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JavaScript Promises 101

Dalibor Belic
Hi there šŸ‘‹ Do you like jazz? I do. Never mind. I'm a full-stack developer, tech enthusiast, former entrepreneur, musician; virtually a generalist. I'm keen on MERNG Stack and the concept of hooks.
惻2 min read

Hey there! Are you new to JavaScript? Well, this post is right for you! Mastering promises is a must if you want to become a JavaScript developer!

A Promise represents a value that's unknown now that may become known in the future; in other words an asynchronous value.
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A Promise is always in one of these states:

  • pending: initial state, neither fulfilled nor rejected.
  • fulfilled: meaning that the operation was completed successfully.
  • rejected: meaning that the operation failed.

Now, let's imagine you're hungry (I am now actually) and you're about to order food using a delivery app. You open the app. You find what you want and click order. At that moment, the restaurant/app makes a Promise that they will deliver you food. While you're waiting, the delivery is pending. In the future, if everything goes according to plan the restaurant/app will resolve to deliver you food at which point your order has been fulfilled. But in some cases, the restaurant/app might reject your order in which case you'll have to order something else. Either way, the original request is finally settled.

More technical explanation

Now, let's explain it in a more technical language. As a developer, you create a Promise to represent an asynchronous value. But, what you'll actually do more often is consuming promises to use the result of an asynchronous operation in your code.

Let's create a Promise.

const food = new Promise((resolve, reject) => {
  if (delivered) {
    resolve("food delivered šŸ„˜");
    // resolve fulfills promise with passed value
  } else {
    reject("you're still starving... šŸ˜­");
    // reject triggers when operation fails
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*šŸ‘‰ executor, f-on that resolves a value or rejects (error)

Now let's consume the Promise.

  .then(value => {
    console.log(value); // food delivered šŸ„˜
  .catch(error => {
    console.log(error); // you're still starving... šŸ˜­
  .finally(() => {
    console.log("all settled!");
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then is a function that handles fulfilment. catch handles rejection; catches the error. And finally, finally is there if you want to run some code no matter what.

I hope this helped you get the basic knowledge, an overview of JavaScript Promises :)

As always, any feedback is greatly appreciated!

Have a great one,

Discussion (5)

meo3w profile image
Phil Hasenkamp

Thank you for sharing! This definitely invites further exploration and use once Iā€™m more experienced. Iā€™m fairly new to JavaScript so Iā€™m still in love with it.

daliboru profile image
Dalibor Belic Author

Good luck with mastering JavaScript, you'll be in love with it even more! And don't give up!

meo3w profile image
Phil Hasenkamp

Thank you! It's really an amazing language. I did my first Stack Overflow answer yesterday dealing with some of the basics of selection. I can't wait to read more of your contributions here!

amtel2013 profile image
Amtel Website Design

I have a question "what happens to finally if the promise fails?"

daliboru profile image
Dalibor Belic Author

It runs when the promise is settled, no matter the outcome.

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