jQueryless Waterfall with custom Deferred

waterfall.js

/* Usage
 * waterfall(this, [Promise1, Promise2, ...])
 *  .fail(function(reason){
 *      console.log('failed')
 *  })
 */

export function waterfall(...params) {
    var steps   = [],
        dfrd    = new Deferred(),
        pointer = 0;

    params.forEach(function (i, a) {
        steps.push(function() {
            var args = [].slice.apply(arguments), d;

            if (typeof(a) === 'function') {
                if (!((d = a.apply(null, args)) && d.promise)) {
                    d = new Deferred()[d === false ? 'reject' : 'resolve'](d);
                }
            } else if (a && a.promise) {
                d = a;
            } else {
                d = new Deferred()[a === false ? 'reject' : 'resolve'](a);
            }

            if(d === undefined) {
                dfrd.reject.apply(dfrd, [].slice.apply(arguments));
            } else {
                d.fail(function() {
                    dfrd.reject.apply(dfrd, [].slice.apply(arguments));
                })
                .done(function(data) {
                    pointer++;
                    args.push(data);

                    pointer === steps.length
                        ? dfrd.resolve.apply(dfrd, args)
                        : steps[pointer].apply(null, args);
                });
            }
        });
    })

    steps.length ? steps[0]() : dfrd.resolve();
    return dfrd;
}

// https://stackoverflow.com/a/18097040/997147
function Deferred(){
  this._done = [];
  this._fail = [];
}

Deferred.prototype = {
  execute: function(list, args){
    var i = list.length;

    // convert arguments to an array
    // so they can be sent to the
    // callbacks via the apply method
    args = Array.prototype.slice.call(args);

    while(i--) list[i].apply(null, args);
  },
  resolve: function(){
    this.execute(this._done, arguments);
  },
  reject: function(){
    this.execute(this._fail, arguments);
  }, 
  done: function(callback){
    this._done.push(callback);
  },
  fail: function(callback){
    this._fail.push(callback);
  }  
}