Do not

reverse_and_create : function(files){
        var sorted = [], instances =[];
        for(var id in files){
            sorted.push(files[id])
        }
        for(var i = 0; i < sorted.length; i++){
            if (sorted[i].id) {
                instances.push(this.create_as_existing(sorted[i]));
            }
        }
        return instances;
    },

    do_not_reverse_and_create : function(files){
        if(files != undefined){
            var instances =[];
            for(var i = 0; i < files.length; i++){
                if (files[i].id) {
                    instances.push(this.create_as_existing(files[i]));
                }
            }
            return instances;
        }
    } 

What a shame!