Here's an example: in Bluebird, cancelation doesn't
  propagate upstream. After registering onCancel in a
  promise constructor, you have to call .cancel() on
  that exact promise object. Calling .cancel() in any
  child promise created with .then() or .catch() will
  not abort the work, rendering the feature useless for
  the most common use case!

  True cancelation must propagate upstream, prevent
  all pending work, and immediately free resources and memory.

From the README

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.

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.

D'oh, thanks

Am bluebird collaborator - confirming cancellation does everything this library does - but also componses and ref-counts to work with one-to-many flawlessly.

Glad it works for you!

Kind of answered in another subthread: https://news.ycombinator.com/item?id=14963524