PricingLoginStart free
JavaScript Academy · Lesson

WeakMap & WeakSet — simple privacy and caches

Learn WeakMap/WeakSet essentials: object-only keys, no size/iteration, privacy helpers, and tiny caches that auto-clean.

Lesson 2 of 38 steps

WeakMap & WeakSet — simple privacy and caches is a free JavaScript Academy lesson on CoddyKit. This is lesson 2 of 3. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the JavaScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The JavaScript Academy course includes 3 lessons in total.

Why Weak collections?

Goal: Add metadata to objects without preventing garbage collection.

  • WeakMap: object → value
  • WeakSet: object membership
  • No size, no iteration
  • Great for privacy & caches
WeakMap & WeakSet — simple privacy and caches — illustration 1

WeakMap essentials

WeakMap only accepts object keys; supports set/get/has/delete; no iteration or size.

// 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
WeakMap & WeakSet — simple privacy and caches — illustration 2

WeakSet essentials

WeakSet holds objects; no duplicates, no iteration, no size property.

// 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 & WeakSet — simple privacy and caches — illustration 3

Privacy via WeakMap

Use a WeakMap to attach hidden data to API objects; when API objects are gone, data can be collected.

// 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)
WeakMap & WeakSet — simple privacy and caches — illustration 4

Per-object cache

Create a per-object cache; entries vanish when the key object is no longer referenced.

// 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
WeakMap & WeakSet — simple privacy and caches — illustration 5

Limits & when not to use

Remember:

  • No iteration or size on WeakMap/WeakSet.
  • Keys must be objects.
  • Use Map/Set if you need to list keys or count items.
WeakMap & WeakSet — simple privacy and caches — illustration 6

WeakMap vs Map quiz

Quick check: WeakMap use case.

WeakMap & WeakSet — simple privacy and caches — illustration 7
Answer it in the interactive lesson

Recap

Recap: Use WeakMap for object→value privacy and caches; use WeakSet for object membership without leaks. Remember: object keys only, no iteration or size.

WeakMap & WeakSet — simple privacy and caches — illustration 8
Free to start

Learn JavaScript with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses
59
Lessons
200
Start learning freeView the full course

Frequently Asked Questions

Is the “WeakMap & WeakSet — simple privacy and caches” lesson free?

Yes — any 3 lessons of the JavaScript Academy learning path, including “WeakMap & WeakSet — simple privacy and caches”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The JavaScript Academy course includes 3 lessons in total.

What will I learn in “WeakMap & WeakSet — simple privacy and caches”?

Learn WeakMap/WeakSet essentials: object-only keys, no size/iteration, privacy helpers, and tiny caches that auto-clean. You practise JavaScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start JavaScript Academy?

No prior experience is required. JavaScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 3.

How long does the “WeakMap & WeakSet — simple privacy and caches” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this JavaScript Academy lesson?

Yes. Every JavaScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Map & Set essentials
  2. WeakMap & WeakSet — simple privacy and caches
  3. Iteration Patterns
← Back to JavaScript Academy