[a-kojeve]

Yeah, but nobody relies on the execution order of disparate callbacks to achieve their results in JS. You're going to get burned in like two seconds if you try to do anything you listed. Lower level concurrency primitives in other languages allow developers to build fragile solutions which work 99% of the time until they deadlock and everything blows up. The concurrency model in JS is very explicit about being "no guaruntees."

[pizlonator]

There are many reasons why complex software fails in the 1% case. Concurrency is not the only reason. In my experience it is not the top reason. Our top hard-to-repro or no-repro crashes in JavaScriptCore are from non determinism introduced by the workload itself. Inside our engine we have many sources of nondeterminism that manifests even with concurrency features disabled.