Experimenting with auto-removed items from WeakSet/WeakMap (via garbage collection) in Node.js when .size doesn't exist?

Question

#1. Workaround for lack of .size property?

In JavaScript, I've never used either WeakSet or WeakMap before, and I don't know that much about garbage collection in general (I'm a PHP + JS developer, so this is the first time I've really needed to think about garbage collection). But I think I have a good use case for WeakMap right now. So I'd like to at least start experimenting with it.

The main thing I want to confirm in my experiments is the automatic removal of objects when they've been garbage collected. This would be easy to test if I could just access a WeakSet.size / WeakMap.size property on the instances to check their size, but they don't exist on the "weak" versions.

If possible, I'm guessing that the results could vary seeing that the size is going to depend on whether the garbage collector has run yet. But that's ok, as none of this experimentation code will be used in production... I just want to confirm that I actually understand how garbage collection and WeakSet/WeakMap are working. The idea of using this feature without being able to test (and therefore fully understand) it makes me very uneasy, and I'm concerned that I'll end up finding out about memory leaks when its too late (in production).

Are there any workarounds or alternatives to deal with the lack of WeakSet.size and WeakMap.size ... at least just for debugging/testing/learning purposes?

If not a .size workaround, is there maybe a way to check the memory usage of my WeakMap collection instances? That would be just as useful, as that's where the main concern is.

The only thing I can think of right now is checking the memory of the entire Node.js process... which doesn't seem very reliable to me.

#2. What is .length for?

Also I'm a bit confused about why there is a .length property on the class constructor/instance prototype of both WeakSet and WeakMap (not on your instances of them).

According to:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap

...both pages say that .length is:

• The value of the length property is 0.

Are they literally just hard-coded to the number 0 at all times? What's the point of that?

Searching for either "WeakSet.length" or "WeakMap.length" on Google (including the double quotes for exact results) yields no further information, only about 20 results for each, which are just mirrors of the MDN pages.

Answer 1

The size won't be available for either WeakSet or WeakMap since their keys are just references to objects, and these are handled by the Garbage Collector. Since the collector cannot be manually controlled (or shouldn't be), it will free the memory of these objects, once they are no longer referenced, at any point during runtime. The workaround you suggest to implement a way to see its current size would not be effective nor recommended considering this.

The length is there since both WeakSet and WeakMap are created through their prototype counterparts. Given how the collector will take care of clearing the object reference at any point,

As for experiment with them, you can try them out in Chrome and expose the garbage collector (and manually calling it) to see how the WeakMap clears itself after an object reference is lost (explained in this answer). Otherwise, you may still see the reference within the WeakMap or WeakSet since devtools usually prevents the garbage collector from running.