<p>At their most basic, promises are a bit like event listeners except:</p>
<ul>
<li>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</li>
<li>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</li>
</ul>
<p>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.</p>
