Finalizers for JavaScript objects

The simple answer is that there is no support for this.

Since asm.js code needs to manage its own memory, everything that interacts with objects stored on the asm side need to respect the memory manager that asm uses rather than the memory manager that the browser uses. The best that you can do is to explicitly call a method on any object referencing internal asm memory whenever you create or destroy a reference to it.

Coming back to this question, I found another answer pointing out that there is a specification for weak references and finalization which some browsers implement. The central component for finalization is FinalizationRegistry.

So depending on which browsers you target, this may be possible now. If you need to support browsers without this feature, using explicit release calls, it might be possible to use the finalizers where supported to detect memory leaks (i.e. objects not explicitly released in JavaScript code) and let the developer know so they can fix this.