var test = function(){
  this.name = 'Lluis';
  this.age = 30;
}

test.prototype = {
  sayMyName: function(){
    console.log(this.name);
  }
}

var obj = new test();

obj.sayMyName(); //prints Lluis

obj.name = 'Alessio';
obj.age = 36;
obj.country = 'Italy';

obj.sayMyName(); //prints Alessio

var str = 'Obj ' + JSON.stringify(obj);
console.log(str); //prints Obj {"name":"Alessio","age":36,"country":"Italy"}