JS控制复选框全部选中和全部不选中

html代码：

<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>

JS代码：

//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;
        }
    }
});