Functions with clearly communicated intent
If you were to have a conversation with a function in your app, would it be like a conversation with somebody you felt you could depend on? Or would it be like talking to someone who seemed a bit iffy? Maybe some borderline multiple personality complex issues? Maybe some overall wishy-washiness, and inability to commit (promise) as to their intent?
I would say, if you are not able to take a look at that function and get a real sense of what it's end result would be, you may be in for some nasty debugging surprises.
At one end of that extreme of being able to promise a predictable outcome we may have a function that returns a boolean based on its internal operation. That's a pretty well quantifiable outcome right?
At the other end we may have a function or lots of functions that return
undefined and here we may start the head-scratching and trying to sort out side effects etc.
Here we may see a preference for pure functions whenever and wherever possible.
Small outcomes that can be predictably promised which compose into the larger app is where it's at!
Oh yeah and Promises (with a capital "P") are cool, too!