File: promise/js/when.js

            /**
            Abstraction API allowing you to interact with promises or raw values as if they
            were promises. If a non-promise object is passed in, a new Resolver is created
            and scheduled to resolve asynchronously with the provided value.
            
            In either case, a promise is returned.  If either _callback_ or _errback_ are
            provided, the promise returned is the one returned from calling
            `promise.then(callback, errback)` on the provided or created promise.  If neither
            are provided, the original promise is returned.
            
            @for YUI
            @method when
            @param {Any} promise Promise object or value to wrap in a resolved promise
            @param {Function} [callback] callback to execute if the promise is resolved
            @param {Function} [errback] callback to execute if the promise is rejected
            @return {Promise}
            **/
            Y.when = function (promise, callback, errback) {
                promise = Promise.resolve(promise);
            
                return (callback || errback) ? promise.then(callback, errback) : promise;
            };