Does anyone know how promises work?

Here is an excerpt from that explains promises better from html5rocks.com (http://www.html5rocks.com/en/tutorials/es6/promises/) :

At their most basic, promises are a bit like event listeners except:

A promise can only succeed or fail once. It cannot succeed or fail twice, neither can it switch from success to failure or vice versa If a promise has succeeded or failed and you later add a success/failure callback, the correct callback will be called, even though the event took place earlier This is extremely useful for async success/failure, because you're less interested in the exact time something became available, and more interested in reacting to the outcome.

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A promise can be:

fulfilled The action relating to the promise succeeded rejected The action relating to the promise failed pending Hasn't fulfilled or rejected yet settled Has fulfilled or rejected.