co VS proposal-async-await

Given that the semantics of async functions are quite similar to generator functions, you can use a library the uses the latter to emulate the former – e.g.: https://github.com/tj/co

Then after a while, we resorted to generator functions and cogenerators, which made async code feel like its synchronous, at the cost of wrapping it in a generator function, yielding every line and introducing a cogenerator library (for example co) to deal with unwrapping the promises like the following example, where we could just yield a promise whenever we encounter it and pretend that the yield does not exist on that line of code.

This evolution served as the inspiration of the async/await syntax introduced in es7, and finally we could just

tc39 async/await proposal has this very question asked From discussion I get that one of the reasons is backward compatibility with old JS code, where await was valid name of a function. So by introducing async it's possible to have await as non-breaking change to syntax since it's only available in async scope. Also async has effect on it's own. As it was mentioned already, async functions have AsyncFunction constructor, instead of just Function. Also async functions always return promise, even if await is never used. If you throw an error or return as usual it'll be wrapped automatically.