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⚠️ Warning: The behavior described here is likely to be quickly obseleted by developments in standardization and implementation. Tread with care.

Q has an async function. This can be used to decorate a generator function such that yield is effectively equivalent to await or defer syntax as supported by languages like Go and C# 5.

Generator functions are presently on the standards track for ES6. As of July 2013, they are only fully supported by bleeding edge V8, which hasn't made it out to a released Chromium yet but will probably be in Chromium 29. Even then, they must be enabled from chrome://flags as "Experimental JavaScript features." SpiderMonkey (used in Firefox) includes an older style of generators, but these are not supported by Q.

Here's an example of using generators by themselves, without any Q features:

function* count() {
    var i = 0;
    while (true) {
        yield i++;

var counter = count();
counter.next().value === 0;
counter.next().value === 1;
counter.next().value === 2;

yield can also return a value, if the next method of the generator is called with a parameter:

var buffer = (function* () {
    var x;
    while (true) {
        x = yield x;

buffer.next(1).value === undefined;
buffer.next("a").value === 1;
buffer.value(2).value === "a";
buffer.next().value === 2;
buffer.next().value === undefined;
buffer.next().value === undefined;

Inside functions wrapped with Q.async, we can use yield to wait for a promise to settle:

var eventualAdd = Q.async(function* (oneP, twoP) {
    var one = yield oneP;
    var two = yield twoP;
    return one + two;

eventualAdd(eventualOne, eventualTwo).then(function (three) {
    three === 3;

You can see more examples of how this works, as well as the Q.spawn function, in the other files in this folder.