WeakMap and WeakSet

Learning objectives#

• Explain what “weak” means
• Store private data per object with WeakMap
• Track objects with WeakSet
• Know their restrictions

Explanation#

A normal Map holds strong references: as long as the Map exists, its keys can never be garbage-collected.

WeakMap and WeakSet hold weak references: if nothing else references the key object, the entry disappears automatically.

Keys must be objects (not strings or numbers), and they are not iterable — you cannot list their contents or get a size.

Typical uses: attaching private or cached data to DOM elements or objects without causing memory leaks.

Code example: Private data and visited tracking#

1const privateData = new WeakMap();
2class Counter {
3  constructor() { privateData.set(this, { count: 0 }); }
4  increment() {
5    privateData.get(this).count++;
6    return privateData.get(this).count;
7  }
8}
9const c = new Counter();
10c.increment();
11console.log(c.increment());
12console.log(Object.keys(c).length);    // nothing visible on the object
13

14const visited = new WeakSet();
15const nodeA = { id: 'A' };
16visited.add(nodeA);
17console.log(visited.has(nodeA), visited.has({ id: 'A' }));
18

19try { new WeakMap().set('key', 1); } catch (e) { console.log(e.name); }

2
0
true false
TypeError

Step-by-step explanation#

1. Each Counter’s data is stored in the WeakMap, keyed by the instance itself.
2. The object has no visible properties, so the data is effectively private.
3. WeakSet.has checks identity — a different object with the same content is not found.
4. Using a string key throws a TypeError: keys must be objects.
5. When c is no longer referenced, its WeakMap entry can be collected automatically.

Real-world use case#

Common mistakes#

Best practices#

• Use WeakMap for per-object caches and metadata
• Prefer #private fields for class privacy in new code
• Use WeakSet to mark objects as processed
• Use Map/Set when you need to list or count entries

Remember this#

Practice exercise#

1const done = new WeakSet();
2function process(obj) {
3  if (done.has(obj)) return 'skipped';
4  done.add(obj);
5  return 'processed';
6}
7const o = {};
8console.log(process(o), process(o)); // processed skipped

Mini quiz#

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Related topics#

My notes#

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