Simple koa like function

run.js

function run(gen) {

    step();

    function step(value) {
        var result = gen.next(value);
        if (result.value instanceof Promise) {
            result.value.then(step);
        } else if (result.value instanceof Array) {
            Promise.all(result.value).then(step);
        } else if (!result.done) {
            step(result.value);
        }
    }

}