<script type="text/javascript">
function co(gen, func) {
    if(!func) return new Promise(ok => co(gen, ok));
    var run = function(arg) {
        var g = gen.next(arg), v = g.value;
        g.done ? func(v) : v(run);
    }; run();
}

function* test() {
    yield ok => setTimeout(ok, 1000);
    document.write("Started");
    yield ok => setTimeout(ok, 1000);
    document.body.innerHTML += "<br />Ended";
    yield ok => setTimeout(ok, 10);
    return "Over";
};
co(test()).then(alert);
</script>