MooTools Promise.Series map/reduce

mapredmoo.js

(function(){ 
  this.Promise = new Class({
    Implements: Class.Thenable,
    initialize: function(promise) {
      if (typeof promise !== 'function')
        throw new TypeError('Promise accepts only a function constructor!');
      try {
        promise(this.resolve.bind(this), this.reject.bind(this));
      } catch (e) {
        this.reject(e);
      }
    }
  });
  this.Promise.Series = new Class({
    Implements: Class.Thenable,
    initialize: function(promises) {
      if (typeOf(promises) != "array") 
        throw new TypeError('Promise Series accepts only an array of functions returning promises as constructor!');
      var ret = new Promise(function(fn, err) {}).resolve(null);
      var results = [];
      return promises.reduce(function(result, promise, index) {
        return result.then(function() {
          return promise().then(function(val) {
            results[index] = val;
          });
        });
      }, ret).then(function() {
        return results;
      });
    }
  });
})();

//USAGE:
new Promise.Series([
    function(){
      return new Promise(function(resolve, reject){
          setTimeout(function(){
              console.log('a resolving.')
              resolve('a');
          },3000);  
       })   
    },
    function(){
        return new Promise(function(resolve, reject){
            setTimeout(function(){
                  console.log('b resolving.')
                  resolve('b');
            },1000);
        })
    }   
    ]).then(function(results){
        console.log('Results in execution order:',results);
    }).catch(function(e){
        console.error('Rejection based on:', e);
});