prms.js

Small promises polyfill (MDN), plus .callback() enhancement

This is a fairly small implementation of Promise (1413 bytes minified, about half that gzipped).

In the browser, if Promise is not in the global scope, it provides an implementation that matches the MDN Promise API, and is A+ compatible.

When included as a module, either the native implementation or the polyfill are returned (and the global scope is not modified).

The following methods are added to either the native implementation or the polyfill:

Promise.callback(function (callback) {...})

An alternative way to construct promises, that gives you a callback function instead of pass/fail. This lets you return a Promise while using async APIs:

var Promise = require('prms');

return Promise.callback(function (callback) {
    fs.readFile(filename, {encoding: 'utf-8'}, callback);
});

Promise.prototype.callback(callback)

This is the equivalent to .then(), but for Node.js-style callbacks. This lets you implement an async API that uses Promises internally. It also makes it easy to construct hybrid promise/callback APIs:

var Promise = require('prms');

function myAsyncApi(arg0, callback) {
    var promise = Promise.resolve(somePrmsApi(...)); // wraps any thenable in Promise
    return promise.then(function (result) {
        return somethingElse(result);
    }).callback(callback);
}

The callback argument has no effect on the result of the promise. In fact callback can be omitted - therefore in the above example, the API can be used in either an async or Promise-y manner.

Because callback is called on both resolution and rejection, it is also a convenient way to perform cleanup:

var Promise = require('prms');

var connection = openSomething();
Promise.resolve(somePromiseApi(...)).then(function (result) {
    ...
}).callback(function () {
    // cleanup code
    connection.close();
});

A series of callbacks will be called in the order they are registered, but their return values are ignored - this means that if your cleanup is asynchronous, it will overlap with subsequent callbacks.