1. Definition

Node.js is a runtime environment that allows you to run JavaScript outside the browser. It is mainly used to build backend applications like servers and APIs.

2. When to Use Node.js

• Building backend/server-side applications
• Creating REST APIs
• Real-time apps (chat, live updates)
• Full-stack development (MERN)

3. Where it is Used

• Web servers
• APIs
• Real-time systems
• Microservices
• CLI tools

4. Why Use Node.js

• JavaScript for both frontend & backend
• Fast (non-blocking)
• Handles multiple requests
• Huge npm ecosystem

Basic Syntax

const http = require("http");

const server = http.createServer((req, res) => {
  res.write("Hello from Node.js");
  res.end();
});

server.listen(3000);

Real Example

const http = require("http");

const server = http.createServer((req, res) => {
  if (req.url === "/api/user") {
    res.write(JSON.stringify({ name: "Manaswini", role: "Developer" }));
  } else {
    res.write("Home Page");
  }
  res.end();
});

server.listen(3000);

Interview Points

• Node.js is a runtime
• Single-threaded event loop
• Non-blocking I/O
• Best for I/O-heavy apps