q.js: difference between resolve() and fulfill()

javascript asynchronous promise q
alecf picture alecf · Aug 16, 2013 · Viewed 10.5k times · Source

I'm still unclear on the difference between calling a resolver's resolve() vs fulfill()? I see both the functions and the terms "resolve a promise" and "fulfill a promise" batted around a lot.

When should I be using each?

Answer

Kris Kowal picture Kris Kowal · Aug 27, 2013

You should use resolve. deferredPromise.resolve(nextPromise) means that anything waiting for deferredPromise will now wait for nextPromise. If nextPromise is not a promise at all, it gets turned into a fulfilled promise which goes on to inform anything waiting for it that the value has become available.

The fulfill method is a bad idea that will be deprecated and eventually go away entirely. fulfill is semantically equivalent to resolve in all useful cases. It’s only reason to exist is that deferredPromise.fulfill(value) is easier for humans to interpret than deferredPromise.resolve(value), since resolve is overloaded to handle both nextPromise and finalValue.

The problem with fulfill existing at all is that deferredPromise.fulfill(rejectedPromise) is semantically paradoxical.

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