Promises are like callback functions, but so much better. Promises are meant to save you from callback hell.
ES6 feature, started out as libraries. Natively supported in all modern browsers.
A promise is a pattern for handling asynchronous operations. The promise allows you to call a method called “then” that lets you specify the function(s) to use as the callbacks.
A promise is an eventual value.
three states
Now of course the more levels of nesting we have, the harder the code is to read, debug, maintain, upgrade, and basically work with. This is generally known as Pyramid of Doom or Callback Hell. Also, if we needed to handle errors, we need to possibly pass in another function to each xhrGET call to tell it what it needs to do in case of an error. If we wanted to have just one common error handler, that is not possible.
.all, which takes an array of resources.