Top React js Interview Questions and Answers

Here are top React js interview questions,

 

1. What is React js?

React js, also called as React, is an open-source JavaScript library used for building user interfaces (UIs) or UI components. It was developed by Facebook and is now maintained by a community of developers. React allows developers to create reusable UI components that can efficiently update when the data changes.

 

2. Explain the key features of React.

- Virtual DOM: React uses a virtual representation of the DOM to improve performance by minimizing actual DOM manipulations.

- Component-Based: React applications are composed of small, reusable components.

- One-Way Data Binding: React enforces a unidirectional flow of data, which makes it easier to understand and debug.

 

3. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript often used with React. It allows you to write HTML-like code within your JavaScript, making it easier to describe the structure of your UI components.

 

4. What is the Virtual DOM in React and how does it work?

The Virtual DOM is a virtual representation of the actual DOM in memory. When data changes, React creates a new virtual DOM tree, compares it with the previous one, and calculates the minimal number of changes needed to update the actual DOM efficiently. This process is known as reconciliation.

 

5. Explain the difference between a functional component and a class component in React.

Functional components are simple JavaScript functions that receive props as arguments and return JSX. Class components are ES6 classes that extend React.Component and have additional features like state and lifecycle methods. With React Hooks, functional components can also manage state and side effects, blurring the distinction.

 

6. What is the significance of state in React?

State is a built-in object in React that allows components to store and manage data that can change over time. When state changes, React re-renders the component, ensuring that the UI is always up-to-date.

 

7. Explain the concept of props in React.

Props (short for properties) are inputs to React components. They are used to pass data from a parent component to a child component. Props are immutable, meaning they cannot be modified by the child component that receives them.

 

8. What is a controlled component in React?

A controlled component is a React component that relies on React state to control its inputs (like forms and input elements). Changes in the input elements are handled through state, making React the single source of truth for the input's value.

 

9. What are React Hooks?

React Hooks are functions that allow functional components to manage state and side effects without using class components. They include `useState`, `useEffect`, `useContext`, and others, making it easier to reuse stateful logic in functional components.

 

10. What is the purpose of `useState` in React?

`useState` is a React Hook that allows functional components to manage state. It takes an initial state value and returns an array with the current state and a function to update that state. For example:

 

jsx

const [count, setCount] = useState(0);

 

11. Explain the concept of React context.

React context is a mechanism for sharing state data between components in a component tree without explicitly passing props through every level of the tree. It's often used for global state management.

 

12. What is React Router, and why is it used?

React Router is a library for adding routing to React applications. It allows you to define routes and their corresponding components, enabling navigation within a single-page application (SPA) without the need for a full page reload.

 

13. What is Redux, and why might you use it with React?

Redux is a state management library for JavaScript applications, often used with React. It provides a predictable state container and enables you to manage the state of your application in a central store. Redux can help manage complex application states and simplify data flow.

 

14. Explain the component lifecycle methods in a class component.

Class components have lifecycle methods like `componentDidMount`, `componentDidUpdate`, and `componentWillUnmount`. These methods allow you to perform actions at different stages of a component's lifecycle, such as fetching data after the component mounts or cleaning up resources when it unmounts.

 

15. What is the key difference between `useEffect` and component lifecycle methods in functional components?

`useEffect` is a React Hook used in functional components to manage side effects and mimic the behavior of component lifecycle methods like `componentDidMount` and `componentDidUpdate`. The key difference is that `useEffect` can manage multiple side effects in a single component and is more declarative.

 

16. What is React's strict mode, and why is it useful?

React's strict mode is a developer tool that highlights potential problems in your application during development. It helps identify and address issues like unsafe lifecycle methods and legacy string ref usage.

 

17. How can you optimize the performance of a React application?

Performance optimization in React can be achieved through various means:

- Memoization: Use `React.memo` to prevent unnecessary re-renders of functional components.

- Code Splitting: Split your code into smaller chunks to reduce initial load times.

- Virtualization: Use libraries like `react-virtualized` to render only the visible portion of long lists.

- PureComponent: Extend `React.PureComponent` or use `shouldComponentUpdate` in class components to prevent unnecessary updates.

 

18. What is the purpose of keys in React lists?

Keys are used to give React a hint about the identity of elements in a list. They help React efficiently update the DOM when items are added, removed, or reordered in a list. Keys should be unique among siblings but can be the same across different lists.

 

19. Explain the concept of Higher-Order Components (HOCs) in React.

Higher-Order Components are functions that take a component as input and return a new enhanced component with additional props or behavior. They are used to share code, logic, and behavior among different components.

 

20. What are React Fragments?

React Fragments are a way to group multiple elements without adding extra nodes to the DOM. They are especially useful when you need to return multiple elements from a component's render method without wrapping them in a parent div.

 

Above are few top React js interview questions. Remember to prepare and expand on these answers.

Good luck with your interview! 👍