Node.js Interview Questions

Beginner Node.js Interview Questions

1. What is a first class function in Javascript?
2. What is Node.js and how it works?
3. How do you manage packages in your node.js project?
4. How is Node.js better than other frameworks most popularly used?
5. Explain the steps how “Control Flow” controls the functions calls?
6. What are some commonly used timing features of Node.js?
7. What are the advantages of using promises instead of callbacks?
8. What is fork in node JS?
9. Why is Node.js single-threaded?
10. How do you create a simple server in Node.js that returns Hello World?
11. How many types of API functions are there in Node.js?
12. What is REPL?
13. List down the two arguments that async.queue takes as input?
14. What is the purpose of module.exports?
15. What tools can be used to assure consistent code style?

Intermediate Node.js Interview Questions

16. What do you understand by callback hell?
17. What is an event-loop in Node JS?
18. If Node.js is single threaded then how does it handle concurrency?
19. Differentiate between process.nextTick() and setImmediate()?
20. How does Node.js overcome the problem of blocking of I/O operations?
21. How can we use async await in node.js?
22. What is node.js streams?
23. What are node.js buffers?
24. What is middleware?
25. Explain what a Reactor Pattern in Node.js?
26. Why should you separate Express app and server?
27. For Node.js, why Google uses V8 engine?
28. Describe the exit codes of Node.js?
29. Explain the concept of stub in Node.js?

Advanced Node.js Interview Questions

30. What is an Event Emitter in Node.js?
31. Enhancing Node.js performance through clustering.
32. What is a thread pool and which library handles it in Node.js
33. What is WASI and why is it being introduced?
34. How are worker threads different from clusters?
35. How to measure the duration of async operations?
36. How to measure the performance of async operations?

Answers:

1. What is a first class function in Javascript?
   
   • In JavaScript, functions are treated as first-class citizens. This means that functions can be assigned to variables, passed as arguments to other functions, and returned from other functions. This allows for higher-order functions and functional programming techniques.
2. What is Node.js and how it works?
   
   • Node.js is a runtime environment that allows you to run JavaScript code outside of a web browser. It is built on the V8 JavaScript engine and uses an event-driven, non-blocking I/O model to handle multiple connections efficiently.
3. How do you manage packages in your node.js project?
   
   • Packages in a Node.js project are managed using npm (Node Package Manager) or yarn. These tools allow you to install, update, and manage dependencies for your project through a package.json file.
4. How is Node.js better than other frameworks most popularly used?
   
   • Node.js is better than other frameworks in several ways:
     • Non-blocking I/O: Node.js uses an event-driven, non-blocking I/O model, which makes it efficient for handling multiple connections simultaneously.
     • JavaScript Everywhere: With Node.js, developers can use JavaScript for both frontend and backend development, leading to a more consistent development experience.
     • Large Ecosystem: Node.js has a vast ecosystem of libraries and modules available through npm, making it easy to find solutions for various tasks.
     • Scalability: Node.js is designed to be scalable, making it suitable for building large-scale applications.
5. Explain the steps how “Control Flow” controls the functions calls?
   
   • Control flow in Node.js is managed through the event loop, which handles asynchronous operations. When a function is called, it is added to the call stack. If the function contains asynchronous operations, those operations are offloaded to the event loop, allowing the main thread to continue executing other code. Once the asynchronous operation is complete, its callback is added to the callback queue, and when the call stack is empty, the event loop processes the callbacks in the queue.
6. What are some commonly used timing features of Node.js?
   
   • Some commonly used timing features in Node.js include:
     • setTimeout(): Executes a function after a specified delay.
     • setInterval(): Repeatedly executes a function at specified intervals.
     • setImmediate(): Executes a function immediately after the current event loop phase.
     • process.nextTick(): Schedules a callback to be invoked in the next iteration of the event loop.
7. What are the advantages of using promises instead of callbacks?
   
   • Promises offer several advantages over callbacks:
     • Improved readability: Promises provide a cleaner and more readable syntax, especially when chaining multiple asynchronous operations.
     • Error handling: Promises allow for centralized error handling using .catch(), making it easier to manage errors in asynchronous code.
     • Avoiding callback hell: Promises help to avoid deeply nested callbacks, leading to more maintainable code.
     • Composability: Promises can be easily composed using methods like Promise.all() and Promise.race().
8. What is fork in node JS?
   
   • In Node.js, fork is a method provided by the child_process module that allows you to create a new Node.js process. It is used to create child processes that can run concurrently with the parent process, enabling parallel execution of code. The forked process can communicate with the parent process using inter-process communication (IPC).
9. Why is Node.js single-threaded?
   
   • Node.js is single-threaded to simplify the programming model and avoid the complexities associated with multi-threading, such as race conditions and deadlocks. By using a single thread for the event loop, Node.js can efficiently handle multiple concurrent connections through non-blocking I/O operations, allowing it to scale well for I/O-bound applications.
10. How do you create a simple server in Node.js that returns Hello World?
    

• You can create a simple server in Node.js using the built-in http module. Here is an example:

const http = require('http');

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World\n');
});

const PORT = 3000;
server.listen(PORT, () => {
  console.log(`Server running at http://localhost:${PORT}/`);
});

11. How many types of API functions are there in Node.js?
    

• There are two types of API functions in Node.js:
  • Asynchronous API functions: These functions perform non-blocking operations and use callbacks or promises to handle the results.
  • Synchronous API functions: These functions perform blocking operations and return results directly, causing the event loop to wait until the operation is complete.

12. What is REPL?
    

• REPL stands for Read-Eval-Print Loop. It is an interactive shell that allows you to execute JavaScript code in real-time. In Node.js, you can access the REPL by simply running the node command in your terminal. It reads your input, evaluates it, prints the result, and loops back to read more input.

13. List down the two arguments that async.queue takes as input?
    

• The async.queue function takes two arguments:
  • worker: A function that processes each task in the queue.
  • concurrency: A number that specifies the maximum number of tasks to be processed concurrently.

14. What is the purpose of module.exports?
    

• The purpose of module.exports is to define what a module exports and makes available for other files to import. By assigning values or functions to module.exports, you can create reusable modules that can be imported using the require() function in other parts of your application.

15. What tools can be used to assure consistent code style?
    

• Some popular tools to assure consistent code style in Node.js projects include:
  • ESLint: A widely used linter that helps identify and fix code style issues.
  • Prettier: An opinionated code formatter that automatically formats your code according to a set of rules.
  • StandardJS: A JavaScript style guide, linter, and formatter all in one.
  • Husky: A tool that allows you to run scripts (like linters) before committing code to ensure code quality.

Intermediate Node.js Interview Questions

16. What do you understand by callback hell?
    

• Callback hell refers to the situation where multiple nested callbacks make the code difficult to read and maintain. It often occurs in asynchronous programming when each callback depends on the result of the previous one, leading to deeply nested structures that are hard to follow.

17. What is an event-loop in Node JS?
    

• The event loop is a core component of Node.js that handles asynchronous operations. It continuously checks the call stack and the callback queue, executing callbacks when the call stack is empty. This allows Node.js to perform non-blocking I/O operations and handle multiple connections efficiently.

18. If Node.js is single threaded then how does it handle concurrency?
    

• Node.js handles concurrency through its event-driven architecture and non-blocking I/O model. While it operates on a single thread, it offloads I/O operations to the system kernel whenever possible. The event loop manages these operations and invokes callbacks when they are complete, allowing Node.js to handle multiple connections concurrently without blocking the main thread.

19. Differentiate between process.nextTick() and setImmediate()?
    

• process.nextTick() schedules a callback to be invoked in the next iteration of the event loop, before any I/O events are processed. It has higher priority and is executed before setImmediate().
• setImmediate() schedules a callback to be executed after the current poll phase of the event loop, allowing I/O events to be processed first. It is executed after process.nextTick() callbacks.

20. How does Node.js overcome the problem of blocking of I/O operations?
    

• Node.js overcomes the problem of blocking I/O operations by using an event-driven, non-blocking I/O model. When an I/O operation is initiated, Node.js offloads it to the system kernel, allowing the main thread to continue executing other code. Once the I/O operation is complete, a callback is added to the callback queue, and the event loop processes it when the call stack is empty. This approach allows Node.js to handle multiple I/O operations concurrently without blocking the main thread.

21. How can we use async await in node.js?
    

• You can use async/await in Node.js by defining an asynchronous function using the async keyword and using the await keyword to pause the execution of the function until a promise is resolved. Here is an example:

async function fetchData() {
  try {
    const data = await someAsyncOperation();
    console.log(data);
  } catch (error) {
    console.error(error);
  }
}
fetchData();

22. What is node.js streams?
    

• Node.js streams are objects that allow you to read data from a source or write data to a destination in a continuous manner. Streams are used for handling large amounts of data efficiently by processing it in chunks rather than loading it all into memory at once. There are four types of streams in Node.js: Readable, Writable, Duplex, and Transform streams.

23. What are node.js buffers?
    

• Node.js buffers are temporary storage areas for binary data. They are used to handle raw binary data in situations where you need to work with streams or perform low-level operations. Buffers are instances of the Buffer class and provide methods for reading and writing binary data.

24. What is middleware?
    

• Middleware is a function that sits between the request and response cycle in a web application. It has access to the request and response objects and can modify them or perform additional operations before passing control to the next middleware function or route handler. Middleware is commonly used for tasks such as authentication, logging, and error handling.

25. Explain what a Reactor Pattern in Node.js?
    

• The Reactor Pattern is a design pattern used in Node.js to handle multiple I/O operations concurrently. It uses an event loop to monitor multiple file descriptors (sockets, files, etc.) and dispatches events to the appropriate handlers when I/O operations are ready to be processed. This pattern allows Node.js to efficiently manage I/O-bound tasks without blocking the main thread.

26. Why should you separate Express app and server?
    

• Separating the Express app and server allows for better modularity and testability. By separating the application logic from the server setup, you can easily test the app independently of the server. It also allows for greater flexibility in configuring the server (e.g., using different ports or protocols) without affecting the application logic.

27. For Node.js, why Google uses V8 engine?
    

• Google uses the V8 engine for Node.js because it is a high-performance JavaScript engine that compiles JavaScript code into machine code, allowing for fast execution. V8 is optimized for speed and efficiency, making it well-suited for server-side applications that require high performance and scalability.

28. Describe the exit codes of Node.js?
    

• Node.js exit codes indicate the status of a process when it terminates. Common exit codes include:
  • 0: Successful completion
  • 1: Uncaught fatal exception
  • 2: Unused (reserved for future use)
  • 3: Internal JavaScript parse error
  • 4: Internal JavaScript evaluation failure
  • 5: Fatal error
  • 6: Non-function internal exception handler
  • 7: Internal exception handler run-time failure

29. Explain the concept of stub in Node.js?
    

• A stub in Node.js is a piece of code that simulates the behavior of a real function or module. Stubs are commonly used in testing to isolate the code being tested from its dependencies. By replacing a real function with a stub, you can control its behavior and return values, allowing you to test specific scenarios without relying on external factors.

Advanced Node.js Interview Questions

30. What is an Event Emitter in Node.js?
    

• An Event Emitter is a core module in Node.js that allows objects to emit and listen for events. It provides a way to implement the observer pattern, where an object (the emitter) can notify other objects (listeners) when certain events occur. The EventEmitter class provides methods such as on(), emit(), and removeListener() to manage event listeners and trigger events.

31. Enhancing Node.js performance through clustering.
    

• Clustering in Node.js allows you to create multiple instances of a Node.js application that can run on different CPU cores. By using the cluster module, you can take advantage of multi-core systems to improve the performance and scalability of your application. Each worker process can handle its own set of requests, distributing the load and increasing the overall throughput of the application.

32. What is a thread pool and which library handles it in Node.js
    

• A thread pool is a collection of worker threads that can be used to perform tasks concurrently. In Node.js, the libuv library manages the thread pool, which is used for handling I/O operations that cannot be performed asynchronously, such as file system operations and DNS lookups. The default size of the thread pool is 4, but it can be configured using the UV_THREADPOOL_SIZE environment variable.

33. What is WASI and why is it being introduced?
    

• WASI (WebAssembly System Interface) is a standardized interface that allows WebAssembly modules to interact with the underlying operating system. It is being introduced to enable WebAssembly to run outside of web browsers, allowing developers to build portable and secure applications that can run in various environments, including server-side applications and edge computing.

34. How are worker threads different from clusters?
    

• Worker threads and clusters are both used to achieve concurrency in Node.js, but they operate differently. Worker threads allow you to run JavaScript code in separate threads within the same process, sharing memory through transferable objects. Clusters, on the other hand, create separate processes that do not share memory, with each process having its own event loop. Worker threads are suitable for CPU-intensive tasks, while clusters are better for scaling I/O-bound applications across multiple CPU cores.

35. How to measure the duration of async operations?
    

• You can measure the duration of async operations in Node.js using the console.time() and console.timeEnd() methods. Here is an example:

console.time('asyncOperation');
await someAsyncFunction();
console.timeEnd('asyncOperation');

36. How to measure the performance of async operations?
    

• You can measure the performance of async operations in Node.js using the performance module. Here is an example:

const { performance } = require('perf_hooks');
const start = performance.now();
await someAsyncFunction();
const end = performance.now();
console.log(`Async operation took ${end - start} milliseconds`);