A WeakMap/WeakSet only holds objects, and holds them weakly: if nothing else references the key, it can be garbage-collected (and removed automatically). That makes them ideal for attaching private data or metadata to objects without causing memory leaks. They're not iterable and have no size.
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<script>const meta=new WeakMap(); const obj={}; meta.set(obj,{secret:42});
document.getElementById('o').textContent='private: '+meta.get(obj).secret;</script></body></html>