obj.变量=？ 应该怎么理解？详情见示例代码注释部分！

<script type="text/javascript">
	window.onload=function(){
		var oDiv=document.getElementsByTagName('div');
		for(var i=0,leg=oDiv.length;i<leg;i++){
			oDiv[i].alpha=30;//此处应该怎么理解？是给oDiv自定义属性？还是声明变量？求大神解疑答惑！
			//oDiv[i].timer=null;
			oDiv[i].onmouseover=function(){
				startMove(this,100);
			}
			oDiv[i].onmouseout=function(){
				startMove(this,30);
			}
		}
	}
	function startMove(obj,iTarget){
		clearInterval(obj.timer);
		obj.timer=setInterval(function(){
			var speed=0;
			if(obj.alpha>iTarget){
				speed=-10;
			}else{
				speed=10;
			}
			if(obj.alpha==iTarget){
				clearInterval(obj.timer);
			}else{
				obj.alpha+=speed;
				obj.style.filter='alpha(opacity:'+obj.alpha+')';
				obj.style.opacity=obj.alpha/100;
			}
		},30)
	}
</script>