Okay. I gave it another read and I think we agree in general, but maybe disagree on how much memory model strangeness is acceptable, and how wide the gap between, “The behaviour can be understood by a developer,” and, “it should be possible to write a sequential JS program that creates an indistinguishable heap,” is likely to be.

The lower level bits round the object model etc. all look very solid.

Although structs may not be necessary to make JS concurrent their limitations might help in reducing where memory model strangeness could creep in.