Tutorial

Promises in JavaScript with ES6 / ES2015

Published on October 31, 2016
Default avatar

By Alligator.io

Promises in JavaScript with ES6 / ES2015

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

Promises are a new feature in the ES6 (ES2015) JavaScript spec that allow you to very easily deal with asynchronous code without resolving to multiple levels of callback functions. Goodbye callback hell!

Here’s how we declare a basic promise. Below you would normally call something asynchronous like an external API for example. Here we’re simulating with a setTimeout function:

let myPromise = new Promise((resolve, reject) => {
  let data;
  setTimeout(() => {
    data = "Some payload";

if (data) {
  resolve(data);
} else {
  reject();
}

Notice how a new promise is initiated with two functions as arguments, a function for its success and a function for its failure. And here’s how you would call the promise to get the payload data from it:

myPromise.then(data => {
  console.log('Received: ' + data);
}).catch(() => {
  console.log("There was an error");
});

The above will log the following at the console:

Received: Some payload

If no data was received, the following would be logged instead:

There was an error

Chaining Promises

Promises can also be chained together with multiple then functions. The return value of a then becomes the value available to the next then in the chain:

myPromise.then(data => {
  console.log('Received: ' + data);
  let moreData = "Another payload";
  return moreData;
}).then(data => {
  console.log(data);
}).catch(() => {
  console.log("There was an error");
});

The above will log the following at the console:

Received: Some payload
Another payload

Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.

Learn more about us


About the authors
Default avatar
Alligator.io

author

Still looking for an answer?

Ask a questionSearch for more help

Was this helpful?
 
Leave a comment


This textbox defaults to using Markdown to format your answer.

You can type !ref in this text area to quickly search our full set of tutorials, documentation & marketplace offerings and insert the link!

Try DigitalOcean for free

Click below to sign up and get $200 of credit to try our products over 60 days!

Sign up

Join the Tech Talk
Success! Thank you! Please check your email for further details.

Please complete your information!

Get our biweekly newsletter

Sign up for Infrastructure as a Newsletter.

Hollie's Hub for Good

Working on improving health and education, reducing inequality, and spurring economic growth? We'd like to help.

Become a contributor

Get paid to write technical tutorials and select a tech-focused charity to receive a matching donation.

Welcome to the developer cloud

DigitalOcean makes it simple to launch in the cloud and scale up as you grow — whether you're running one virtual machine or ten thousand.

Learn more
DigitalOcean Cloud Control Panel