Async is pretty good “green threads” on its own. Coroutines can be better, but they’re really solving an overlapping set of problems. Some the same, some different.

In JavaScript async doesn’t have a good way to nice your tasks, which is an important feature of green threads. Sindre Sorhus has a bunch of libraries that get close, but there’s still a hole.

What coroutines can do is optimize the instruction cache. But I’m not sure goroutines entirely accomplish that. There’s nothing preventing them from doing so but implementation details.