为啥使用obj.style[filter]在浏览器里会报错，obj.style[filter]和obj.style.filter不是一样的吗？报错说obj.style[filter]未定义？

if (atr=="opacity") {

obj.style[atr]=(cur+speed)/100;

console.log(obj.style.filter);

console.log(obj.style[filter]);

obj.style.filter='alpha(opacity:'+cur+speed+')';

  //obj.style[filter]='alpha(opacity:'+cur+speed+')';

}else{

obj.style[atr]=cur+speed+"px";

}