my_promise_impl.js

function MyPromise(task) {
  this._state = 'init';
  this._resolve_handler = null;
  this._reject_handler = null;
  this._resolved_value = null;
  this._rejected_value = null;
  this._resolve = this._resolve.bind(this);
  this._reject = this._reject.bind(this);
  task(this._resolve, this._reject);
  return this;
}

MyPromise.resolved = function (val) {
  const p = new MyPromise(res=>res(val));
  p._state = 'resolved';
  return p;
}

MyPromise.prototype = {
  _resolve: function(val) {
    this._state = 'resolved';
    this._resolved_value  = val;
    if (this._resolve_handler) {
      this._resolve_handler(val);
    }
  },
  _reject: function(err) {
    this._state = 'rejected';
    this._rejected_value = err;
    if (this._reject_handler) {
      this._reject_handler(err);
    }
  },
  then: function(resolved, rejected) {
    return new MyPromise((res, rej)=> {
      if (this._state === 'init') {
        this._resolve_handler = function (val) {
          res(resolved(val));
        };  
        this._reject_handler = function (err) {
          res(rejected(err));
        }
      } else if (this._state === 'resolved') {
        res(resolved(this._resolved_value));
      } else if (this._state === 'rejected') {
        res(rejected(this._rejected_value));
      }  
    });
  }
}


p = new MyPromise((res, rej)=> {
  setTimeout(()=> {
    rej('hello')
  })
});
p.then(val => {
  console.log('console.log('+val+')');
  return 123;
}, err=> console.log(err)).then(console.log)