jQuery Validate Plugin - How to create a simple custom rule?

You can create a simple rule by doing something like this:

jQuery.validator.addMethod("greaterThanZero", function(value, element) {
    return this.optional(element) || (parseFloat(value) > 0);
}, "* Amount must be greater than zero");

And then applying this like so:

$('validatorElement').validate({
    rules : {
        amount : { greaterThanZero : true }
    }
});

Just change the contents of the 'addMethod' to validate your checkboxes.

$(document).ready(function(){
    var response;
    $.validator.addMethod(
        "uniqueUserName", 
        function(value, element) {
            $.ajax({
                type: "POST",
                url: "http://"+location.host+"/checkUser.php",
                data: "checkUsername="+value,
                dataType:"html",
                success: function(msg)
                {
                    //If username exists, set response to true
                    response = ( msg == 'true' ) ? true : false;
                }
             });
            return response;
        },
        "Username is Already Taken"
    );

    $("#regFormPart1").validate({
        username: {
            required: true,
            minlength: 8,
            uniqueUserName: true
        },
        messages: {
            username: {
                required: "Username is required",
                minlength: "Username must be at least 8 characters",
                uniqueUserName: "This Username is taken already"
            }
        }
    });
});

// add a method. calls one built-in method, too.
jQuery.validator.addMethod("optdate", function(value, element) {
        return jQuery.validator.methods['date'].call(
            this,value,element
        )||value==("0000/00/00");
    }, "Please enter a valid date."
);

// connect it to a css class
jQuery.validator.addClassRules({
    optdate : { optdate : true }    
});

Custom Rule and data attribute

You are able to create a custom rule and attach it to an element using the data attribute using the syntax data-rule-rulename="true";

So to check if at least one of a group of checkboxes is checked:

data-rule-oneormorechecked

<input type="checkbox" name="colours[]" value="red" data-rule-oneormorechecked="true" />

addMethod

$.validator.addMethod("oneormorechecked", function(value, element) {
   return $('input[name="' + element.name + '"]:checked').length > 0;
}, "Atleast 1 must be selected");

And you can also override the message of a rule (ie: Atleast 1 must be selected) by using the syntax data-msg-rulename="my new message".

NOTE

If you use the data-rule-rulename method then you will need to make sure the rule name is all lowercase. This is because the jQuery validation function dataRules applies .toLowerCase() to compare and the HTML5 spec does not allow uppercase.

Working Example

$.validator.addMethod("oneormorechecked", function(value, element) {
  return $('input[name="' + element.name + '"]:checked').length > 0;
}, "Atleast 1 must be selected");

$('.validate').validate();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.14.0/jquery.validate.min.js"></script>

<form class="validate">
    red<input type="checkbox" name="colours[]" value="red" data-rule-oneormorechecked="true" data-msg-oneormorechecked="Check one or more!" /><br/>
    blue<input type="checkbox" name="colours[]" value="blue" /><br/>
    green<input type="checkbox" name="colours[]" value="green" /><br/>
    <input type="submit" value="submit"/>
</form>

Thanks, it worked!

Here's the final code:

$.validator.addMethod("greaterThanZero", function(value, element) {
    var the_list_array = $("#some_form .super_item:checked");
    return the_list_array.length > 0;
}, "* Please check at least one check box");