Coroutine microimplementation

co.js

export default function co(gen) {
  return function (...args) {
    const iter = Function.prototype.apply.call(gen, this, args);
    const resume = (value) => {
      try {
        const result = iter.next(value);
        const promise = Promise.resolve(result.value);

        return result.done ? promise : promise.then(resume);
      } catch (error) {
        return Promise.reject(error);
      }
    };

    return resume(undefined);
  };
}

example.js

import co from './co';

function sleep(ms, value) {
  return new Promise((resolve) => {
    setTimeout(() => resolve(value), ms);
  });
}

const asyncAdd = co(function* genAdd(a, b) {
  console.log('Sleeping');
  console.log(`Slept for ${yield sleep(3000, '3 seconds')}`);
  return a + b;
});

asyncAdd(3, 4).then(console.log);