Top 10 Junior React Developer Interview Questions and Answers

1. What is React, and what problem does it solve?

React is a JavaScript library for building user interfaces. It solves the problem of efficiently updating and rendering user interfaces in response to changes in application state. React uses a virtual DOM and a component-based architecture to make UI development more manageable and performant.

2. Explain the concept of components in React.

Components are the building blocks of a React application. They are reusable, self-contained pieces of the user interface. Components can be functional (stateless) or class-based (stateful) and can receive and manage their own props and state. They encourage code reusability and maintainability.

3. What is JSX, and why is it used in React?

JSX (JavaScript XML) is a syntax extension for JavaScript used in React to describe the structure of UI components. It allows developers to write HTML-like code within JavaScript, making it more readable and maintainable. JSX is transformed into JavaScript code that React can understand.

4. What is the virtual DOM in React?

The virtual DOM is a lightweight, in-memory representation of the actual DOM (Document Object Model). React uses the virtual DOM to improve performance by minimizing direct manipulation of the real DOM. When changes occur, React calculates the most efficient way to update the DOM, reducing the number of DOM operations and improving rendering speed.

5. How does state differ from props in React?

State and props are both used to manage data in React components, but they have distinct roles. State is used for data that can change and is specific to a component. It's managed internally within the component. Props, on the other hand, are used to pass data from parent to child components and are immutable within the child component. They are a way to configure a component with external data.

6. What are lifecycle methods in React, and why are they important?

Lifecycle methods are special methods in React components that allow you to hook into component creation, updating, and destruction. They provide a way to execute code at specific points in a component's lifecycle. For example, componentDidMount is often used to fetch data from a server after a component has been added to the DOM.

7. What is the significance of key in React lists?

The key prop is used to uniquely identify elements in a list of components. It helps React efficiently update and re-render lists by associating each element with its specific representation in the previous render. Using a key ensures that React can identify added, removed, or reordered elements in a list.

8. What is the purpose of Redux in React, and when should you use it?

Redux is a state management library for React applications. It provides a centralized store to manage application state, making it easier to share data between components. Redux is particularly useful in large and complex applications where managing state through props and component hierarchies becomes cumbersome.

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

Performance optimization in React involves various strategies, including using the virtual DOM, avoiding unnecessary re-renders, implementing shouldComponentUpdate, lazy loading, code splitting, and optimizing network requests. Additionally, tools like React DevTools can help identify and address performance bottlenecks.

10. What is a higher-order component (HOC) in React?

A higher-order component is a design pattern in React that allows you to reuse component logic by wrapping components with other components. HOCs are used for cross-cutting concerns such as authentication, data fetching, or routing. They enhance the functionality of a component without altering its structure.