Top 10 Senior JavaScript Developer Interview Questions & Answers in 2024

1. How does the Virtual DOM work in React, and what advantages does it offer in terms of performance optimization?

React's Virtual DOM is a lightweight copy of the actual DOM. When state or props change, React first updates the Virtual DOM, then compares it with the previous version to identify the minimal changes needed, and finally updates the real DOM efficiently. This process minimizes browser reflows and repaints, improving overall performance.

2. Explain the concept of Higher-Order Components (HOCs) in React. Provide an example of using an HOC for code reusability and explain its benefits.

Higher-Order Components (HOCs) are functions that take a component and return a new component with enhanced functionality. They promote code reusability and logic sharing among components. Example:

const withLogging = (WrappedComponent) => {
  return class extends React.Component {
    componentDidMount() {
      console.log(`Component ${WrappedComponent.name} is mounted`);
    }

    render() {
      return <WrappedComponent {...this.props} />;
    }
  };
};

const EnhancedComponent = withLogging(MyComponent);

This HOC adds logging functionality to any component it wraps.

3. Discuss the differences between 'null' and 'undefined' in JavaScript. When and why might you use one over the other?

• null: A deliberate assignment representing no value or no object. It is an intentional absence of any object value.
• undefined: A variable that has been declared but not assigned a value. It also represents the absence of a value.

Choosing between them depends on the context. 'null' is typically used to represent the absence of an object, while 'undefined' indicates the absence of a value assignment.

4. How can you optimize the performance of a web application built with Angular? Discuss strategies like Ahead-of-Time (AOT) compilation, lazy loading, and tree shaking.

• Ahead-of-Time (AOT) compilation: Compiles Angular templates during the build process, improving runtime performance.
• Lazy loading: Defers loading of non-essential parts of the application until they are needed, reducing initial load time.
• Tree shaking: Eliminates dead code and unused dependencies during the build process, resulting in a smaller bundle size.

5. Discuss the importance of the 'useEffect' hook in React. How does it differ from 'componentDidMount' and 'componentDidUpdate' lifecycle methods?

The 'useEffect' hook in React allows developers to perform side effects in functional components. It combines the functionality of 'componentDidMount,' 'componentDidUpdate,' and 'componentWillUnmount' lifecycle methods. 'useEffect' runs after every render and enables the management of asynchronous operations, subscriptions, and other side effects in a declarative way.

6. What is Redux, and how does it manage the state in a React application? Discuss the core principles of Redux, including actions, reducers, and the store.

Redux is a state management library for JavaScript applications, commonly used with React. Its core principles include:

• Actions: Plain JavaScript objects describing state changes.
• Reducers: Functions that specify how the state changes in response to actions.
• Store: An object that holds the application state and allows access to it.

Redux enables a predictable state container that simplifies state management in complex applications.

7. Explain the differences between GraphQL and REST APIs. In what scenarios might GraphQL be a better choice, and what tools can developers use to work with GraphQL?

• GraphQL: A query language for APIs that allows clients to request only the data they need. Offers a single endpoint and a more flexible data retrieval model.
• REST: Uses multiple endpoints for different resources and can lead to over-fetching or under-fetching of data.

GraphQL might be a better choice in scenarios where data requirements are dynamic, or there is a need for real-time updates. Developers can use tools like Apollo Client or Relay to work with GraphQL in JavaScript applications.

8. Discuss the advantages and disadvantages of using Webpack in a JavaScript project. How does it contribute to bundling, code splitting, and optimizing assets?

• Advantages: Efficient bundling, code splitting for lazy loading, and asset optimization.
• Disadvantages: Initial setup complexity and potential configuration challenges.

Webpack is a powerful tool that simplifies the management of project assets, improves loading performance, and facilitates code splitting to enhance the user experience.

9. Explain the concept of WebSockets and their role in real-time applications. Provide an example of using WebSockets in JavaScript to achieve bidirectional communication.

WebSockets enable bidirectional communication between a client and a server over a single, long-lived connection. Example:

const socket = new WebSocket('wss://example.com/socket');

socket.addEventListener('open', (event) => {
  socket.send('Hello, server!');
});

socket.addEventListener('message', (event) => {
  console.log('Received:', event.data);
});

// Handle other events: close, error

This example establishes a WebSocket connection and sends/receives messages between the client and server.

10. Discuss the principles of Progressive Web Apps (PWAs) and their benefits. How can developers implement PWA features in a JavaScript application?

Progressive Web Apps (PWAs) combine the best of web and mobile applications, providing a reliable, fast, and engaging user experience. Key principles include responsiveness, connectivity independence, and app-like interactions. Developers can implement PWA features using service workers for offline support, a manifest file for app-like behavior, and optimizing performance for various devices.