runtime.js

One spine: the evolution of JavaScript

Pick an era, expand a concept, and see where it lands in today's runtimes. Every snippet can be dropped straight into the sandboxed console — the event loop panel then shows real, instrumented execution, not a looping animation.

Pre-ES6

ES51995 – 2014

One function scope, one binding keyword, and callbacks all the way down. Everything that came later is a reaction to this era's sharp edges.

Interview gotchas

15 genuinely tricky, well-known snippets — event-loop ordering, closures in loops, and `this`-binding traps. Open one, read the gotcha, then drop it straight into the sandboxed console to watch it really run.

Event loop

Closures & scope

this binding

Promises & async

Coercion & equality

Built-in traps

One spine: the evolution of JavaScript

Pre-ES6

var & hoisting

Callbacks & the pyramid

Prototypal inheritance

Strict mode & sloppy this

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