js的对象，如何实现方法共享，数据分离？

(function(){
    function Person(firstName, lastName, age){ 
        //私有变量： 
        var _firstName = firstName; 
        var _lastName = lastName; 
 
        //公共变量: 
        this.age = age; 
 
        //方法： 
        this.getName = function() { 
            return (firstName + " " + lastName); 
        }; 
        Person.prototype.sayHello = function() { 
            return ("我是" + firstName + " " + lastName); 
        }; 

        var f = function(){
            return _firstName;
        };
        Person.prototype.get = function() { 
            return f;
        }; 
    }; 
     
    var gs = new Person("Bill", "Gates", 53); 
    var jb = new Person("Steve", "Jobs", 53); 
    
    document.write("<br>getName:");
    document.write(gs.getName() + "=" + jb.getName());//结果是正确的,方法分离，所以数据也分离

    document.write("<br>sayHello:");  
    document.write(gs.sayHello()  + "=" + jb.sayHello()); //结果有问题，都是“Steve Jobs”了，主要是方法共享引起，但数据只认最后一次，这是与getName()的方法区别

    document.write("<br>方法:"); 
    document.write(gs.getName ==  jb.getName); //这是false
    document.write(","); 
    document.write(gs.sayHello == jb.sayHello); //这是true
})();

this.sayHello = function(a,b) {
            return function() {
                return ("我是" + a + " " + b)
            };
        }(firstName,lastName);

document.write(gs.sayHello == jb.sayHello);

//上面方法的代码可能变量用错了，可能有误导： 
        this.getName = function() { 
            return (_firstName + " " + _lastName + ":" + this.age); 
        }; 
        Person.prototype.sayHello = function() { 
            return ("我是" + _firstName + " " + _lastName + ":" + this.age); 
        }; 

Person.prototype.sayHello = function() {
            return function() {
                return ("我是" + this.getName())
            };
        }();

Person.prototype.sayHello = function() {
            return ("我是" + this.getName())
        };