Node.js Basics

Node.js Architecture and Event Loop

Node runs your JavaScript on a single thread and hands slow operations — file reads, network calls, DNS — to the operating system or a thread pool. The event loop picks up completed operations and runs their callbacks, which is how one thread serves thousands of connections.

What is Architecture and Event Loop in Node.js?

Why Architecture and Event Loop matters

The event loop explains Node’s greatest strength and its sharpest edge: it handles enormous I/O concurrency, but any long synchronous computation freezes the entire server for every user.

Architecture and Event Loop example

Execution order surprises beginners

JavaScript

console.log('1 — synchronous');

setTimeout(() => console.log('4 — timer'), 0);

Promise.resolve().then(() => console.log('3 — microtask'));

process.nextTick(() => console.log('2 — nextTick'));

// Output: 1, 2, 3, 4

How this works

Synchronous code runs first. Then process.nextTick callbacks, then promise microtasks, and only afterwards does the event loop move on to timers — which is why a zero-millisecond timeout still runs last.

Key points to remember

Microtasks (promises, nextTick) run between every phase, not only at the end.
The libuv thread pool defaults to four threads and handles fs and crypto work.
Blocking the loop blocks every connected client, not just one.

Event loop phases in order

Phase	Handles
timers	setTimeout and setInterval callbacks
pending callbacks	deferred system callbacks
poll	incoming I/O events — where Node waits
check	setImmediate callbacks
close	close events such as socket.on("close")

Common mistakes with Architecture and Event Loop

A large synchronous JSON.parse or loop, which stalls all requests.
Using fs.readFileSync inside a request handler.
Assuming setTimeout(fn, 0) runs immediately.