What is Node.js and How Does it Work?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   V8 compiles JavaScript

   Your JS is parsed and JIT-compiled to machine code by the V8 engine for fast execution.

2. Step 2

   Code runs on one main thread

   Synchronous code executes top to bottom on a single thread — no thread-per-request model.

3. Step 3

   I/O is offloaded

   Slow operations (file, network, DB) are handed to libuv, which uses OS async facilities and a thread pool.

4. Step 4

   The event loop waits

   When an operation finishes, its callback is queued and the event loop picks it up when the stack is clear.

5. Step 5

   Callbacks resume work

   The queued callback runs, delivering the result, then the loop moves to the next ready task.

Code Example

const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello from Node.js\n');
});

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

const fs = require('fs');

// Asynchronous: does not block the event loop
fs.readFile('data.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

console.log('This logs first, before the file is read');

Follow-up Questions

• How does the Node.js event loop work internally?
• What is the difference between blocking and non-blocking code?
• Why is Node.js not ideal for CPU-intensive tasks?
• What role does libuv play in Node.js?
• How do you scale Node.js across multiple CPU cores?