Comment by kmeisthax

> This is a promise (JavaScript) or future (Java, Rust, etc). The concept dates to Baker and Hewitt in 1977, but it took the C10K pressure of the 2010s to push it into mainstream programming.

Almost. JavaScript adopted async because it was a programming language designed to slot into someone else's event loop. Other programming languages, at least on the server, that needed lightweight threading didn't bother with any of this, they just shipped their own managed stacks. But UI code practically demands to own its own event loop and requires everything else live as callbacks inside of it. And JavaScript, because it was designed to live in a browser, inherited these same semantics.