Top Node.js Interview Questions (2026)

Preparing for a Node.js interview? You should also understand system design concepts and backend scalability to crack top product-based companies.

Node.js is one of the most popular backend technologies used in 2026 for building scalable, high-performance applications. Companies like Netflix, PayPal, and Uber use Node.js for handling millions of requests efficiently.

This guide covers the most important Node.js interview questions with explanations and examples to help you crack backend interviews easily.

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1. What is the Event Loop in Node.js?

The event loop is the core mechanism that allows Node.js to handle multiple operations asynchronously using a single thread without blocking execution.

It continuously checks the call stack and callback queue to execute pending tasks.

console.log("Start");

setTimeout(() => {
  console.log("Async Task");
}, 0);

console.log("End");

Output:
Start → End → Async Task

2. What is Middleware in Node.js?

Middleware functions are executed during the request-response cycle and can modify request/response objects or terminate the request.

app.use((req, res, next) => {
  console.log("Request received");
  next();
});

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3. What is Asynchronous Programming?

It allows execution of tasks without blocking the main thread using callbacks, promises, and async/await.

4. What are Promises in Node.js?

Promises represent the eventual completion or failure of an asynchronous operation and help avoid callback hell.

fetchData()
  .then(data => console.log(data))
  .catch(err => console.error(err));

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5. What is Express.js?

Express.js is a minimal Node.js framework used to build scalable APIs and backend applications.

const express = require("express");
const app = express();

app.get("/", (req, res) => {
  res.send("Hello World");
});

app.listen(3000);

6. What is Callback Hell?

Callback hell occurs when multiple nested callbacks make code difficult to read and maintain.

7. What is async/await?

async/await is a modern way to handle asynchronous operations in a synchronous style.

async function getData() {
  const data = await fetchData();
  console.log(data);
}

8. What is the difference between require and import?

require is used in CommonJS modules, while import is used in ES modules.

9. What is a Stream in Node.js?

Streams are used to handle reading/writing data in chunks instead of loading everything into memory.

10. What is Buffer in Node.js?

Buffer is used to handle binary data directly in memory.

11. What is Cluster Module?

Cluster module allows you to create multiple processes to handle load efficiently.

12. What is Rate Limiting?

Rate limiting is used to control the number of requests from a client to prevent abuse.

13. What is JWT Authentication?

JSON Web Tokens are used for secure authentication between client and server.

14. What is CORS?

CORS allows servers to specify who can access resources from different origins.

15. What is REST API?

REST APIs follow a stateless architecture using HTTP methods like GET, POST, PUT, DELETE.

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