props in React.js?
   • Answer: Props (short for properties) are used to pass data from parent components to child components. They are immutable and serve as a way to communicate between components.
8. Differentiate between state and props in React.js.
   • Answer: State is mutable and managed within a component, while props are immutable and passed from parent components to child components. State is used for internal component data, and props are used for communication between components.
9. How does React handle forms and form elements?
   • Answer: React handles forms using controlled components. Input elements are controlled by React state, and changes are handled through event handlers, allowing React to manage the form’s state.
10. Explain the purpose of the key prop in React lists.
    • Answer: The key prop is used to give each element in a React list a unique identifier. It helps React efficiently update and re-render lists by identifying which items have changed, been added, or been removed.

Advanced React.js Concepts interview question:

11. What are React lifecycle methods?
    • Answer: React lifecycle methods are methods that are called at different stages in the life of a component, such as componentDidMount, componentDidUpdate, and componentWillUnmount. They allow developers to perform actions at specific points in a component’s life.
12. Explain the concept of higher-order components (HOC) in React.js.
    • Answer: Higher-order components are functions that take a component and return a new component with additional functionality. They are used for code reuse, logic abstraction, and enhancing component behavior.
13. What is the purpose of React hooks, and how do they differ from class components?
    • Answer: React hooks are functions that allow functional components to use state and lifecycle features. They include useState, useEffect, and others. Hooks provide stateful logic in functional components, eliminating the need for class components.
14. Describe the use of the useEffect hook in React.
    • Answer: The useEffect hook in React is used for side effects in functional components. It allows you to perform actions like data fetching, subscriptions, or manually changing the DOM after the component has rendered.
15. What is React context, and how is it used for state management?
    • Answer: React context is a feature that allows data to be passed through the component tree without having to pass props manually at every level. It is often used for managing global state.
16. Explain the purpose of the useReducer hook in React.
    • Answer: The useReducer hook in React is used for managing more complex state logic. It is an alternative to useState when state transitions depend on the previous state.
17. What is React Router, and how is it used for navigation in React applications?
    • Answer: React Router is a library for handling navigation in React applications. It enables the creation of single-page applications with dynamic, client-side routing.
18. What is code splitting in React, and how is it implemented?
    • Answer: Code splitting in React involves breaking the bundle into smaller chunks and loading them on demand. It is implemented using dynamic imports or tools like React.lazy and Suspense.
19. What are React fragments, and why are they useful?
    • Answer: React fragments are a way to group multiple elements without introducing an additional parent container in the DOM. They help avoid unnecessary div elements when rendering components.
20. Explain the concept of controlled and uncontrolled components in React forms.
    • Answer: Controlled components have their state managed by React, with their values bound to React state. Uncontrolled components store their state in the DOM, and their values are not controlled by React.

Testing in React.js interview question:

21. How can you perform unit testing in React.js?
    • Answer: Unit testing in React.js can be performed using testing frameworks like Jest and testing utilities like React Testing Library or Enzyme.
22. What is snapshot testing in Jest, and how does it work with React components?
    • Answer: Snapshot testing in Jest involves capturing a snapshot of a component’s rendered output and comparing it with future renders. It helps detect unintentional changes in the UI.
23. Explain the purpose of the render method in React Testing Library.
    • Answer: The render method in React Testing Library is used to render React components in a testing environment. It returns an object with methods for querying and interacting with the rendered components.
24. How can you mock API calls in React.js testing?
    • Answer: API calls in React.js testing can be mocked using tools like jest.mock for Jest or by creating manual mocks. This allows isolated testing of components without making actual network requests.
25. What is end-to-end testing in React.js, and how is it done?
    • Answer: End-to-end testing in React.js involves testing the complete application workflow. Tools like Cypress, Puppeteer, or Selenium can be used for React.js end-to-end testing.

React Ecosystem interview question:

26. What is Redux, and how does it work with React.js?
    • Answer: Redux is a state management library for JavaScript applications. It can be used with React.js to manage the global state of an application in a predictable way.
27. Explain the concept of middleware in Redux.
    • Answer: Middleware in Redux is a way to intercept actions before they reach the reducer. It is often used for tasks such as logging, asynchronous operations, or routing.
28. What is the purpose of React Hooks in the context of functional components?
    • Answer: React Hooks allow functional components to use state and lifecycle features without converting them to class components. They include hooks like useState and useEffect.
29. What is GraphQL, and how does it relate to React.js?
    • Answer: GraphQL is a query language for APIs. It enables clients to request the data they need, and nothing more. React.js applications can use GraphQL for efficient and flexible data fetching.
30. How does server-side rendering (SSR) work in React.js?
    • Answer: Server-side rendering in React.js involves rendering React components on the server before sending the HTML to the client. It improves initial page load performance and search engine optimization (SEO).