[code-e]

But the linked bluebird docs says:

  As an optimization, the cancellation signal propagates 
  upwards the promise chain so that an ongoing operation e.g. 
  network request can be aborted.

[Mitranim]

Interesting! This must be new. Bluebird didn't have upstream cancelation last time I checked. Now it might actually be viable. Should update the readme. Thanks!

Unfortunately it's still not viable in the browser due to its size, promises too easily get converted into non-cancelables, and worst of all, async/await forces native promises. If you want cancelable coroutines, you're forced to roll a generator-based implementation such as what Posterus provides.