Single Thread vs Multi Thread

Node runs your JavaScript on one main thread (the event loop). All synchronous code, all callbacks, all event handlers — they all share that one thread.

While JS runs on one thread, I/O happens in C++ on libuv's thread pool or via the OS kernel asynchronously. So while JS is "idle" waiting for the disk or network, Node can accept thousands of other requests.

js Run in compiler
// BAD — blocks the entire process for ~1 second
function fib(n) { return n < 2 ? n : fib(n-1) + fib(n-2); }
console.log(fib(42));   // every other request waits!

• Worker Threads (built-in, since Node 10) — run JS in parallel threads, message-passing via postMessage.
• Cluster module — spawn N child processes, share a port via the master.
• child_process.spawn — run external programs (FFmpeg, Python, shell).

js
// worker.js — heavy fib in a thread
const { parentPort } = require('worker_threads');
parentPort.on('message', n => parentPort.postMessage(fib(n)));