promises.js

// A function that takes a number and returns a Promise for nothing but
// some logging side effects.
const vow = num => new Promise(resolve => {
  console.log(`start ${num}`);
  setTimeout(() => {
    console.log(`done  ${num}`);
    resolve();
  }, Math.random() * 500);
});

// These promises will all start at about the same time
// and finish in an unpredictable way.
vow(1);
vow(2);
vow(3);

// These promises will each start when the previous one is done.
function* seq() {
  yield vow(4);
  yield vow(5);
  yield vow(6);
}

// A cheap and hacky coroutine.
const run = generator => {

  const iterate = result => result.done
    ? Promise.resolve(result.value)
    : Promise.resolve(result.value)
        .then(p => iterate(generator.next(p)),
              e => iterate(generator.throw(e)));

  try {
    return iterate(generator.next());
  } catch (e) {
    return Promise.reject(e);
  }

};


setTimeout(() =>
  run(seq());
}, 3000);