利用jQuery.Validate异步验证用户名是否存在（推荐）

<script type="text/JavaScript" src="../js/jQuery-1.3.2.min.js"></script>
<script type="text/javascript" src="../js/jquery.validate.min.js"></script>
<script type="text/javascript" src="../js/messages_cn.js"></script>

<form name="form1" id="form1" method="post" action="">
<dl>
 <dt>用户名：</dt>
 <dd><input name="txtUserName" id="txtUserName" type="text" class="input1" /></dd>
</dl>
</form>

$(function() {
//表单验证JS
  $("#form1").validate({
    //出错时添加的标签
    errorElement: "span",
    rules: {
      txtUserName: {
        required: true,
        minlength: 3,
        maxlength: 16,
        remote: {
          type: "post",
          url: "/tools/ValidateUserName.ashx",
          data: {
            username: function() {
              return $("#txtUserName").val();
            }
          },
          dataType: "html",
          dataFilter: function(data, type) {
            if (data == "true")
              return true;
            else
              return false;
          }
        }
      }
    },
    success: function(label) {
      //正确时的样式
      label.text(" ").addClass("success");
    },
    messages: {
      txtUserName: {
        required: "请输入用户名，3-16个字符（字母、数字、下划线），注册后不能更改",
        minlength: "用户名长度不能小于3个字符",
        maxlength: "用户名长度不能大于16个字符",
        remote: "用户名不可用"
      }
    }
  });
});

data: {
  username: function() {
  return $("#txtUserName").val();
}