This chapter covered a lot of details about the Promise object introduced in ES6 to help JavaScript programmers deal with synchronization issues that have plagued the language for years. With asynchronicity comes callbacks—lots of callbacks. This creates a callback hell that we want to avoid at all costs.
Promises help us deal with synchronization issues by implementing a simple interface that's generic enough to resolve any value. Promises are always in one of three states—pending, fulfilled, or rejected, and they only change their state once. When these state changes happen, callbacks are triggered. Promises have an executor function, whose job is to set up the asynchronous actions that use a promise resolver or rejector function to change the state of the promise.
Much of the value that promises bring to the table is about how they help us simplify complex scenarios. Because, if we only had to deal with a single asynchronous action that runs a callback with a resolved value, promises...