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JavaScriptHardNew
What are WeakMap and WeakSet and how do they differ from Map and Set?
Weak collections hold object keys weakly — no iteration, GC-friendly.
Ideal Answer
WeakMap and WeakSet are collection types that hold weak references to objects, allowing those objects to be garbage-collected when no other references exist.
| Map / Set | WeakMap / WeakSet | |
|---|---|---|
| Keys / values | Any type | Objects only |
| Iteration | Yes (for...of, .size) |
No — not enumerable |
| Garbage collection | Strong refs prevent GC | Weak refs — entries disappear with object |
| Use case | General caching, dedup | Private/metadata attached to objects |
JavaScript
const wm = new WeakMap()
let obj = { id: 1 }
wm.set(obj, 'metadata')
obj = null // { id: 1 } can now be GC'd; WeakMap entry goes with itTypical use cases
- Storing private data or metadata on DOM nodes / class instances without leaking memory
- Caching computed results keyed by object identity
- Tracking object presence without preventing cleanup
WeakSet — stores only objects (like a Set of weakly held object references). Useful for tagging objects ("already visited") without retaining them.
You cannot get .size or list entries — the trade-off for automatic memory management.