Demystifying Async Programming in Javascript - co() simple solution

co-simple-solution.js

function co(generator) {
  return new Promise((resolve, reject) => {
    const g = generator();

    function next(nextVal) {
      const ret = g.next(nextVal);
      if (ret.done) {
        return resolve(ret.value);
      }

      ret.value.then(next);
    }
    next();
  });
}