async-5.js

function spawn(genF, self) {
    return new Promise(function (resolve, reject) {
        var gen = genF.call(self);
        function step(nextF) {
            var next;
            try {
                next = nextF();
            } catch (e) {
                // finished with failure, reject the promise
                reject(e);
                return;
            }
            if (next.done) {
                // finished with success, resolve the promise
                resolve(next.value);
                return;
            }
            // not finished, chain off the yielded promise and `step` again
            Promise.resolve(next.value).then(function (v) {
                step(function () { return gen.next(v); });
            }, function (e) {
                step(function () { return gen.throw(e); });
            });
        }
        step(function () { return gen.next(undefined); });
    });
}