function Person(national,age)
    {
      this.age = age;  //实例对象，每个示例不同
      Person.national = national;  //类对象,所用实例公用
      var bb = 0; //局部变量，外面不能访问（类似局部函数）
    }

var p = new Person("中国", 29);
      document.writeln("age:" + p.age);
      document.writeln("object national:" + p.national);
      document.writeln("Class national:" + Person.national);
      document.writeln("local var:" + p.bb);

      var p2 = new Person("美国", 31);
      document.writeln("</br>");
      document.writeln("age:" + p2.age);
      document.writeln("object national:" + p2.national);
      document.writeln("Class national:" + Person.national);
      document.writeln("local var:" + p2.bb);

      document.writeln("</br>");
      document.writeln("Class national:" + Person.national);
      //age:29 object national:undefined Class national:中国 local var:undefined 
      //age:31 object national:undefined Class national:美国 local var:undefined 
      //Class national:美国