React Top 100 Interview Questions and Answers – Part 1

React, a powerful JavaScript library for building user interfaces has become a cornerstone of modern web development.

Whether you are a seasoned developer or just starting your journey, mastering React can significantly enhance your career prospects. Preparing for a React job interview requires a solid understanding of both basic and advanced concepts.

This comprehensive guide will walk you through the top 100 React interview questions and answers, helping you to confidently showcase your expertise and land that coveted job.

In this article, we cover a wide range of topics, from fundamental principles like components and state management to more advanced subjects like hooks, context API, and performance optimization.

Each question is designed to test your knowledge and problem-solving abilities, ensuring you are well-prepared for any challenge that comes your way.

Whether you’re aiming for a junior developer position or a senior role, these questions will help you demonstrate your proficiency and understanding of React to potential employers.

So, let’s dive in and start preparing for your next big interview!

1. What is React?

• it’s a JavaScript library for building user interfaces, particularly single-page applications.
  
• React was developed by Facebook and later open-sourced. It allows developers to create reusable UI components and manage their state efficiently.
  
• React follows a component-based architecture, where the UI is composed of independent and reusable components.
  
• These components can have their own state and lifecycle methods, making it easier to build complex user interfaces.

2. What are the key features of React?

• Virtual DOM: React uses a virtual DOM to improve performance by minimizing the need to directly manipulate the actual DOM.
• Component-Based Architecture: React follows a component-based approach, allowing developers to build encapsulated components.
• JSX: JSX is a syntax extension for JavaScript that allows mixing HTML with JavaScript.
• One-way Data Binding: React uses one-way data flow to maintain the state of the components.

3. What is JSX?

• JSX is a syntax extension for JavaScript that allows mixing HTML with JavaScript.
  
• It provides a concise and readable way to describe the structure of UI components.

4. Explain the concept of Virtual DOM in React.

• The Virtual DOM is a lightweight copy of the actual DOM maintained by React.
  
• When the state of a component changes, React creates a new virtual DOM representation, compares it with the previous one, and calculates the minimal set of changes needed to update the actual DOM.

5. Explain the difference between state and props in React.

• State is internal to a component and can be changed within the component.
  
• Props (short for properties) are passed to a component from its parent and are immutable within the component.

6. How do you create a React component?

7. What are fragments in React?

• Fragments allow you to return multiple elements from a component’s render method without wrapping them in a container element like a div. 
• They improve code readability and reduce unnecessary nesting in the DOM.

8. What are the differences between class components and functional components in React?

• Class components are ES6 classes that extend from React.Component and have a render method, whereas functional components are functions that take props as an argument and return JSX.
• Class components can have state and lifecycle methods, while functional components can use hooks to achieve similar functionality.
• Functional components are easier to read, write, and test compared to class components.

9. What are React hooks?

• React hooks are functions that enable functional components to use state and other React features without writing a class.
  
• Examples of React hooks include useState, useEffect, useContext, etc.

10. What are React lifecycle methods?

• React lifecycle methods are special methods that are automatically invoked at various stages of a component’s lifecycle, such as mounting, updating, and unmounting.
   
• Examples include componentDidMount, componentDidUpdate, and componentWillUnmount.

11. How do you perform conditional rendering in React?

12. What are keys in React and why are they important?

• Keys are special attributes that provide a unique identity to each component in React lists. They are important for efficient list rendering and reconciliation.
   
• When a list changes (e.g., items are added, removed, or reordered), React uses keys to determine which items have changed, added, or removed.

13. How do you handle events in React?

14. What is the purpose of the useEffect hook in React?

• The useEffect hook is used to perform side effects in functional components.
  
• It allows you to perform actions such as data fetching, subscriptions, or manually changing the DOM after the component has rendered.

15. Provide an Example of UseEffect

16. What is the difference between controlled and uncontrolled components in React?

• Controlled components:
  1. Their value is controlled by React state. 
  2. Changes to the value are handled by React, usually through event handlers.
     
• Uncontrolled components:
  1. Their value is managed by the DOM itself. 
  2. You can get their current value using a ref.

17. How do you pass data between components in React?

• Data can be passed between components in React through props.
   
• Parent components can pass data as props to child components.

18. Provide an Example of Passing Props between Parent and Child?

Below is the parent component

Here’s the child component

19. What is Context in React?

• Context provides a way to pass data through the component tree without having to pass props down manually at every level.
   
• It’s particularly useful for passing down global data such as user authentication or theme information.

20. What are higher-order components (HOCs) in React?

• Higher-order components are functions that take a component and return a new component with additional functionality. 
• They enable code reuse, logic abstraction, and composition in React applications.

This is how we wrap the component

21. How do you handle forms in React?

22. What are refs in React and when would you use them?

• Refs provide a way to access DOM nodes or React elements created in the render method. 
• They are often used to interact with child components, manage focus, or integrate with third-party libraries.

23. What is the purpose of the useMemo hook in React?

• The useMemo hook is used to memoize expensive calculations in functional components. 
• It returns a memoized value that only changes when one of the dependencies has changed, optimizing performance by avoiding unnecessary recalculations.

24. What is the purpose of the useReducer hook in React?

• The useReducer hook is used to manage complex state logic in functional components. 
• It is an alternative to useState and is particularly useful when the next state depends on the previous one and requires more complex updates.

25. How do you optimize performance in React applications?

Performance optimization in React can be achieved by using techniques such as:

• Memoization (using useMemo and useCallback hooks)
• Code splitting
• Virtualization for large lists
• Reducing unnecessary renders
• Using production builds and minimizing bundle size
• Implementing server-side rendering (SSR) or static site generation (SSG) for improved initial load times.

The interview questions and answer series is continued in next parts.

Kindly check out to continue your learning.

Good Luck.