So if the promise still exists, that means that there is an active call out for a promise using this key already, so we'll just return it. We know this because when the function that is executing finishes, we delete the promise from the map.

A purpose of this might be, let's say you rebuild some structure very frequently, like hundreds of times per second or whatever. You can call this function for a given key as many times as you want while that async process is executing, and it will only execute it once, always returning the same promise.