Most Popular React Interview Questions and Answers

React is commonly used to build various applications. According to Stack Overflow's 2022 developer survey, it ranks 2nd among all web frameworks!

React's popularity continues to grow day by day. As a result, companies' need for React Developers has also started to increase.

If you are going to take a React Developer interview, we recommend studying the technical questions that may come. Technical interview questions are nothing to fear, especially if you have prepared for possible questions in advance…

Practicing makes you feel more confident during the interview. Therefore, we have listed the most popular React interview questions and answers asked in interviews for React Developers.

 

1. What is React?

React, developed by Facebook in 2011, is a JavaScript library for building web and mobile user interfaces.

React allows developers to create reusable UI components. It also has a large React community.

 

2. What is React used for?

React is used for creating, updating, and managing user interfaces in web applications. React uses a component-based approach, handling each part of the application individually and dynamically reacting to changes in these parts. This allows user interfaces to be built more quickly and efficiently.

 

3. What are React components?

The parts of a user interface created using React are called React components.

✔️ Components break down a user interface into reusable pieces.

✔️ Prevents code duplication.

✔️ Makes the code easier to understand.

✔️ Saves time and effort.

✔️ Facilitates testing of the interface.

 

4. What are functional components and class components in React?

In React, there are two types of components: class components and functional components.

• Functional components are the most basic type of React component. They work for rendering and use Props for data fetching or display. They are simple and fast.
• Class components are more complex React components that allow developers to manage component lifecycle methods and state. They come with advanced features and are used for more complex operations.

⚠️ Both types of components are used in React. The choice is made based on the needs of the component.

 

5. What is Event Handling in ReactJs?

ReactJs Event Handling is a feature used for managing events such as clicking a UI button, hovering over a UI element, or using keyboard commands with the UI. It allows managing events that occur when a user interacts with a UI element in the React JavaScript library.

 

6. What is JSX?

1. JSX is an HTML-like syntax that allows writing HTML-style code in JavaScript.
2. It is not necessary to use JSX to build websites or applications with React. However, using JSX reduces code complexity.
3. JSX increases the performance of React applications.
4. JSX allows object definitions that look like HTML.

Example of JSX code 👇:

 

7. Why can't browsers read JSX?

Current browsers can only read JavaScript objects. To enable the browser to read JSX, the JSX file is transformed into a JavaScript object using JSX transformers like Babel. The objects are then readable by the browser.

 

8. What is the Virtual DOM in React?

React Virtual DOM is a structure used by React instead of the real DOM (Document Object Model).

The Virtual DOM is a copy of the real DOM and allows React to model the UI elements it uses.

 

9. How does the React Virtual DOM work?

1️⃣ Web browsers create a structure called the Document Object Model (DOM) that shows how the site or application is organized.

2️⃣ React uses the Virtual DOM, which is a copy of the actual DOM of the site.

3️⃣ React determines which parts of the real DOM need to be changed by using the Virtual DOM. It takes the changes from the Virtual DOM and updates the real DOM accordingly.

 

10. What are the differences between the Real DOM and the Virtual DOM?

 

11. How are errors debugged in React?

Debugging tools are used to debug errors in React code. For example, DevTools for Google Chrome can be used.

Debugging tools help in viewing JavaScript errors and console outputs that occur in the browser.

 

12. What are the advantages of using React when building user interfaces?

• ✅ Improves application performance through the Virtual DOM model.
• ✅ Enhances efficiency with JSX.
• ✅ Components are reusable across multiple projects.
• ✅ Supported by an open-source community.
• ✅ Has a large number of plugin tools.

 

13. What are the disadvantages of React?

Some of the disadvantages of React are as follows:

1. React JS is a JavaScript library and may not be sufficient for complex projects.
2. It may take some time to understand React.
3. React can be a challenging library for users who are not experienced in JavaScript and UI development.
4. Some of React’s documentation may not be sufficient.
5. React is constantly being updated. This means that existing features are continuously changing, and developers have to keep up with the ever-updating new features.

 

14. What is Redux?

Redux is a library that complements React. It is compatible with React and other JavaScript frameworks and libraries.

Redux helps developers build testable web applications easily. It also includes debugging tools.

 

15. What is the concept of Flux in React?

Flux is a data management structure developed by Facebook. It is not a framework or library. In React applications, it is used to ensure the one-way flow of data and to organize data management.

 

16. What is React Router?

React Router is a JavaScript library that provides URL routing and page flows for React applications. It helps in converting React applications into dynamic and interactive web pages, thereby enhancing the usability and performance of the application.

 

17. What are the <> </> tags used for in React?

In React, <> </> empty tags are supported by the JSX (JavaScript XML) syntax. These empty tags can be used to facilitate the rendering of components.

 

18. What happens when there are multiple lines of expression?

A JSX expression is used.

 

19. What is the purpose of the render() function in React?

Every React component contains a render() function. If multiple HTML elements need to be created, they should be grouped within a single wrapping tag such as <form>, <group>, <div>.

 

20. What is StrictMode?

StrictMode is a tool added in React's version 16.3 to highlight potential problems in an application.

• ✅ Allows defining components with unsafe lifecycle methods.
• ✅ Provides warnings about deprecated API usage.
• ✅ Facilitates the debugging process.

 

21. What is the concept of Hooks in React?

React Hooks are structures that allow the use of React features without writing a class.

 

22. How can two or more components be combined?

 

23. What are Props and State?

Props and State are two different mechanisms in React components that provide data fetching and rendering. They control the behavior and data of the component.

• ✅ Props define the values inside a React component. These values can be read by the component but cannot be changed.
• ✅ State defines the data inside a React component.

 

24. How is the state of a component updated?

The state of a component can be updated using this.setState() 👇.

25. What is an Arrow function in React and how is it used?

Arrow function is the use of the arrow function structure in JavaScript within React components. They are also known as 'fat arrow' functions (=>).

These functions allow components to properly bind their context. Arrow functions are especially useful when working with higher-order functions.

 

26. What are stateful and stateless components in React?

👉 Stateful components, store information about changes in the component's state in memory. They keep track of past, present, and possible future changes in state and have the authority to change the state.

👉 Stateless components calculate the internal state of components. They do not have the authority to change the state and do not contain information about past, present, or possible future state changes.

 

27. What are the lifecycle methods of React components?

1. componentWillMount() – A lifecycle method called during the "mount" phase before the component is created.
2. componentDidMount() – A method that runs only once after the initial rendering on the client side.
3. componentWillReceiveProps() – A method called when receiving new props from the parent class before any processing.
4. shouldComponentUpdate() – Used to determine whether the component should update, aiding in performance improvements.
5. componentWillUpdate() – A method called just before the creation occurs in the DOM.
6. componentDidUpdate() – Called after the component has updated. It provides access to the new updated component states.
7. componentWillUnmount() – Used to clean up memory leaks.

 

28. What is an event in React?

In React, an event is a situation that occurs as a result of user interactions on the browser, such as hovering over the mouse, clicking the mouse, pressing a key, etc.

Handling these events is similar to handling events on DOM elements. However, there are some syntactical differences:

 

1️⃣ Events are named using camelCase instead of lowercase.

2️⃣ Events are passed as functions instead of strings.

 

29. How is code made modular in React?

Code can be made modular by using export and import features 👇.

 

30. How are forms created in React?

React forms are similar to HTML forms. In React, states are found in the component's state property. These states are updated through setState(). Therefore, elements cannot directly update their states, and submissions are handled by a JavaScript function.