var Greeter = function(who){
  var toGreet = who;

   this.sayHello = function(){
    alert("Hello, "+ toGreet + "!");
  }
};

var greeter1 = new Greeter("World");
var greeter2 = new Greeter("John");
greeter1.sayHello(); //-> Hello World!
greeter2.sayHello(); //-> Hello John!