javascript判断表单不能为空！

<div method="post" action="/mail.php"  onsubmit="return check()" >            <table width="500" border="0" cellspacing="0" cellpadding="0" align="center">                  <tr>                    <td align="right">联系人<font color="red">*</font></td>                    <td><input name="q1" type="text" /></td>                  </tr>                  <tr>                    <td align="right">联系电话<font color="red">*</font></td>                    <td><input name="q2" type="text" /></td>                  </tr>  <tr>                    <td align="right">地  区<font color="red">*</font></td>                    <td><input name="q3" type="text" /></td>                  </tr>                  <tr>                    <td> </td>                    <td><input name="submit" type="submit" value="提交申请" ></td>                  </tr>                </table></form>            </div>        </div><!--by:xue--><script language="javascript">function check(){if(document.contactForm.q1.value==""){alert("联系人不能为空!");this.contactForm.q1.focus();return false;}else if(document.contactForm.q2.value==""){alert("电话不能为空!");this.contactForm.q2.focus();return false;}else if(document.contactForm.q3.value==""){alert("地区不能为空!");this.contactForm.q3.focus();return false;}else contactForm.submit();}</script>