Maybe if you use a WeakMap with a transferrable object and assuming that if you postMessage a transferrable object and then get it back later and still have the WeakMap recognize it as a key. I'm not very sure that last part would work. I don't think GC-links reach through iframes in a way that would enable that.

WeakMaps are more limited than most people seem to expect. They're powerful tools that enable many new powerful patterns, but they don't expose the workings of the garbage collector. They're very different from Java's WeakHashMap for example. In Javascript, the only way you can tell whether the browser is using a garbage collector or not is to try to run out of memory. Without crashing, there's no way for some Javascript code to observe the runtime's GC behavior at all by design.

Object identity is not preserved with Transferring or regular structured cloning. Transferring is really about the resource represented it held onto by the object being transferred.

If object identity (for the purpose of WeakMap) was preserved, your idea would provide a way to observe GC behavior. Which wouldn't automatically be a complete disaster, though I am personally dead set against it except maybe in some sort of privileged context. Weak references keep getting proposed, and there are some pretty compelling use cases, but there is also some very major risk that we could never back out of once weakrefs were available, and they could permanently hold back future GC performance. (You could easily break deployed web applications by improving GC behavior.)

(Source: I implemented Transferring in general and Transferring ArrayBuffers in particular in Spidermonkey, and I work on the GC engine. And it's nice to see a comment on HN like the parent that gets it right for once!)