JS控制复选框全部选中和全部不选中

<input type="checkbox" id="select_all" style="margin-left:20px">全选
<br/>
<div style="height:420px;overflow-y:scroll">
<table id="con_table" style="width:100%">
<tr style="background-color: #eee">
<td title="ischoose">选择</td>
</tr>
<tr>
<td>
<input type="checkbox"/>
</td>
<td>
<input type="checkbox"/>
</td>
</tr>
</table>
</div>

//全选和全不选
$("#select_all").bind("click", function () {
var chk = this.checked;
var ck_list = $("#con_table").find("input[type='checkbox']");
var len = ck_list.length;
for (var i = 0; i < len; i++) {
if (chk == false) {
ck_list[i].checked = false;
}
else {
ck_list[i].checked = true;
}
}
});