Read the Docs
You know a spec is good when the docs are this concise: MDN Promise Documentation
Promises are a way to avoid 👹👹👹 CALLBACK HELL 👹👹👹. You know you’ve done it.
Cut down those Promise trees!
Firebase emits promises, so most of the time you won’t have to create your own, you’ll just consume the Promise API that Firebase produces. A typical bit of code might look like this.
The bit of code above is something that I use in production to handle the checkout process for my shopping cart. It’s the most critical bit of code in my entire app and, therefore, livelihood. Every CheckoutService function returns a Promise, so I can chain them and avoid callback hell.
Anyway, the key to understanding promises, is to recognize that they’re just a fancy, flat way of handling callbacks. They’re just callbacks. That’s it.
Roll Your Own
To make this abundantly clear, let’s roll a promise of our own.
This example has a function named waitForIt. waitForIt takes an argument N and waits N milliseconds before continuing. waitForIt returns a Promise. The Promise takes an executor function, which has two arguments, resolve, and reject. The executor function then uses setTimeout to wait N milliseconds before calling resolve.
Because waitForIt returns a Promise, we can register callbacks against that Promise using .then(). The beauty of .then() is that it can return it’s own Promise, which can then register its own .then() callback.
Read through the following example to see some of the fun stuff that Promises can do.
Lines 1–3 are a new Promise that waits 1000 milliseconds before resolving itself with the number 10.
Lines 4–7 receive the number 10 from the earlier promise and immediately returns a resolves promise using Promise.resolve();
Lines 8–11 do the exact same thing as the previous .then() callback, but they don’t wrap the result in Promise.resolve(). Returning a value out of a .then() callback and returning Promise.resolve() has the same result.
Lines 12–21 are the fanciest yet. They use Promise.all() to take an array of 10 Promises and wait for all of them to resolve before continuing on. Promise.all() itself returns a Promise, and that Promise resolves with an array of it’s child Promises’ results. See line 23.
Lines 22–25 logs out the array of results from the previous Promise and then throws a rejection with Promise.reject();
Lines 26–28 will never execute, because an earlier link in the *.then() *chain returned a rejected Promise.
Lines 29–31 will catch the rejected Promise in the preceding chain and log it out.
This file relies on two libraries that both emit Promises, *firebase *and axios. The code uses axios to get a list of Star Wars characters from the Star Wars API—SWAPI—and save them to Firebase. It also queries the Firebase collection via the Firebase REST API to get a JSON list of the saved Star Wars characters.
That’s All Folks
There’s not much more to Promises. It’s a small API, and you’ll use it a lot, so read the docs, get comfortable with using Promises instead of callbacks, and let me know if you have any questions!