Map, Set, WeakMap, and WeakSet

Purpose

Map is a general key/value collection accepting any value as a key. Set stores unique values. WeakMap/WeakSet are their weakly-held counterparts, restricted to object keys that don’t prevent those objects from being garbage collected.

Syntax

const scores = new Map();
scores.set('Ada', 95);
scores.set('Grace', 88);
scores.get('Ada'); // 95
scores.size; // 2
scores.has('Ada'); // true

const uniqueTags = new Set(['js', 'web', 'js']); // duplicates collapse
uniqueTags.size; // 2

const privateData = new WeakMap();
privateData.set(someElement, { clicks: 0 }); // cleaned up automatically once someElement is unreachable

Behavior notes

Map preserves insertion order when iterated, and any value — including an object or function — can be a key, unlike a plain object (string/Symbol keys only).
Set membership checks (.has()) and Map lookups (.get()/.has()) are both efficient — not a linear scan the way Array.prototype.includes() is.
WeakMap/WeakSet cannot be iterated, have no .size, and only accept objects as keys — all deliberate restrictions, since exposing iteration or size would leak information about garbage collection timing, which isn’t observable elsewhere in the language.
[...map] (spread) produces an array of [key, value] pairs; [...set] produces an array of its values.

Common mistakes

Defaulting to a plain object for general key/value storage out of habit, when the keys aren’t known string identifiers or a Map’s ordering/size guarantees would help.
Trying to iterate a WeakMap/WeakSet or read its size — both are unsupported by design.

Runtime notes

Map and Set (ES2015), WeakMap and WeakSet (ES2015), all core ECMAScript with identical behavior across every current browser and Node version.