Edit in JSFiddle

var Starship = function(reg){
    this.registry = "NCC-" + reg;
};

Starship.prototype.spedometer = function() {
     $("#log").append($("<li />").text(this.registry + ' at ' + this.warpFactor_ || 0));
};

var WarpCore = {
    warpFactor_: 0,
    warpFactor: function(num) {
        if(num === undefined) { //Getter
            return this.warpFactor_;
        } else {
            this.warpFactor_ = num;
            return this;
        }
    },
    cruise: function(){
        return this.warpFactor(5);
    }
};

var enterprise = $.extend(new Starship('1701-D'), WarpCore);

enterprise.spedometer();
enterprise.warpFactor(8)
enterprise.spedometer();
enterprise.cruise()
enterprise.spedometer();