1.1.0 • Public • Published

custom-promise Build Status

A small, useful, secure and customizable A+ promise library.

  • Small: About 500 bytes minified and gzipped. About 300 bytes when built only for A+ compliance.
  • Useful: catch, resolve, reject, all, race, old IE support.
  • Secure: No private state exposed.
  • Customizable: Include only necessary functionality with the Customizer!


Include the fully-featured library with npm:

npm install custom-promise

In Node, load it via require:

var p = require('custom-promise');

In browsers, load it via <script>:

<script src="node_modules/custom-promise/modules/p.script.js"></script>

Alternatively, create a custom build with the Customizer or tools/build.js.

Promise API

Access the promise API through the exported function p and its methods.


Returns a promise. The function executor is immediately called with resolve and reject functions as arguments. Call resolve with one argument to fulfill the promise with that value. Call reject with one argument to reject the promise with that reason.

promise.then(onFulfilled, onRejected)

Registers callbacks to receive a promise's eventual value or the reason why it cannot be fulfilled, and returns a promise resolving with the return value of these callbacks.


Registers just a rejection callback, and returns a promise resolving with the return value of this callback.


Returns a promise fulfilled with value. If value has a then method, it is assumed to be a promise, and the returned promise inherits the state of value.


Returns a promise rejected with reason.


Returns a promise resolving the values in collection. If collection is array-like (has a length property), the promise is fulfilled with an array, otherwise it is fulfilled with an object. Each value in collection must be fulfilled (internally) by p.resolve before the returned promise is fulfilled. If any value in collection is rejected, the returned promise is rejected.


Returns a promise resolving with the first value to resolve in collection via p.resolve (internally). If any value in collection is rejected, the returned promise is rejected.


Use p.resolve to create a promise and then to handle its fulfillment:

p.resolve('Hello World!').then(function (value) {

Manage promises with p(). Here, a promise is created, then randomly fulfilled or rejected, and fulfillment and rejection callbacks handle the outcome:

p(function (resolve, reject) {
  setTimeout(function () {
    if (Math.random() < 0.5) {
    } else {
}).then(function (value) {
}, function (reason) {

Managing promises is often avoidable. Prefer using promises returned by other APIs, or use p.resolve and p.reject to create promises.

Use p.all to await the completion of multiple promises:

]).then(function (results) {
  results.forEach(function (result) {
    // Handle the crowd's excited response
}).catch(function (error) {
  // Recover from a vegetable assault

When order is unimportant, pass an object to p.all instead:

  user: getUser(),
  friends: getFriends()
}).then(function (results) {
  var user = results.user;
  var friends = results.friends;

When only the value of one promise in a set of promises matters, use p.race with an [array-like] object:

]).then(function (result) {
  console.log('Did ' + result + ' as it was faster');

Custom Build API

Programmatically make custom builds with tools/build.js.


Returns a customized implementation of p as a string. The following options are available:

  • catch: Provide the catch method on promises.
  • resolve, reject, all, race: Provide these methods on p.
  • task: Customize the task function. The default is setTimeout. Alternatives like setImmediate or process.nextTick may be used if they will be available globally in target environments.
  • ie: Workaround old IE bugs.
  • node: Export a Node.js module.


Create a custom build with catch and ie support, and save it to build/p.custom.js:

var fs = require('fs');
var build = require('custom-promise/tools/build');
fs.writeFileSync('build/p.custom.js', build({
  catch: true,
  ie: true

Use cases

This library aims to provide reliable promises in as few bytes as possible. It is suited for situations where network latency is a concern (e.g. web browsers) and for users concerned with behavioral correctness. Its small size makes it a good candidate for inclusion within other libraries.

This library is not concerned with competitive performance (minimizing task delay, CPU cycles and memory), as that could cost bytes and compromise security. (However, you can customize the task function with the Customizer.) Other promise libraries may be better suited for especially stressful scenarios.

This library does not provide a polyfill for the Promise constructor or its methods. However, being A+-compliant, the promises are interoperable. Also, Promise and p have approximately the same interface, so this implementation could reasonably substitute for Promise.




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