Yes, but that doesn't give you a great way to deal with attaching and canceling multiple callbacks simultaneously. If you want to perform several independent asynchronous actions after some other operation finishes, you have to collect them into a single giant callback in one location. And as you mention, the function has to keep track of some piece of mutable state in order to decide whether it's cancelled or not before firing off those operations. So why not give the function a mutable array of asynchronous operations to perform to begin with, which you can then add to at any time? And at that point, you already have a something very close to a promise.