function Student(name,age){
  this.name=name;
  this.age=age;
}

function Man(name,age){
  this.name=name;
  this.age=age;
}

var stu=new Student("dc",24);
var man1=new Man("mm",34);

alert (stu instanceof Student);  //返回true

alert(man1 instanceof Student);  //返回false

alert(man1 instanceof Man);       //返回true

alert (typeof(stu));     //返回object

alert(typeof(man1)); //返回 object

<script>
var str=new String("hello");
alert(typeof(str));//返回string(无论引用什么类型都返回的是object)
alert(str instanceof  String);//返回true,明确对象特定类型
</script>

<script type="text/javascript">
var a = function () {};
var b = function () {};
b.prototype = new a;
var c = new b;
var d = new a;
alert(c instanceof a); //true
alert(d instanceof a); //true
alert(a instanceof Function); //true
alert({} instanceof Function); //false
a.prototype = {}; //改变原型链
alert(c instanceof a); //false
alert(d instanceof a); //false
</script>

测试文件名为
demo.htm
<script type="text/javascript">
var a = new window.ActiveXObject('Microsoft.XMLHTTP');
a.open('get', 'demo.htm', false);
a.send(null);
alert(typeof a.responseBody); //unknown
</script>