Event Loop & Async

What is the Event Loop?

JavaScript in Node.js runs on a single thread. Yet it can handle thousands of simultaneous connections without blocking. The secret is the event loop — a mechanism that delegates slow work (file reads, network calls) to the operating system, then processes the results when they arrive.

At its core, the runtime has three key structures:

Call stack — where synchronous code executes, one frame at a time.
Microtask queue — holds Promise callbacks and queueMicrotask callbacks. Drains completely after every task.
Macrotask queue (task queue) — holds setTimeout/setInterval callbacks. One macrotask is processed per event loop iteration.

The loop runs continuously: pick a macrotask → execute it → drain all microtasks → pick the next macrotask.

What this module covers

Lesson	Topic
Call Stack	Synchronous execution and why blocking matters
Callbacks	The original async pattern and Node’s error-first convention
Promises	States, chaining, and combinators
Async/Await	Reading async code like synchronous code
Microtasks & Macrotasks	Queue priorities and drain order
Timers	`setTimeout`, `setInterval`, and timer precision

A first async demo

The snippet below runs three pieces of code. Notice they do not execute top-to-bottom in the order written — the synchronous log fires first, the Promise callback fires next (microtask), and the setTimeout callback fires last (macrotask).

// What runs first?
console.log('A — sync');
Promise.resolve().then(() => console.log('B — microtask (Promise)'));
setTimeout(() => console.log('C — macrotask (setTimeout)'), 0);

Run it to confirm the order:

JavaScript

console.log('A — sync');
Promise.resolve().then(function() { console.log('B — microtask (Promise)'); });
setTimeout(function() { console.log('C — macrotask (setTimeout)'); }, 0);

The output is always A, then B, then C — the event loop guarantees this ordering.

In what order does the event loop process queues after executing a task? Macrotasks first, then microtasks Microtasks first, then the next macrotask Both queues are processed simultaneously Microtasks and macrotasks alternate one by one

Which of the following is placed in the microtask queue? A setTimeout callback A setInterval callback A Promise .then callback A DOM click event handler