Promises

ES6 / 2015

ES20152015

The largest single revision of the language. Block scoping, lexical `this`, class syntax, and a standardised Promise — the foundation of every later feature.

A standard object for a value that is not here yet — and, crucially, a standard scheduling rule: reactions run on the microtask queue, always after the current synchronous run to completion.

console.log("1 sync");

setTimeout(() => console.log("4 macrotask"), 0);

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

console.log("2 sync");

Microtask draining across runtimes

V8	Owns the microtask queue itself; the host decides when checkpoints run. `queueMicrotask` and promise reactions share one FIFO queue.
Node.js	libuv drives phases; microtasks drain after each macrotask AND between each `process.nextTick` batch — nextTick jumps ahead of promises.
Deno	Tokio event loop over V8, no nextTick queue in native code — promise ordering matches the browser far more closely.
Bun	JavaScriptCore instead of V8; microtask semantics are spec-identical, but timer resolution and task batching differ measurably.

runtime.js

One spine: the evolution of JavaScript

ES6 / 2015

let / const and the TDZ

Arrow functions & lexical this

Classes & extends

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