循环传值_闭包

<p id="btn1" itemid='1'>1</p>
    <p id="btn2" itemid="2">2</p>
    <p id="btn3" itemid="3">4</p>
    <p id="btn4" itemid="4">4</p>
    <button id="subbtn">提交</button>
    <script type="text/javascript">
    var btn = document.getElementsByTagName('p');
    var itemid = [];
     
    for(var i=0,len=btn.length;i<len;i++)
    {
        
        (function(index){
        var  _this = btn[index];
            _this.onclick = function(){
            _this.style.background = '#ccc';
            if(_this.className!=='checked')
            {
                
            }
            _this.className = 'checked';
            itemid.push(_this.getAttribute('itemid')); 
        }})(i)
    }
    subbtn.onclick = function(){
        alert(itemid.join(','));
    }
    </script>