document.getElementsByClassName获取的元素集合拿不到id?

Question

<div class="a">das</div>
<div class="a">fsdg</div>

var del=document.getElementsByClassName('a');

for(var i=0;i<del.length;i++){
   console.log(del[i].id);
}

console.log(del[i].id);为什么打印不出id?