co-example.js

const sleep1 = (time = 1) => {
  return fn => {
    setTimeout(() => fn(null, time), time * 1000);
  };
}

const sleep2 = (time = 1) => new Promise(resolve => {
  setTimeout(() => resolve(time), time * 1000);
});

co(function* () {
  const a = yield 1;
  console.log('a = ', a);
  const b = yield sleep1(2);
  console.log('b = ', b);
  const c = yield sleep2(3);
  console.log('c = ', c);
  
  return 4;
}).then(res => {
  console.log('done', res);
}, err => console.error('error', err));

co.js

const co = (fn, ...args) => new Promise((resolve, reject) => {
  if (typeof fn !== 'function')
    return reject(new TypeError('must be a function'));
  let value, done, gen = fn.apply(null, args);
  (function next(v) {
    try{
      ({ value, done } = gen.next(v));
    }catch(e) {
      return reject(e);
    }
    const _next = x => (done ? resolve : next)(x);
    switch(true){
      case (typeof value === 'function'):
        value((err, result) => err ? reject(err) : _next(result)); // thunkify style
        break;
      case (typeof value === 'object' && typeof value.then === 'function'):
        value.then(_next, reject); // thenable
        break;
      default: // otherwise
        _next(value);
        break;
    }
  })();
});