It's a shame that WeakMap and WeakSet aren't enumerable. Ephemeron tables in Lua are enumerable, so creating a data structure that detects when an object is gone is possible. And because Lua supports finalizers, you can trigger an event the moment the object is garbage collected.[1][2]
[1] Lua also supports reviving objects. For example, if your finalizer leaks the object to the outside environment. Implementing this can be tricky, which is perhaps why JavaScript doesn't permit it. So many limitations in JavaScript exist because a particular feature would be too difficult to implement in existing implementations.
[2] Of course, because Lua supports finalizers it's possible to detect GC without using ephemeron tables. But if you can't control the target object's finalizer, then ephemeron tables are handy for doing this. And you can use your own finalizer on the ephemeron table value object to detect GC, though it'll trigger one or two GC cycles behind when the key was collected.
[1] Lua also supports reviving objects. For example, if your finalizer leaks the object to the outside environment. Implementing this can be tricky, which is perhaps why JavaScript doesn't permit it. So many limitations in JavaScript exist because a particular feature would be too difficult to implement in existing implementations.
[2] Of course, because Lua supports finalizers it's possible to detect GC without using ephemeron tables. But if you can't control the target object's finalizer, then ephemeron tables are handy for doing this. And you can use your own finalizer on the ephemeron table value object to detect GC, though it'll trigger one or two GC cycles behind when the key was collected.