es6-promise

What is es6-promise?

The es6-promise package is a polyfill for the ECMAScript 6 Promise. It provides a way to handle asynchronous operations in JavaScript by allowing you to associate handlers with an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future.

What are es6-promise's main functionalities?

Creating a Promise

This feature allows you to create a new Promise object. The constructor of Promise takes a function that takes two arguments, resolve and reject, which are functions themselves. You perform the asynchronous operation within this function and call resolve upon successful completion with the result, or reject with an error.

var promise = new Promise(function(resolve, reject) {
  // do something asynchronous which eventually calls either:
  //   resolve(someValue); // fulfilled
  // or
  //   reject("failure reason"); // rejected
});

Using a Promise

This feature demonstrates how to use a Promise. Once a Promise has been created, you can attach success and failure handlers to it using the .then method. The first function passed to .then is called if the Promise is resolved, and the second is called if it is rejected.

promise.then(function(value) {
  // success
}, function(value) {
  // failure
});

Chaining Promises

Promises can be chained, meaning that the result of one promise can be used to create another promise, forming a chain of promises. This is useful for performing a series of asynchronous operations in sequence.

new Promise(function(resolve, reject) {
  setTimeout(() => resolve(1), 1000);
}).then(function(result) {
  console.log(result); // 1
  return result * 2;
}).then(function(result) {
  console.log(result); // 2
  return result * 2;
}).then(function(result) {
  console.log(result); // 4
  return result * 2;
});

Other packages similar to es6-promise

bluebird

Bluebird is a fully featured promise library with focus on innovative features and performance. It is known for its rich API and being one of the fastest promise libraries. Compared to es6-promise, Bluebird offers more utilities such as .map, .reduce, and .filter for promises, making it a more comprehensive solution for handling asynchronous operations.

q

Q is an earlier promise library that influenced many aspects of the Promise specification. It provides a robust set of features for creating and composing promises. While es6-promise aims to provide a polyfill for native ES6 Promises with minimalistic API, Q offers additional features like promise cancellation, better error handling, and progress notifications.

README

ES6-Promise (subset of rsvp.js)

This is a polyfill of the ES6 Promise. The implementation is a subset of rsvp.js, if you're wanting extra features and more debugging options, check out the full library.

For API details and how to use promises, see the JavaScript Promises HTML5Rocks article.

Downloads

• es6-promise
• es6-promise-min

Node.js

To install:

npm install es6-promise

To use:

var Promise = require('es6-promise').Promise;

Usage in IE<9

catch is a reserved word in IE<9, meaning promise.catch(func) throws a syntax error. To work around this, you can use a string to access the property as shown in the following example.

However, please remember that such technique is already provided by most common minifiers, making the resulting code safe for old browsers and production:

promise['catch'](function(err) {
  // ...
});

Or use .then instead:

promise.then(undefined, function(err) {
  // ...
});

Auto-polyfill

To polyfill the global environment (either in Node or in the browser via CommonJS) use the following code snippet:

require('es6-promise').polyfill();

Notice that we don't assign the result of polyfill() to any variable. The polyfill() method will patch the global environment (in this case to the Promise name) when called.

Building & Testing

• npm run build to build
• npm test to run tests
• npm start to run a build watcher, and webserver to test
• npm run test:server for a testem test runner and watching builder

Changelog

2.3.0

• #121: Ability to override the internal asap implementation
• #120: Use an ascii character for an apostrophe, for source maps