javascript回车触发旋钮事件

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
 <HEAD>
  <TITLE> New Document </TITLE>
<SCRIPT LANGUAGE="JavaScript">
<!--
    function show(event) {
        var ev = event || window.event;
        //回车键对应的ASCII是13，Tab对应的是9
        //判断按下回车建的控件类型，不能是reset，submit，textarea和空
        if (ev.keyCode == 13 && ev.srcElement.type != "submit" &&
            ev.srcElement.type != "reset" && ev.srcElement.type != "textarea" &&
            ev.srcElement.type != "" ) {
            ev.keyCode = 9;
        }
    }
    //键盘按下事件
    document.onkeydown = show;
//-->
</SCRIPT>
 </HEAD>

 <BODY>
 <form action="" method="post" name="inputForm">
<input type="text" name="names"><br>
<input type="text" name = "txtNames"><br>
<input type="button" value="提交">
 </form>
 </BODY>
</HTML>