WeakMapとWeakSet — シンプルなプライバシー保護とキャッシュ

なぜWeakコレクションなのか

WeakMapの基本

// WeakMap: keys must be objects
const wm = new WeakMap();

const user = { name: "Ayla" };
wm.set(user, { visits: 1 });

console.log("has user?", wm.has(user));
console.log("get meta:", wm.get(user));

// Note: no wm.size, no wm.keys() — not iterable

WeakSetの基本

// WeakSet: store objects without duplicates
const ws = new WeakSet();

const obj1 = {};
const obj2 = {};

ws.add(obj1);
ws.add(obj2);
ws.add(obj1); // duplicate ignored

console.log("has obj1?", ws.has(obj1));

// Note: no ws.size and no iteration

WeakMapによるプライバシー保護

// Privacy helper: store private data outside the object
const _priv = new WeakMap();

function makeCounter() {
  const api = {};
  _priv.set(api, { value: 0 });

  api.inc = function () {
    const box = _priv.get(api);
    box.value = box.value + 1;
    console.log("value", box.value);
  };

  // No public field exposes value directly
  return api;
}

const c = makeCounter();
c.inc();
c.inc();
// console.log(c.value) -> undefined (nothing public)

オブジェクトごとのキャッシュ

// Cache computed data per object without leaks
const cache = new WeakMap();

function computeLength(doc) {
  // Return cached result if present
  if (cache.has(doc)) {
    return cache.get(doc);
  }
  // Fake "expensive" compute
  const result = (doc.text || "").length;
  cache.set(doc, result);
  return result;
}

const d1 = { text: "hello" };
const d2 = { text: "world!" };

console.log("len d1:", computeLength(d1));
console.log("len d1 cached:", computeLength(d1));
console.log("len d2:", computeLength(d2));

// If d1/d2 become unreachable, their cache entries can disappear automatically

制限と使わない場合

WeakMapとMapのクイズ

まとめ