co "re-impelented"

co.js

function asyncCalc (num, delay) {
  delay = delay || 500;

  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      resolve(num);
    }, delay);
  });
}

run(function* () {
  var a = yield 4;
  var b = yield asyncCalc(50, 500);
  var c = yield [
    asyncCalc(50, 500),
    asyncCalc(50, 200)
  ];

  return {
    sum: a + b + c[0] + c[1]
  };
})
  .then(function (result) {
    console.log(result);
  });

run.js

function run (fn) {
  var iterator = fn();

  return new Promise(function (resolve, reject) {
    function next (result) {
      if (result.done) {
        resolve(result.value);
        return;
      }

      // is promise
      if (typeof result.value.then === 'function') {
        result.value.then(function (res) {
          next(iterator.next(res));
        }).catch(reject);
      }

      // is array of promise -> paralell
      else if (Array.isArray(result.value)) {
        Promise.all(result.value).then(function (res) {
          next(iterator.next(res));
        }).catch(reject);
      }

      else {
        next(iterator.next(result.value));
      }
    }

    next(iterator.next());
  });
}