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Nyala is a promise library for javascript. It was partially inspired by an article by Michael Fogus. It is written in CoffeeScript, and targets both the browser and node.js. For more information, see the manual, or look at some examples.

Although written in CoffeeScript, Nyala explicitly targets both JavaScript and CoffeeScript environments, this readme uses JavaScript, but the manual also offers CoffeeScript.


Promises are a way to deal with asynchrony in your code. JavaScript, by nature, lends itself very well to writing asynchronously structured applications. Promises help you manage them. Instead of passing callbacks to all functions that will perform an asynchronous action, you retrieve a Promise from them and work with that, making it easier to break your application up into manageable chunks. The following example

someFunc('foo', 'bar', function(error, data) {
    if (!error) {
    } else {

could become

promise = new Promise(function(keepCallback, breakCallback) {
    someFunc('foo', 'bar', function(error, data) {
        if (error) {
        } else {



While this may seem more tedious, at first, there are several benefits to this approach. For instance, Promises can be passed around in your application, and, if applicable, be reused. It is also possible to assign more than one kept handler.

Promise Chaining

When doing asynchronous things in series, you'll end up indenting for every link in the chain. It is not uncommon to see javascript like this:

foo('foo', function(foo) {{
        bar.baz(function(quux) {

With more than a couple of steps in your chain this quickly becomes messy and unmanageable. Nyala offers you a PromiseChain class, which executes all promises serially. The previous example could become:

chain = new PromiseChain;

chain.add(function(keepCallback) {
    foo('foo', keepCallback);
}).add(function(bar, keepCallback) {

chain.kept(function(quux) {


Just like a Promise, these chains can be passed around, and different kept handlers can be attached. Since every step in the chain is also a separate Promise, they can be managed separately, not violating Demeter's law.


Another problem with asynchronous code is what to do when you need the results of a couple of functions, but those do not depend on one another. In node you might for instance need to get the contents of a couple of files, in no particular order.

You could chain the calls, but that would not be very efficient. Keeping track of their respective states manually is also rather strenuous. Nyala lets you group Promises into "bursts". A burst will execute all Promises at once, and "keep" its own promise, when all promises keep theirs.

burst = new PromiseBurst;

burst.add(function(keep) {
}).add(function(keep) {

// You can also add Promises, that already have a `kept` handler
promise = new Promise(function(keep) {
    // Do something useful with the results

burst.kept(function() {
    // All three have been executed
    // you can use burst.eachResult(function(){}); to get at the results