Event loop ordering

Async Era

ES2017+2017 – 2019

`async`/`await` made the microtask queue an everyday concern. Asynchronous code finally reads top-to-bottom while still yielding to the loop at each await.

Microtasks drain completely before the next macrotask. A promise chain scheduled inside a timer still finishes before the following timer fires.

setTimeout(() => {
  console.log("timer 1");
  Promise.resolve().then(() => console.log("  micro inside timer 1"));
}, 0);

setTimeout(() => console.log("timer 2"), 0);

Promise.resolve().then(() => console.log("micro before any timer"));
console.log("sync");

Where the loop actually lives

V8	Ships no event loop at all. It exposes a microtask queue and expects the embedder to pump it. `// no timers or I/O phases — // the embedder pumps the microtask queue`
Node.js	libuv phases: timers → pending → poll → check (`setImmediate`) → close. `setImmediate` fires after I/O, `setTimeout(fn, 0)` before it. `setImmediate(() => {}); // check phase: after I/O, before close process.nextTick(() => {}); // before ANY promise microtask`
Deno	Rust + Tokio. No `setImmediate` in the native API surface (only via the Node compatibility layer); web timers are the primitive. `await new Promise((r) => setTimeout(r, 0)); // web timers only — no setImmediate`
Bun	Custom loop on JavaScriptCore. Implements Node's `setImmediate` for compatibility; timer latency is generally lower under load. `setImmediate(() => {}); // implemented for Node compatibility`

runtime.js

One spine: the evolution of JavaScript

Async Era

async / await

Unhandled rejections

Interview gotchas

sync → microtask → macrotask

await resumes before a queued .then

Closures in loops: var vs let

The classic IIFE capture fix

Function declarations vs var hoisting

Detaching a method loses `this`

`this` inside a timer callback

The Promise executor runs synchronously

Promise.all rejects on the first failure

The six falsy loose-equality traps

NaN is never equal to itself

map(parseInt) doesn't parse numbers

Array#sort defaults to string sort

typeof null is 'object'

Object spread is only shallow