本人js新手，编写 按下回车键时自动切换焦点 功能时，出现一些错误，不知道哪出错，请前辈指出，谢谢

<table width="86%" align="center" border="0">
    <form name="form4" onreset="return AllReset()">
     <tr>
        <td align="right">商品名称：</td>
        <td align="left">
            <input  type="text" name="txt1"  onkeydown="MyEnter('txt3')"/>
        </td>
    </tr>
    <tr>
        <td align="right">所属类别：</td>
        <td align="left">
            <select name="txt2">
                <option value="家用电器">家用电器</option>
                <option value="礼品工艺">礼品工艺</option>
            </select>
        </td>
    </tr>
    <tr>
        <td align="right">商品编号：</td>
        <td align="left">
            <input  type="text" name="txt3" id="txt3" />
        </td>
    </tr>
    <tr>
        <td height="22" colspan="2" align="center">
            <input type="submit" value="提交" onclick="form4.submit()"/>
            <input type="reset"  value="重置"/>
        </td>
    </tr>
    </form>
</table>


js代码：
function MyEnter(idStr)
{
    var ev = event ? event :(window.event ? window.event : null);
    if(ev.keyCode==13)
        document.getElementById(idStr).focus();
}