js generators async patterns

function callAsync(fn) {
    // get iterator
    var gen = fn();

    // used as a callback to currently yielded functions 
    function next() {
        // get next function
        var ret = gen.next();

        // recursion exit condition
        if (ret.done) { return; }

        // call async function and pass next() as aa callback so it will ask the iterator to yield next when callback is called
        ret.value(next);
    }

    // initialise recursion
    next();
}

function foo(timeout) {
    return function (done) {
        setTimeout(() => {
            console.log(`done ${timeout}`);
            done();
        }, timeout);
    }
}

callAsync(function* () {
    var a = yield foo(1000);
    var b = yield foo(2000);
    console.log('done all');
});