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Something obvious (in retrospect) about ES6 promises

I’ve been pretty excited about the new features of EcmaScript 6 (ES6, or just “modern JavaScript”) for a while, but yesterday it really struck me how entirely different some of them make the experience of writing JS code!

First: A promise1

Promises are one thing that’s new in ES6. They encode, in a neat little state machine, how an asynchronous action might progress. From the mozilla docs:

A Promise is in one of these states:

  • pending: initial state, not fulfilled or rejected.
  • fulfilled: meaning that the operation completed successfully.
  • rejected: meaning that the operation failed.

A short example of using the (equally new) fetch function (and the equally, equally new arrow function syntax) for accessing HTTP content:

    then((response) => response.text()).
    then((txt) => console.log(txt.split("\n")[0]));

Which would make an HTTP request to this blog’s Atom feed, returning a promise; then when that promise resolves with a response, we request the response body, which returns a promise in turn. When the second promise resolves, we print the first line of the body.

At first glance, this is much easier to follow than the callback hell we all had to deal with before. But wait - there’s more!

As you’d expect from a properly asynchronous tool, you can call .then on promises even if they’re resolved. (Because things might happen faster or slower than your computer can execute the next JS statement, of course!)2

And that brings us to the neat thing that I saw for the first time yesterday.


I was pair-programming with somebody yesterday, and we were musing about chaining HTTP requests. We’d written a thing that was firing off all sorts of requests using fetch simultaneously, and waited for them all to resolve using Promise.all. However, we wanted to fire the requests off one after the other.

So, without blinking, my pair writes this code:

    (p, url) =>
        p.then(() => fetch(url).then(handleResponse)),

What. Uh. This does the right thing, but huh? A bunch of insights have let do this short piece of code:

  • Promise.resolve() returns a promise that is already in the resolved state. But as mentioned before, it can have .then called on itself3. And so will every promise returned by .fetch.

  • .then in turn returns a promise, which lets us chain them together.

  • .reduce will run a function across an array’s contents and the previous function’s return value.

And so, using the resolved promise as a zero element, this piece of code gathers up requests, one after the other.

Kinda amazing.

Suddenly, Burritos

I made a promise, but allow me to drift off into maths appreciation briefly: Promises, combined with some algebra (and operators like reduce that take advantage of the algebraic nature of stuff) allow you to express realistically cool things in a tidy way.

I’d encourage you to go forage in the mozilla docs for more ES6 features (fetch alone is worth a lot)! Look for Object.assign and other gems!

But more importantly, think about what you could build if you had a sensible and well-integrated state machine abstraction for your most complex software task.

  1. The promise is that I won’t use the word “monad”4 ↩︎

  2. It’s worth noting that .then also returns a promise. ↩︎

  3. it’s “thenable”, in ES6 parlance, which I find hilarious. ↩︎

  4. Well, oops. This time doesn’t count. Also, you’re reading footnotes. ↩︎