Map

JavaScript · Reference cheat sheet

📋 Overview

Map is a keyed collection that preserves insertion order and accepts any value as a key (including objects). Prefer Map over plain objects when keys are unknown, non-strings, or you need reliable size / iteration.

🔧 Core concepts

Create: new Map(), new Map([[k, v], ...]).
Mutate: set, get, has, delete, clear.
Size: map.size (not a method).
Iterate: keys(), values(), entries(), forEach, for...of.
Key equality: SameValueZero (NaN equals NaN; +0 / -0 equal).

const m = new Map();
m.set("a", 1).set("b", 2);
m.get("a"); // 1
m.size; // 2

💡 Examples

// Object keys by reference
const user = { id: 1 };
const scores = new Map();
scores.set(user, 100);
scores.get(user); // 100

// Build from object
const fromObj = new Map(Object.entries({ x: 1, y: 2 }));

// To object (string keys only)
const obj = Object.fromEntries(fromObj);

// Frequency map
function frequencies(list) {
  const freq = new Map();
  for (const item of list) {
    freq.set(item, (freq.get(item) ?? 0) + 1);
  }
  return freq;
}

// Default get
function getOr(map, key, factory) {
  if (!map.has(key)) map.set(key, factory());
  return map.get(key);
}

// Iterate
for (const [k, v] of scores) console.log(k, v);

// Group into Map
function groupBy(list, keyFn) {
  const map = new Map();
  for (const item of list) {
    const key = keyFn(item);
    getOr(map, key, () => []).push(item);
  }
  return map;
}

// WeakMap — keys must be objects; not iterable; GC-friendly
const privateState = new WeakMap();
function attach(el) {
  privateState.set(el, { clicks: 0 });
}

⚠️ Pitfalls

Object keys are by reference, not deep equality.
JSON.stringify(map) → "\{\}" — convert with Object.fromEntries or entries array.
map["a"] = 1 does not use Map storage — always use .set / .get.
WeakMap is not iterable and cannot use primitive keys.
Converting to object fails for non-string/symbol keys.

set.md — Set collections
objects.md — plain object maps
iteration.md — iterating Maps
for_of.md — for...of entries
json.md — serialization
arrays.md — Object.groupBy

Map

Map

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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