Promises in Javascript: I’ll tell you about it later

Beginner Javascript coders use promises for Ajax calls and the like, but we don’t really understand what is going on under the hood. I recently watched Kyle Simpson’s Advanced Javascript course on Pluralsight and, among other things, took a bit of a deep-dive into promises. Naturally, I still didn’t understand what the hell was going on, so I wrote this blog post.

What is a Promise?

Simply put, a promise is a placeholder that waits until it receives more information to continue. Kyle had a brilliant metaphor of buying fast food. You give them money, they give you a receipt (a promise), and then later when your food is ready, you exchange that receipt for your dinner. Promises work the same way. You tell them when they can expect to be “resolved” and when they are, they continue with whatever it was you told them to continue with.

You can instantiate a promise like so: const myPromise = new Promise(resolve, reject) where resolve is a function you want to happen when the promise gets its data and reject is what to do if the Promise receives an error.

Resolving Promises

It’s easier to think backwards a bit. Your resolve function is what you want to happen when your promise is resolved. Let’s have something stupid-simple:

function resolveIt (resolve) {
console.log('resolving in 5 seconds');
setTimeout(() => {
}, 5000);

In five seconds, our program will resolve the promise and log to the console. Note that we are ignoring the reject parameter — our program will fail silently if it fails.

Now we can stick it into a promise:

const promise = new Promise(resolveIt);

promise actually returns a lot of useful information:

> Promise {[[PromiseStatus]]: "pending", [[PromiseValue]]: undefined}

Note that it is pending and it will continue to be pending until that resolve function is called. Note also that we did not have to pass in resolve and reject parameters to our resolveIt function — the Promise seems to take care of that automatically.

So great. We wait five seconds, promise is resolved and our return statement looks like:

> Promise {[[PromiseStatus]]: "resolved", [[PromiseValue]]: undefined}

But what if we wanted to do something as soon as that promise was resolved? Say, we had requested information from a database or an API and needed our program to execute some functions after that data was returned. Enter .then().

then is then

.then() is a super-handy function that runs when a promise is resolved.

If we wait our five seconds and run .then() on our promise, it will trigger immediately — we are running a callback on an already-resolved promise:

promise.then( () => { console.log('then') } )

(I’m using ES6 syntax here. If that’s confusing, you may as well stop now and research fat arrow functions because this stuff is everywhere.)

That will just go ahead and log then immediately.

But if we are very quick, and run that line before our five seconds is up, the then log will wait until the five seconds has elapsed.

Of course, in the real world, we don’t have to jump in quick before we set the .then() — we wouldn’t wait to call the .then() function at all, but just chain it naturally onto the promise. I want to demonstrate here that our promise is “pending” during those five seconds.


I can make all of that much shorter:

const promise = Promise.resolve('yo!')
.then(() => { console.log('success!')})

We immediately call the resolve() function, and since we resolved promise, then .then() call runs immediately.

Note also that we can see the .reject() behavior in this format:

const rejected = Promise.reject('yo!')
.then(() => { console.log('failure!')})

Note that this throws an error because Javascript expects you to handle the rejection. It complains that you do nothing with 'yo!'.

One more note: the arguments passed to the resolve() and reject() functions will automatically pass down into the .then():

const promise = Promise.resolve('yo!')
.then((text) => { console.log(text)})

logs yo!.

And last-last note, if you are going to immediately resolve or reject your promise, skip all of that nonsense and just get right to the execution. This is for breakdown purposes.

Further Reading

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