Mastering React.js: Top 20 Interview Questions You Need to Know
Tags: react, reactjsdevelopment, javascript, interview
Published: 2023-09-30 07:36:56

React.js has become the go-to library for building user interfaces, thanks to its flexibility, performance, and vibrant community. Whether you're a seasoned developer or just starting with React, it's crucial to be well-prepared for technical interviews. In this blog post, we'll explore the top 20 React.js interview questions that can help you land your dream job.

## 1. **What is React.js?**
React.js is an open-source JavaScript library for building user interfaces or UI components. It allows developers to create reusable UI components that update in real-time when data changes.

## 2. **What are the key features of React.js?**
React.js boasts features like virtual DOM, component-based architecture, one-way data binding, and JSX syntax, enabling efficient UI development.

## 3. **Explain Virtual DOM in React.js.**
Virtual DOM is a lightweight copy of the real DOM. React uses it to improve performance by reducing the number of direct manipulations to the actual DOM.

## 4. **What is JSX?**
JSX (JavaScript XML) is a syntax extension for JavaScript recommended by React. It allows mixing HTML with JavaScript, enhancing readability and ease of writing components.

## 5. **What are React Components?**
React components are independent, reusable pieces of UI. They can be class components or functional components.

## 6. **What is the difference between functional and class components?**
Functional components are simpler and used for simple UI representation, while class components have additional features like state and lifecycle methods.

## 7. **What is State in React?**
State is an object that holds data which affects the component's behavior and renders when it changes. It is mutable and should be changed using the `setState` method.

## 8. **What are Props in React?**
Props (short for properties) are a way to pass data from parent to child components. They are read-only and help in maintaining the unidirectional flow of data.

## 9. **Explain React Lifecycle Methods.**
React components have lifecycle methods that get called at different phases of a component's life. They include `componentDidMount`, `componentDidUpdate`, and `componentWillUnmount`.

## 10. **What is Redux?**
Redux is a predictable state container for JavaScript applications. It helps manage the state of an entire application in a single immutable state tree.

## 11. **What are React Hooks?**
Hooks are functions that let developers "hook into" React state and lifecycle features from functional components. They include `useState`, `useEffect`, and `useContext`.

## 12. **What is the significance of `useEffect` in React Hooks?**
`useEffect` is a hook that performs side effects in functional components. It can mimic componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods.

## 13. **What is Context in React.js?**
Context provides a way to pass data through the component tree without having to pass props down manually at every level. It's especially useful for passing theme data, user authentication, etc.

## 14. **Explain Error Boundaries in React.**
Error boundaries are React components that catch JavaScript errors anywhere in a component tree and log those errors, and display a fallback UI instead of crashing the component tree.

## 15. **What is React Router?**
React Router is a standard library for routing in React. It enables the navigation among views of various components in a React Application, allows changing the browser URL, and keeps UI in sync with the URL.

## 16. **What is the significance of `key` prop in React lists?**
The `key` prop is a special string attribute you need to include when creating lists of elements. It helps React identify which items have changed, are added, or are removed.

## 17. **What is React Fiber?**
React Fiber is a set of internal algorithms for rendering updates in React 16 and above. It enables React to interrupt rendering to work on other tasks and resume later, improving the user experience.

## 18. **What is the Virtual DOM reconciliation process?**
Reconciliation is the process through which React updates the real DOM to keep it in sync with the virtual DOM. It's a way of making changes to the UI without sacrificing performance.

## 19. **How do you optimize React performance?**
Performance optimization techniques in React include using production builds, minimizing state usage, utilizing shouldComponentUpdate, and lazy loading components.

## 20. **What are the differences between React and Angular?**
React is a JavaScript library focused on building user interfaces, while Angular is a complete web application framework. React uses a virtual DOM, while Angular uses a real DOM. React is maintained by Facebook, and Angular is maintained by Google.

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These questions cover a wide range of topics in React.js and can serve as a valuable resource for your preparation. Understanding these concepts thoroughly will not only help you in interviews but also make you a more proficient React developer.

Best of luck with your interviews! Happy coding!