第二个函数有问题求大神指导

<!DOCTYPE html>
<html>

<head>
<meta charset="UTF-8">
<title></title>
<script>
var j = 1

function changeColor() {
var tabl = document.getElementById("tabl");
//获得表的行数
var count = tabl.tBodies[0].rows.length;
for(var i = 0; i < count; i++) {
//遍历每行
if(i % 2 == 0) {
//偶数行
tabl.tBodies[0].rows[i].style.backgroundColor = "blue";
} else {
//奇数行
tabl.tBodies[0].rows[i].style.backgroundColor = "red";
}
}
}

function checkAll() { //写第二个函数后，第一个函数的效果不显示？？？
//得到总复选框的控制权
var selectAll = document.getElementById("selectAll");
//判断总复选框是否被选择
var ids = document.getElementsByName("ids");
var len = ids.length;
if(selectAll.checked == true) {
//已被选择，将下面的复选框选择
for(var u = 0; u < len; u++) {
ids[u].checked = true;
} else {
//未被选择，将下面的复选框选择
for(var u = 0; u < len; u++) {
ids[u].checked = false;
}
}
}
}
</script>
</head>

<body onload="changeColor()">
<table border="1" width="80%" align="center" id="tabl">

<thead>
<tr>
<td>
<input type="checkbox" id="selectAll" name="ids1" onclick="checkAll" />
</td>
<th>分类的ID</th>
<th>分类的名称</th>
<th>分类的描述</th>
<th>操作</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>1</td>
<td>手机数码</td>
<td>手机数码</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>2</td>
<td>电脑办公</td>
<td>电脑办公</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>3</td>
<td>烟酒糖茶</td>
<td>烟酒糖茶</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>4</td>
<td>笔记本</td>
<td>笔记本</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>5</td>
<td>美女</td>
<td>美女</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>6</td>
<td>五金用品</td>
<td>五金用品</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
<tr>
<td>
<input type="checkbox" name="ids" />
</td>
<td>7</td>
<td>饮料</td>
<td>饮料</td>
<td>
<a href="">修改</a>|
<a href="">删除</a>
</td>
</tr>
</tbody>
<tr>
<td colspan="5">
<input type="button" name="add" value="添加" />
<input type="button" name="del" value="删除" />
</td>
</tr>
</table>
</body>

</html>