关于function.call()的问题

<html>
<body>
   <script>
    var arry=[1,2,3,4,5,6];
    alert((function(){
     for(var i=0,ret=arry[0]; i<arry.length; i++)
     {
      ret = Math.max(ret,arry[i])
     }
     return ret;
    })());

    alert((function(){ return Math.max.apply(null,arry); })())
    </script>

<input type="button" id="btnSubmit" value="Click me!" />

    <script language="javascript" type="text/javascript">
        function ButtonManager(buttonId, message)
        {
        debugger;
            this._message = message;
            document.getElementById(buttonId).onclick = createDelegate(this,this.ShowMessage); //结果：hello 原因：通过function.call()方法，把当前的函数指针赋给ShowMessage
            //document.getElementById(buttonId).onclick = this.ShowMessage;  //结果：undefined 原因：此时ShowMessage里面的this对象window对象
        }

        ButtonManager.prototype.ShowMessage=function()
        {
            alert(this._message);
        }

        function createDelegate(object, method)
        {
            var delegate = function()
                {
                    method.call(object);
                }
             return delegate;
        }

        var btn = new ButtonManager("btnSubmit", "Hello!");


        var x=10;
    var o ={x:15};
    function foo()
    {
    alert(this.x);
    }
    foo();
    foo.call(o);
    </script>

</body>
</html>

补充：当使用了fn.call(obj)方法以后，fn方法中的this所指向的对象即为obj对象。

//***********************************//

function classA()
     {
      alert(this.msg);
     }
     function classB(name,msg)
     {
      alert(name);
      this.msg = msg;
      classA.call(this);
     }
     classB("kro","我叫什么？")

//***********************************//