Naive Promise

naivePromise.js

// creates a promise object which will either resolve or reject
const Promise = function (executor) {    
  const _thens = [];
  let _catch;

  const promise = {
    then: cb => {
      _thens.push(cb);
      return promise;
    },
    catch: errHandler => {
      _catch = errHandler
      return promise;
    }
  }

  const fulfill = data => {
    while (_thens.length) {
      try {
        data = _thens.shift()(data);
      } catch (err) {
        _catch && _catch(err);
        break;
      }
    }
  };

  const reject = err => {
    _catch && _catch(err);
  };

  executor(fulfill, reject);

  return promise;  
};

// TEST
Promise((fulfill, reject) => {
  setTimeout(() => {
    if (Math.random() < 0.5) {
      fulfill('this is the value');
    } else {
      reject('err');
    }
  }, 100)
})
  .then(val => {
    console.log(val);
    return 'i am chainable';
  })
  .then(val => {
    console.log(val);
  })
  .catch(console.error)