20 Essential ReactJS Interview Questions and Answers for Experienced Developers

What is ReactJS? Answer: ReactJS is an open-source JavaScript library used for building user interfaces or UI components. It was developed by Facebook and is now maintained by a community of developers.
What are the advantages of using ReactJS? Answer: Some advantages of using ReactJS are:

Improved performance and speed due to the use of a virtual DOM
Modular and reusable code through the use of components
Better debugging and error handling due to its one-way data flow
Better scalability and maintainability of code

How do you create a React component? Answer: A React component can be created in two ways:

Class Component: A class component extends React. Component class and has a render method that returns the component's JSX code.
Function Component: A function component is a simpler way to create a component that takes in props and returns JSX code.

What is JSX? Answer: JSX is a syntax extension for JavaScript that allows us to write HTML-like code in our JavaScript files. It is used to describe the UI elements in React components.
What is the difference between state and props in ReactJS? Answer: State is used to store and manage internal data within a component, while props are used to pass data from a parent component to a child component.
How do you handle events in ReactJS? Answer: Events are handled using event handlers in ReactJS. We can define event handlers as methods in the component class and bind them to the component's instance using the this keyword.
What is the difference between controlled and uncontrolled components in ReactJS? Answer: Controlled components are those that have their state controlled by React, while uncontrolled components have their state managed by the DOM. In controlled components, the state is passed down as props, and the component only updates when the state changes.
What is Redux? Answer: Redux is a state management library for JavaScript applications. It provides a predictable and centralized way of managing application state, making it easier to develop, test, and maintain complex applications.
What are Higher Order Components (HOC) in ReactJS? Answer: Higher Order Components (HOC) are components that wrap other components and add additional functionality to them. They are used for code reuse and can add features like authentication or data fetching to a component.
How do you optimize performance in ReactJS? Answer: Performance can be optimized in ReactJS by:

Using a virtual DOM
Avoiding unnecessary re-renders using shouldComponentUpdate() or React.memo()
Using server-side rendering
Lazy-loading components and data
Minimizing the use of state and props in components.

What is React Fiber and how does it improve performance? Answer: React Fiber is a rewrite of the React reconciliation algorithm that allows for more efficient and flexible updates to the virtual DOM. It improves performance by prioritizing updates based on their importance and allowing for incremental rendering.
What is the purpose of the shouldComponentUpdate() method? Answer: The shouldComponentUpdate() method is used to optimize performance by allowing components to avoid unnecessary re-renders. It returns a Boolean value that indicates whether the component should be updated or not based on changes to its props or state.
How do you handle asynchronous data in ReactJS? Answer: Asynchronous data can be handled using techniques like promises, async/await, or callbacks. The data can be fetched using APIs like fetch or Axios, and then stored in the component's state or passed down as props to child components.
What is the purpose of Redux middleware, and how is it used? Answer: Redux middleware is used to intercept and modify Redux actions before they reach the reducers. It is used to implement features like logging, error handling, or asynchronous actions. Middleware functions are applied to the Redux store using the applyMiddleware() method.
How does React Router work, and what are its key components? Answer: React Router is a library used for client-side routing in React applications. It maps URLs to components and allows for navigation without reloading the page. Its key components include <Router>, <Route>, and <Link>, which are used to define routes and links in the application.
How do you implement server-side rendering in ReactJS, and what are its benefits? Answer: Server-side rendering (SSR) can be implemented in ReactJS using libraries like Next.js or React-Router-Config. SSR allows for faster initial rendering and improved SEO, as the HTML is generated on the server and sent to the client as a fully-formed page.
What are React hooks, and how do they simplify component logic? Answer: React hooks are functions that allow components to use state and other React features without using class components. They simplify component logic by reducing the amount of boilerplate code needed to manage state and handle side effects.
What is the difference between React and React Native, and when would you choose one over the other? Answer: React is a library for building web applications, while React Native is a framework for building mobile applications for iOS and Android. React Native uses a different set of components and APIs that are optimized for mobile development. React would be chosen for web development, while React Native would be chosen for mobile development.
How do you test React components, and what are some popular testing frameworks? Answer: React components can be tested using libraries like Jest and Enzyme, which provide tools for testing component behavior, rendering, and state changes. Testing frameworks like Mocha and Chai can also be used for testing React components.
What are some common performance issues in React applications, and how can they be addressed? Answer: Common performance issues in React applications include unnecessary re-renders, inefficient data fetching, and excessive use of state and props. They can be addressed using techniques like shouldComponentUpdate(), lazy-loading, and server-side rendering. Profiling and performance monitoring tools like React DevTools and Chrome DevTools can also be used to identify and address performance issues.