100 React Interview Questions and Answers

Introduction

• Focus on React and Redux for beginners
• Useful for revision purposes
• Mentions a friend's YouTube channel for tech content

Question 1: What is React JS?

• Open-source JavaScript library for building user interfaces, particularly single-page applications (SPAs)
• Based on creating reusable UI components

Question 2: Major Features of React

• Virtual DOM: Improves performance by minimizing direct DOM manipulation.
• JSX: JavaScript XML allows writing HTML in React components.
• Components: Reusable blocks that make React more manageable.
• One-way Data Binding: Makes applications easier to understand.
• High Performance: Due to the virtual DOM concept.
• Unidirectional Data Flow: Provides better control over applications.

Question 3: What is Virtual DOM?

• Lightweight in-memory representation of the real DOM.
• Maintains a copy of the actual DOM structure to optimize rendering.
• Uses a diffing algorithm to minimize updates only to changed elements.

Question 4: What are Components in React?

• Building blocks of a React application.
• Two types: Class-based and Functional components.

Question 5: Class-Based Components

• Use class keyword, extend React.Component, implement render method.
• Example provided for creating a simple class component.

Question 6: Functional Components

• Can be created using function keyword or ES6 arrow function.
• Returns JSX directly, simpler than class components.

Question 7: What is JSX?

• Stands for JavaScript XML, allows writing HTML in JavaScript.
• React will convert JSX into React elements internally.

Question 8: Exporting and Importing Components

• Components can be exported as default or named exports.
• Highlighted the importance of exporting for reuse in other parts of the app.

Question 9: Nested Components

• React allows nesting components for better organization.
• Parent and child relations explained with examples.

Question 10: What is State in React?

• Represents a part of an app that can change.
• Key point: Each component can have its own state.

Question 11: Updating State

• Class components use setState, while functional components use useState hook.

Question 12: What are Props?

• Props are used to pass data to child components.
• One-way data flow; props are immutable.

Question 13: Difference Between State and Props

• State is mutable and managed within the component.
• Props are immutable and passed from parent to child.

Question 14: Lifting State Up

• Lifting state refers to moving state to the closest common ancestor to share data across children.

Question 15: Children Prop

• Special property used to pass nested content in components.

Question 16: Default Props

• Setting default values for props to prevent errors when props are not provided.

Question 17: Fragments in React

• Used to group multiple elements without adding extra nodes to the DOM.

Question 18: Styling in React

• Various methods: inline styles, CSS stylesheets, CSS modules, etc.

Question 19: Conditional Rendering

• Using if statements, ternary operators, or logical && operators to render components based on conditions.

Question 20: Rendering Lists

• Utilize the map function to render lists of data.

Question 21: Key Prop

• Unique identifier for list items to help React track changes.

Question 22: Why Not Use Index as Key?

• Using indexes as keys can lead to performance issues and unexpected behavior.

Question 23: Handling Buttons

• Demonstrated how to create a button with onClick handlers.

Question 24: Handling Inputs

• Controlled and uncontrolled components defined with examples.

Question 25: Lifecycle Methods in React

• Explained phases: Mounting, Updating, Unmounting.

Question 26: Popular Hooks in React

• Mentioned popular hooks: useState, useEffect, useContext, etc.

Question 27: Use State Hook

• Manages state in functional components with initial values and an updater function.

Question 28: Use Effect Hook

• Manages side effects like data fetching, subscriptions, etc.

Question 29: Data Fetching

• Implemented using useEffect for API calls and managing loading states.

Question 30: Prop Drilling

• Explained concept of passing props through multiple levels and how to avoid it using Context API.

Question 31: Context API

• Used to share values between components without prop drilling.

Question 32: Use Context Hook

• Consuming context values in functional components.

Question 33: Updating Context Values

• How to update state in context using hooks.

Question 34: Multiple Contexts in a Component

• Using multiple contexts within a component through useContext hook.

Question 35: Advantages of Context API

• Simplifies state management and avoids prop drilling.

Question 36: Use Reducer Hook

• Alternative to useState for managing complex state logic.

Question 37: Passing Additional Arguments to Reducers

• Additional properties can be dispatched within the action object.

Question 38: Handling Side Effects with Use Reducer

• Side effects can be managed within useEffect while using useReducer.

Question 39: Using Refs in React

• Accessing and interacting with DOM elements using useRef.

Question 40: Forward Ref

• Passing refs through components to access child component instances.

Question 41: Handling Forms

• Creating controlled components to handle form data.

Question 42: Custom Hooks

• Reusable functions for stateful logic across components.

Question 43: Implementing Lazy Loading

• React.lazy and Suspense to load components lazily.

Question 44: Testing in React

• Using Jest and React Testing Library for testing components.

Question 45: Simulating User Events

• Using fireEvent or userEvent to simulate interactions.

Conclusion

• Covered 100 questions to help with React interviews, encouraging practical implementation and revision.