promises.js

var Promise = function(url) {
  var returned_values = {
    complete : null,
    fail : null
  };

  var xhr = new XMLHttpRequest();
  xhr.open("GET", url, true, null, null);

  this.getData = function() {
    return returned_values.complete;
  }

  this.getError = function() {
    return returned_values.fail;
  }

  this.when = function(callback) {
    xhr.onreadystatechange = function() {
      if(this.readyState == 4) {
        returned_values.complete = JSON.parse(xhr.response);

        callback.call(null);
      }
    }
    
    xhr.onerror = function() {
      returned_values.fail = xhr.statusText;
    }

    xhr.send(null);

    return this;
  }
};