how can i fix it, Simple JavaScript Inheritance problem

how can i fix it, Simple JavaScript Inheritance problem

see this first plz: http://ejohn.org/blog/simple-javascript-inheritance/

I am studying the "Simple JavaScript Inheritance" :
look this sample below:

var Person = Class.extend({
  name:['a','b','c']
});

var Ninja = Person.extend({
  dance: function(){
    return this.name;
  }
});

var c = new Ninja();
var d = new Ninja();
d.name[0]="z";

//result
document.write(c.dance());// z,b,c
document.write(d.dance());// z,b,c

//what i want
document.write(c.dance());// a,b,c
document.write(d.dance());// z,b,c

how can i fix it

sb help me