Map, Set and their weak cousins in JavaScript

Maps take any key and remember insertion order; WeakMap holds no reference.

const byObject = new Map();
const key = { id: 1 };
byObject.set(key, "object key").set("id", "string key");
console.log(byObject.get(key), byObject.size, [...byObject.keys()].length);

const seen = new Set([1, 2, 2, 3]);
console.log(seen.size, seen.has(2), [...seen]);

const a = new Set([1, 2, 3]);
const b = new Set([2, 3, 4]);
console.log([...a].filter((x) => b.has(x)));

const cache = new WeakMap();
cache.set(key, { computed: true });
console.log(cache.has(key), cache.get(key).computed);

How it works

1. An object key in a plain object becomes a string; in a Map it does not.
2. Set deduplicates by SameValueZero.
3. A WeakMap entry vanishes when its key is collected.

Keywords and builtins used here

• Map
• Set
• WeakMap
• const

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Type this snippet

Step 4 of 8 in Objects & collections, step 30 of 43 in Language basics.

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