Promise JS

global.js

new Promise(function (reject) {

}).then(function (reject) {
  reject('this is an error');
}).then(function () {
  console.log('never run');
}).catch(function (err) {
  console.log('err', err);
}).finally(function () {

});

promise.js

function Promise (func) {
  this.hasError = false;
  this.error = null;

  var that = this,
    reject = function (err) {
      that.hasError = true;
      that.error = err;
    };

  (func || function(){})(reject);
}

Promise.prototype.then = function (func) {
  var that = this,
    reject = function (err) {
      that.hasError = true;
      that.error = err;
    };

  if (!this.hasError)
    (func || function(){})(reject);

  return this;
};

Promise.prototype.catch = function (func) {
  if (this.hasError)
    (func || function(){})(this.error);

  return this;
};

Promise.prototype.finally = function (func) {
  (func || function(){})();

  return this;
};