Out of interest, what other libraries did you consider?
A break down of pros and cons for each would be really useful, and I'm not sure anyone has done that already (at least I can't find any after a quick search).
jQuery.Deferred: Too coupled to jQuery, doesn't chain, not async
Q: Too monolithic
promise-stream: inadequate chaining support and node-specific
when.js: synchronous delivery, much larger API surface area than Promises/A
For what it's worth, promise-tests was written by Domenic Denicola, a contributor to Q, who seemed happy to have a lightweight entrant that implemented promises correctly.
Thanks for that, it's a good snapshot of the current state of promises.
The readme in the repository has some good discussion around the different features you mention here, but it would be really interesting to see a discussion around the design of this library in counterpoint to the other options available. Some other time maybe!
With regards to the promise-tests, I think multiple implementations validate a concept, and a set of tests for a specification is much more useful when multiple implementations can pass it. If the specification is extended you have multiple implementations to compare the impact against, and if the implementations' behaviour differ in notable ways then you expose holes in the test suite.
Multiple implementations of a specification, particularly when they all pass a common test suite, is a good thing.
Q: Too monolithic
promise-stream: inadequate chaining support and node-specific
when.js: synchronous delivery, much larger API surface area than Promises/A
For what it's worth, promise-tests was written by Domenic Denicola, a contributor to Q, who seemed happy to have a lightweight entrant that implemented promises correctly.