I'm referring to Os level threads, but functionally it's no different if we talk about forking with shared memory. Given that the vast majority of computing 15 years ago was single processor and multi-process or multi-thread for ansynchronous behavior (or use of select), it's all functionally equivalent, and many of the same problems discovered many decades ago for multi-process programs apply.

If I have a main program, and I fork a child to handle a task, and use shared memory to communicate, how is that any different than Javascript executing and and async call that sets or returns a value? The Javascript runtime has the same behavior as the OS scheduler in this, and if there's a single processor, it necessarily will only execute one instruction at a time. There's still plenty of pitfalls to worry about and that's why locks were (and are) useful, and why they are included with OS thread implementations (which are really just a nice API on forks and shared memory often dealt with by the OS for additional benefit).