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JavaScript's GC won't collect an object as long as any reference exists. Weak collections let you hold a reference that DOESN'T prevent collection.

WeakMap — keys are objects, weakly held:

Differences from Map:

• Keys must be objects (not primitives)
• Not iterable — no keys(), values(), entries(), forEach
• No size property
• Entries can vanish at any time

Use cases:

• Caching computed data per object: const cache = new WeakMap()
• Private data: const privateData = new WeakMap(); then privateData.get(this) inside methods
• DOM event listeners that should clean up when nodes are removed

WeakSet — same idea, no values:

WeakRef (modern, ES2021) — explicit weak reference to an object:

FinalizationRegistry — get a callback when an object is GC'd:

Caveats:

• GC timing isn't deterministic — never rely on a specific moment
• Don't use these for critical cleanup (file handles, network connections) — use explicit close()
• Mostly useful for caches, observers, debugging memory leaks