js 全选问题

<input type="checkbox" id="checkAll"/>全选/全不选
<div id="itemContainer">
    <input type="checkbox" />A
    <input type="checkbox" />B
    <input type="checkbox" />C
</div>
<script type="text/javascript">
window.onload = init;
 
function init() {
    var checkAllNode = document.getElementById("checkAll"),
        itemContainerNode = document.getElementById("itemContainer"),
        items = itemContainerNode.getElementsByTagName("input");
    checkAllNode.onclick = function() {
        for(var i = 0, j = items.length; i < j; i++) {
            items[i].checked = this.checked;
        }
    }
}
</script>