js 多层数量加减，不小于0

<script type="text/javascript">
function disNum(dom){
var a = dom.nextSibling.nextSibling.innerHTML;
if(a==0){
return;
}
dom.nextSibling.nextSibling.innerHTML--;
}

function incNum(dom){
dom.previousSibling.previousSibling.innerHTML ++;
}

</script>