JSworkinges2015
Map and WeakMap
Keyed data, done right.
A plain object pretends to be a dictionary, but its keys are always strings and it carries inherited baggage. Map is the real thing: any value as a key, honest size, guaranteed insertion order — and WeakMap attaches private metadata without leaking memory.
Object keys vs Map keys
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The object stores only one entry (Object.keys(obj).length is 1) because the numeric key 1 was coerced to the string '1' and overwrote it, while the Map keeps both as distinct keys and reports size 2 — proof that Map preserves key type.