gistfile1.txt

var Test = (function () {
    function Test(){
        this.value = "base";
    };
    Test.prototype = {
        method : function(){
            console.log(this.value);
        }
    };
    return Test;
})();

// mixin
(function () {
    Test.prototype.___method = Test.prototype.method;
    Test.prototype.appendMethod = function(){
        console.log("append");
    };

    Test.prototype.method = function(){
        this.___method();
        console.log("override");
    };
})();

var object = new Test();
object.appendMethod();
object.method();