spawn-logging.js

function spawn(gen) {
  const it = gen();

  function _co(method, arg) {
    let res;

    try {
      console.log('***TRY***', method, arg);
      res = it[method](arg);
    } catch(err) {
      console.log('***CATCH***', err.message);
      return Promise.reject(err);
    }

    if (res.done) {
      if (method === 'throw') {
        console.log('***DONE THROW***', arg);
        return arg;
      } else {
        console.log('****DONE VAL***', res.value);
        return res.value;
      }
    } else {
      return Promise.resolve(res.value)
        .then((val) => {
          console.log('***THEN RESOLVE***');
          return _co('next', val);
        }, (err) => {
          console.log('***THEN ERR***', err);
          return _co('throw', err);
        });
    }
  }

  const ret = _co('next');
  console.log('***RET SPAWN PROMISE***', typeof ret.then);
  
  return ret;
}