WeakMap and WeakSet

In short: JavaScript frees memory automatically — once nothing in the program can reach an object any more, the garbage collector may remove it. A normal Map or Set counts as a reference, so anything stored in one can never be collected, even after every other part of the program has let go of it. WeakMap and WeakSet hold their objects weakly: if the only thing still pointing at an object is a weak collection, the object — and its entry — can vanish. In exchange, their keys must be objects, and neither collection can be looped over or asked for its size, since entries may disappear at any moment.

Attach a note to a song object

A WeakMap works like a Map for reading and writing, but its key must be an object — here, the song itself:

This behaves exactly like a normal Map so far: get and has work the same way. The difference only shows once nothing else references song — at that point the object, and this entry, become eligible for collection, something a normal Map would never allow.

Try a string as a WeakMap key

Keys must be objects, because a string or number could be recreated at any time and would never truly "go away" — weak references need something with a real, trackable lifetime:

This throws TypeError: Invalid value used as weak map key. Swap the string for an object — new WeakMap().set({}, 1) — and it works fine.

Mark songs that have already been rendered

A WeakSet is the "mark, don't store a value" version — perfect for remembering which objects have already been processed:

This prints false then true. If song later leaves the playlist and nothing else references it, both the object and its mark in rendered become eligible for collection — no cleanup code required.

The trap: expecting Map-like listing

WeakMap and WeakSet deliberately leave out anything that would let you see every entry, because entries can vanish mid-list:

forEach is undefined — it does not exist on WeakMap at all — while get is a real function. Only get, set, has and delete exist on WeakMap; only add, has and delete exist on WeakSet.

Quick practice

🎯 Your turn

A player caches a computed waveform for each song object in a Map. Over a long session memory keeps growing even after songs are removed. Explain why, and why a WeakMap fixes it, including one limitation you accept in exchange.

Write your explanation and submit it — there's no code to run here, just your reasoning.