关于this关键字 和setInterval的问题

[code]<div id="test">
            <div id="0">aaa</div>
            <div id="1">bbb</div>
            <div id="2">ccc</div>
            <div id="3">ddd</div>
        </div>
<script type="text/javascript">
            function ChildChangeListener(){
                this.lastlength=document.getElementById("test").childNodes.length;
                this.start();
            }
//间隔半秒监听移除事件
            ChildChangeListener.prototype.start=function(){
                setInterval(this.listener,500);
            }
            ChildChangeListener.prototype.listener=function(){
                var ccl=this;
                var currlength=document.getElementById("test").childNodes.length;
                if(ccl.lastlength-currlength!=0){
                    alert(this.lastlength+","+currlength);
                    this.lastlength=currlength;
                }
            }
            function startit(){
                remove();
            }
//间隔1秒移出一个节点
            function remove(){
                var test = document.getElementById("test");
                if(test.childNodes.length>0){
                    test.removeChild(test.firstChild);
                    setTimeout("remove()",1000);
                }
            }
            function add(){
                var div=document.createElement("div");
                div.innerHTML="new div";
                var test = document.getElementById("test");
                test.appendChild(div);
            }
        </script>
        <script type="text/javascript">
            var li=new ChildChangeListener();
        </script>
<input type="button" onclick="startit();" value="delete"></button>
        <input type="button" onclick="add();" value="add"></button>[/code][code]<div id="test">
            <div id="0">aaa</div>
            <div id="1">bbb</div>
            <div id="2">ccc</div>
            <div id="3">ddd</div>
        </div>
<script type="text/javascript">
            function ChildChangeListener(){
                this.lastlength=document.getElementById("test").childNodes.length;
                this.start();
            }
//间隔半秒监听移除事件
            ChildChangeListener.prototype.start=function(){
                this.listener();
            }
            ChildChangeListener.prototype.listener=function(){
            
               var currlength=document.getElementById("test").childNodes.length;
               var num1=this.lastlength+1
                if(num1-currlength!=0){
                    alert(this.lastlength+","+currlength);
                 
                }
            }
            
//间隔1秒移出一个节点
            function remove(){
                var test = document.getElementById("test");
                if(test.childNodes.length>0){
                    test.removeChild(test.firstChild);
                    setTimeout("remove()",1000);
                }
            }
            function add(){
                var div=document.createElement("div");
                 div.appendChild(document.createTextNode("new div"));
                var test = document.getElementById("test");
                test.appendChild(div);
            }
             var li=new ChildChangeListener();
            
            </script>
<input type="button" onclick="remove();" value="delete"></button>
        <input type="button" onclick="add();" value="add"></button>[/code]
上面的2段代码在ChildChangeListener.prototype.start=function(){
                this.listener();}
这里有区别
ChildChangeListener.prototype.start=function(){
                setInterval(this.listener,500);            }
为什么第一段代码,显示this.length是UNDERFIND