How to iterate over a weakmap?

Things are moving and it will soon be possible to create iterable week maps thanks to weak refs. See the iterable WeakMap example in the tc39 weakrefs proposal.

(note that it is already possible with nodejs v12.?.? using --harmony-weak-refs flag)

Is it possible to nevertheless loop over entries in some way?

No, as you say, the contents of a WeakMap are not accessible by design, and there is no iterability.

If not … how does the Chrome console do this?

The console uses the debugging API of the JS engine, which allows access to the internals of objects (also to promise states, wrapped primitives, etc.) and many more.