jQuery Validation插件:如何检查元素是否有效？

<form id="myForm">
 <label for="username">用户名：</label>
 <input type="text" id="username" name="username" required>
  
 <label for="password">密码：</label>
 <input type="password" id="password" name="password" required>
  
 <input type="submit" value="提交">
</form>

$(document).ready(function() {
  $("#myForm").validate({
    rules: {
      username: {
        required: true,
        minlength: 4
      },
      password: {
        required: true,
        minlength: 6
      }
    },
    messages: {
      username: {
        required: "请输入用户名",
        minlength: "用户名必须包含至少4个字符"
      },
      password: {
        required: "请输入密码",
        minlength: "密码必须包含至少6个字符"
      }
    }
  });
});

var validator = $("#myForm").validate();
if (validator.element("#username")) {
  console.log("用户名有效");
} else {
  console.log("用户名无效");
}