gistfile1.js

// Spawn function inspired by Q.async at http://wiki.ecmascript.org/doku.php?id=strawman:async_functions#reference_implementation 
// Takes an ES6 generator function and produces a WinJS.Promise
function spawn(generatorFunc) {
    return new WinJS.Promise(function (c, e) {
        var generator = generatorFunc();
        var callback = continuer.bind(void 0, 'send');
        var errback = continuer.bind(void 0, 'throw');
        function continuer(verb, valueOrErr) {
            var promisedValue;
            try {
                promisedValue = generator[verb](valueOrErr);
            } catch (err) {
                if (isStopIteration(err)) { return c(err.value); }
                return e(err);
            }
            return promisedValue.then(callback, errback);
        }
        return callback(undefined);
    });
}

// Helpers for the ES6 Generator abstraction - these are provided by the environment in ES6
function StopIteration(value) { this.value = value; }
function isStopIteration(e) { return e instanceof StopIteration; }

// Hand rolled generator implementation - aims to do the same as:
// function* myGen() {
//     console.log('starting');
//     yield WinJS.Promise.timeout(1000);
//     console.log('resuming from 1 sec sleep');
//     yield WinJS.Promise.timeout(2000);
//     console.log('resuming from 2 sec sleep');
//     return "I'm done!!!";
// }
function myGen() {
    var state = 0;
    return {
        send: function (v) {
            switch (state) {
                case 0:
                    console.log('starting');
                    state = 1;
                    return WinJS.Promise.timeout(1000);
                case 1:
                    console.log('resuming from 1 sec sleep');
                    state = 2;
                    return WinJS.Promise.timeout(2000);
                case 2:
                    console.log('resuming from 2 sec sleep');
                    state = -1;
                    throw new StopIteration("I'm done!!!");
            }
        },
    }
}

// Spawn the generator to get back a promise for it's completion
var p = spawn(myGen)

// 3 seconds later, this should get invoked with "I'm done!!!"
p.then(function(x) {
    console.log(x);
})