Object metadata
Attach cached or computed metadata to objects without extending their lifetime.
JS Puzzles · Const
WeakMap GC
WeakMap associates values with object keys without preventing those keys from being garbage-collected. Its entries cannot be enumerated, and cleanup timing is controlled by the JavaScript engine.
How it works
A WeakMap stores values under object keys without strongly retaining those keys. When a key has no other references, the engine may collect the key and its associated value; cleanup timing is implementation-dependent, and the collection cannot be enumerated.
Syntax
new WeakMap(); weakMap.set(key, value); weakMap.get(key); weakMap.has(key); weakMap.delete(key)Parameters
| Name | Type | Description |
|---|---|---|
key | object | The object used as the weak key. |
value | any | The value associated with the key; it may be any JavaScript value. |
Returns
WeakMap methods return the associated value, a boolean for has/delete, or the WeakMap instance for set; the page specifically lists get, set, delete, and has.
Key points
- Unlike Map, WeakMap does not keep its object keys alive solely because they are keys.
- A key can become collectible when no other strong reference reaches it.
- When the key becomes unreachable elsewhere, its entry is eventually removed along with the key.
- WeakMap is useful for attaching auxiliary data whose lifetime follows an object.
- set(key, value) adds or replaces the value associated with an object key.
- get(key) retrieves the value associated with the key.
Common mistakes
- Do not rely on immediate cleanup: the JavaScript engine chooses when garbage collection occurs.
- You cannot inspect all entries or use keys(), values(), or entries() to observe which keys remain.
- Primitive values such as strings and numbers cannot be used as keys.
Example: Associate metadata without preventing collection
const metadata = new WeakMap();
let user = { name: "John" };
metadata.set(user, { lastLogin: new Date() });
console.log(metadata.get(user));
user = null; // The object and associated metadata may be collected later.The metadata is associated with the object without keeping that object alive.
When to use it
Ephemeral associations
Store auxiliary data for objects managed by another collection or subsystem.
Leak-resistant caching
Avoid memory retention when associated objects become unreachable.
Related
MapWeakSetgarbage collectionweak references