A promise library for professionals.

pro.js

function Pro(callback) {
  this.isResolved;
  this.isRejected;
  this.thens = [];
  this.catches = [];

  var resolve = function (data) {
    if (this.isResolved || this.isRejected) return;
    this.thens.forEach(function (fn) {
      fn(data);
    });
    this.isResolved = true;
  }.bind(this);
  
  var reject = function (error) {
    if (this.isResolved || this.isRejected) return;
    this.catches.forEach(function (fn) {
      fn(error);
    });
    this.isRejected = true;
  }.bind(this);
  
  callback(resolve, reject);
}

Pro.prototype.then = function (fn) {
  if (this.isResolved) {
    fn('oops, we lost ur data');
  } else {
    this.thens.push(fn);
  }
  return this;
};

Pro.prototype.catch = function (fn) {
  if (this.isRejected) {
    fn(new Error('sorry, you missed your chance'));
  } else {
    this.catches.push(fn);
  }
  return this;
};

// usage:
var promise = new Pro(function (resolve, reject) {
  if (Math.random() < 0.333333) {
    reject(new Error('1 in 3! Explosion!'));
  }
  window.setTimeout(function () {
    resolve(1);
  }, 2000);
});

promise
  .then(function (data) {
    console.log('resolved with data:', data);
  })
  .catch(function (err) {
    console.error(err.message);
  });