var Obj = function(a){ this.a = a;};
Obj.prototype.test = function(b){ return ++b; };

var Obj2 = function(){

    //actual function returned by call
    function MyConstructor( a ){ this.a = a; }

    //a function we'll use on this object
    function test(b){ return ++b; }

    //get the prototypa reference
    var _p = MyConstructor.prototype;

    //assign as constructor
    _p.constructor = MyConstructor;

    //assign the function to the prototype
    MyConstructor.prototype.test = test;

    //now Obj2 = Myconstructor
    return MyConstructor;
}();// <- call

function main(){

    var i, o, v, nb = 10000000;

    console.time( "obj1" );
    v = 0;
    for(i=0;i<nb;i++)
    {
        o = new Obj( "a" );
        v = o.test(v);
    }
    console.timeEnd( "obj1" );
    console.log( "v = ", v );

    console.time( "obj2" );
    v = 0;
    for(i=0;i<nb;i++)
    {
        o = new Obj2( "a" );
        v = o.test(v);
    }
    console.timeEnd( "obj2" );
    console.log( "v = ", v );

}
main();