Map and Set — When Objects and Arrays Aren't Enough

Objects and arrays cover most needs — Map and Set cover the cases where they get awkward.

Set — uniqueness for free

const unique = [...new Set([1, 1, 2, 3, 3])];   // [1,2,3] — THE dedupe idiom

const seen = new Set();
seen.add(id);
seen.has(id);      // O(1) — vs arr.includes() which is O(n)
seen.size;

Any "have I seen this before?" logic wants a Set — visited nodes in graphs, selected IDs, permission flags.

Map — a better dictionary

const m = new Map();
m.set("a", 1);
m.set(someObject, "metadata");   // ANY key type — objects, functions...
m.get(someObject);
m.size;                           // objects need Object.keys(o).length
for (const [k, v] of m) { ... }   // iterates in INSERTION order, guaranteed

Decision rules

• String keys + JSON round-trips + dot access → plain object is fine
• Non-string keys, frequent add/delete, need .size, order matters → Map
• Frequency counters: object is fine; Map is cleaner
• WeakMap: keys are weakly held — attach metadata to DOM nodes without preventing garbage collection (framework internals; know it exists for interviews)