co in 16 lines

co.js

function co(genFunc) {
  let genObj = genFunc();
  run();

  function run(promiseResult = undefined) {
    let item = genObj.next(promiseResult);
    if (!item.done) {
      // A promise was yielded
      item.value
        .then(result => run(result))
        .catch(error => {
          genObj.throw(error);
        });
    }
  }
}