You Don't Know JS: ES6 & Beyond

Chapter 4. Async Flow Control

It’s no secret if you’ve written any significant amount of JavaScript that asynchronous programming is a required skill. The primary mechanism for managing asynchrony has been the function callback.

However, ES6 adds a new feature that helps address significant shortcomings in the callbacks-only approach to async: Promises. In addition, we can revisit generators (from the previous chapter) and see a pattern for combining the two that’s a major step forward in async flow control programming in JavaScript.

Promises

Let’s clear up some misconceptions: Promises are not about replacing callbacks. Promises provide a trustable intermediary—that is, between your calling code and the async code that will perform the task—to manage callbacks.

Another way of thinking about a Promise is as an event listener, upon which you can register to listen for an event that lets you know when a task has completed. It’s an event that will only ever fire once, but it can be thought of as an event nonetheless.

Promises can be chained together, which can sequence a series of asychronously completing steps. Together with higher-level abstractions like the all(..) method (in classic terms, a “gate”) and the race(..) method (in classic terms, a “latch”), promise chains provide an approximation of async flow control.

Yet another way of conceptualizing a Promise is that it’s a future value, a time-independent container wrapped around a value. This container can be reasoned about ...