[Animats]

In this example, though, the library's C++ sort is calling out to Javascript code during the sort. That code can alter the object being sorted. That's where the trouble comes from. The sort function needs exclusive mutable access to the object being sorted. But Javascript doesn't support such access control.

This is a general problem with cross-language data access. The languages may not have the same data model. It's especially bad when one side has garbage collection, and the other side has to have explicit GC-aware code.