When We Should Use WeakMap and WeakSet

Hi,

I am not sure if I have a good perception of this, but I assume that Map is fairly widely used, whereas WeakMap is not as common. WeakMap is often described as being useful for caching, but I think it is better understood as a specialized version of Map.

The major differences between Map and WeakMap are:

1. Map is iterable, while WeakMap is not.

2. The keys of a WeakMap must be objects or symbols.

Notice that this description does not mention garbage collection. I guess WeakMaps are designed for garbage collectability, and it’s one of their biggest benefits. However, we could also view this capability as a by-product rather than the primary reason for using them.

When to Use WeakMap

If you are using a Map, but you do not need to iterate over it and all the keys are objects, you could use a WeakMap instead. I am not 100% sure of the actual performance differences, but it feels like WeakMap can be more efficient than Map in cases where either could be used.

How Jotai Uses WeakMap

In the Jotai codebase, we use WeakMap wherever possible. In particular, for atoms, we use WeakMap to ensure that atom values are garbage-collected when their references are no longer used.

We even have unit tests to verify this behavior. If you are interested, feel free to check them out: Jotai Memory Leak Tests

A Note on WeakSet

The same applies to WeakSet. If we use a Set only for objects and do not need to iterate over them, we can use WeakSet instead.

Happy coding.