Is Q a superset of a native Promise?

[andykais]

Hi, new to using Q here in an existing codebase. Could someone clarify this for me? A Q promise has all the functionality of a native promise, with no changed behavior, correct? There are only additional methods that allow more functionality?

#836 mentioned that you can coerce a promise from a Q promise with

Promise.resolve(Q.resolve(1))

but I want to know if a Q promise can simply be used in-place of a native promise.

[andykais]

follow up question, what is the interopability between native promises and Q promises? Are these both legal

const nativePromise = (n) => new Promise(resolve => setTimeout(resolve, n))
const qPromise = (n) => {
  const deferred = q.defer()
  setTimeout(() => deferred.resolve(), n)
  return deferred.promise
}

const qPromiseFromNative = Q.resolve(nativePromise(100))
const nativePromiseFromQ = Promise.resolve(qPromise(100))

[benjamingr]

No, but it is pretty close, for example native promises have more guarantees on timings

[andykais]

Thanks, I was able to discover some differences with unhandledRejections as well. A native promise will become unhandled at the end of one node event loop. A Q promise does not have that guarantee. Q uses its own custom nextTick, so it makes sense.

If you wish to elaborate on their differences I would be interested. If not, feel free to close this issue. I see that there are subtle differences now.

[benjamingr]

@andykais well, I (helped) spec how unhandledRejection behaves in Node ( https://gist.github.com/benjamingr/0237932cee84712951a2 ) and I added the events to Q ( #643 ).

I don't think you should rely on the differences between Q and Node here :]