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Promises/A+ logo Build Status Promise.js

A Promises/A+ library, originally developed for the ArangoDB client.

Main features

  • Conforming to the Promises/A+ specification.
  • Small footprint, no other dependencies or frameworks.
  • Simple design, fast, easy to understand, integrate & debug.
  • Runtime independent so that it works in browsers, nodejs and perhaps even in rhino.
  • Supports CommonJS and AMD loaders, or exports a global Promise().
  • Flexible fulfillments that allows multiple fulfillment values catched with then(value) or spread(arguments).
  • Easy to create promised wrappers by using when(task) to bundle functions/processes into a promise.


Include this promise.js library into your project using an AMD or CommonJS module loader. Without any loader the library gets exported as a global Promise(). A minified built file of the library is included in dist/promise.js.


Example a simple webpage using promises.

<!doctype html>
        <script src="promise.js"></script> 
    <h1 id="h1"></h1>
    <h2 id="h2"></h2>
    <h2 id="h3"></h2>
        document.getElementById('h1').innerHTML = "Fulfill a promise";
            var doc = document.getElementById('h2');
            doc.innerHTML = a;
            return "World!";
            var doc = document.getElementById('h3');
            doc.innerHTML = b;

Example of how to use promises for Ajax.

<!doctype html>
        <script src="./promise.js"></script> 
    <h1 id="h1"></h1>
    <h2 id="h2"></h2>
    <div id="content"></div>
        document.getElementById('h1').innerHTML = "Ajaxed promise";
        function getAjax(url) {
         var promise = new Promise(),
         xhr = new XMLHttpRequest, timer;
            /* This will be called after getAjax(...) returns */
            xhr.onreadystatechange = function() {
             if(xhr.readyState === 4 && xhr.status) {
             if(xhr.status > 399) promise.reject(xhr.statusText);
                    else promise.fulfill(xhr.responseText);   
            /* send an asynchronous XmlHttp GET request */
            /* set a timeout that fires after 5 secs */
            timer = setTimeout(function() { 
                promise.reject("operation timedout!"); 
            /* return this promise */
         return promise;
        /* When the response arives it calls either the fulfilled block or the rejected block. */
            /* validates JSON data */
            var data = JSON.parse(value);
         document.getElementById('h2').innerHTML = "Fulfilled";
            document.getElementById('content').innerHTML = JSON.stringify(data);
         document.getElementById('h2').innerHTML = "Rejected";
            document.getElementById('content').innerHTML = reason;
/* chaining promises, (undefined blocks are ignored) */
promise = new Promise();
    return value + 1;
    if(value < 5) throw "value is to small";
    console.log("value:", value);
    /* catch thrown error here */
/* fulfill with a random value */
/* Multiple fulfillment values can be placed inside an array */
promise().fulfill([1,2,3]).then(function(value) {
    console.log("a(%s) b(%s) c(%s)", value[0], value[1], value[2]);
/* Use spread() to receive the individual elements as arguments */
promise().fulfill([1,2,3]).spread(function(a,b,c) {
    console.log("a(%s) b(%s) c(%s)", a, b, c);
/* You can also fulfill with multiple fullfilment arguments. */
promise().fulfill("abc",123,{abc:123}).spread(function(a,b,c) {
    console.log("a(%s) b(%s) c(%s)", a, b, c);

Example how to create a promise around a process making it run asynchronously in your code.

var promisedSync = Promise().when(function(){
    var retval = syncProc();
    /* if syncProc() throws the error will */
    /* end up in the rejection handler.    */
    if(retval) this.fulfill(retval);
    else this.reject("Operation failed");
/* Process the result whenever its ready */

You may also wrap a promise around multiple functions or promises.

var tasks = [];
    console.log("Tasks returned", values);
}, function(error){
    console.log("Tasks failed", error);

Test & build

Install the development requirements using npm i -d. Then run make for executing the tests and building/minifying into dist.


Copyright (c) 2012 Kaerus (, Anders Elo <anders @ kaerus com>.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.