Map and WeakMap

This chapter follows Set in JavaScript.

Map is an ordered key/value store. Keys can be any value (objects, primitives, symbols). WeakMap only accepts object keys (and, since ES2023, symbols); keys are held weakly so entries disappear when nothing else references the key object.

Map essentials

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const map = new Map([
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  ["name", "Ada"],
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  ["year", 1815],
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]);
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map.set({ id: 1 }, "metadata");
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map.get("name"); // "Ada"
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map.size; // 3

Insertion order is preserved when iterating—unlike ordinary objects, where integer-like keys sort first.

When Map beats {}

• Keys are objects, functions, or mixed types—not only strings/symbols.
• You need reliable size and frequent add/delete without touching prototype pollution concerns.
• You iterate entries in insertion order and do not need JSON serialization of the map itself.

Plain objects remain fine for fixed, string-keyed records and JSON payloads.

WeakMap essentials

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const cache = new WeakMap();
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let obj = { id: 1 };
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cache.set(obj, { computed: 42 });
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cache.get(obj); // { computed: 42 }
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obj = null; // entry may be collected; no way to list it

Typical uses: private metadata on DOM nodes or class instances, caches keyed by request objects, and breaking reference cycles without leaking memory.

WeakMap vs Map vs Set

For ordered unique values, use Set. For numeric indexed lists, use Arrays.

Continue with Typed arrays and ArrayBuffer.