Question

jquery.validate里的this.optional(element)这个函数什么意思

Answer 1

jQuery.validate的optional(element)，用于表单控件的值不为空时才触发验证。
当element为空时this.optional(element)=true，用于在该控件为非必填项目时可以通过验证，及条件可以不填但是不能填错格式。
例如：
jQuery.validator.addMethod("division", function(value, element) { return this.optional(element) || value % 2 == 0 && value % 3 == 0; }, "必须能被2和3整除");

如果值为空时也要触发验证，移除optional(element)。
例如：
$f.addMethod("division", function(value, element) { return value % 2 == 0 && value % 3 == 0; }, "必须能被2和3整除");

Answer 2

A validation method implements the logic to validate any element. Provided are a set of default validation methods, like required. Except required itself and equalTo, all validation methods declare an element valid when it has no value at all. That way an email field is optional, unless also required is specified. you can specify an element input to contain a valid email address, or nothing at all. Use $.validator.addMethod to implement custom methods.

判断element是否是可选的，也就是是否指定了required

Answer 3

就是相当于定义一个变量，当前js作用域的一个变量的调用this后面是变量名