jQuery Validation plugin: validate check box

You can validate group checkbox and radio button without extra js code, see below example.

Your JS should be look like:

$("#formid").validate();

You can play with HTML tag and attributes: eg. group checkbox [minlength=2 and maxlength=4]

<fieldset class="col-md-12">
  <legend>Days</legend>
  <div class="form-row">
    <div class="col-12 col-md-12 form-group">
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="1" required="required" data-msg-required="This value is required." minlength="2" maxlength="4" data-msg-maxlength="Max should be 4">Monday
        </label>
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="2">Tuesday
        </label>
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="3">Wednesday
        </label>
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="4">Thursday
        </label>
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="5">Friday
        </label>
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="6">Saturday
        </label>
        <label class="checkbox-inline">
          <input type="checkbox" name="daysgroup[]" value="7">Sunday
        </label>
        <label for="daysgroup[]" class="error">Your error message will be display here.</label>
    </div>
  </div>
</fieldset>

You can see here first or any one input should have required, minlength="2" and maxlength="4" attributes. minlength/maxlength as per your requirement.

eg. group radio button:

<fieldset class="col-md-12">
  <legend>Gender</legend>
  <div class="form-row">
    <div class="col-12 col-md-12 form-group">
        <label class="form-check-inline">
          <input type="radio" name="gendergroup[]" value="m" required="required" data-msg-required="This value is required.">man
        </label>
        <label class="form-check-inline">
          <input type="radio" name="gendergroup[]" value="w">woman
        </label>
        <label class="form-check-inline">
          <input type="radio" name="gendergroup[]" value="o">other
        </label>
        <label for="gendergroup[]" class="error">Your error message will be display here.</label>
    </div>
  </div>
</fieldset>

You can check working example here.

• jQuery v3.3.x
• jQuery Validation Plugin - v1.17.0

You had several issues with your code.

1) Missing a closing brace, }, within your rules.

2) In this case, there is no reason to use a function for the required rule. By default, the plugin can handle checkbox and radio inputs just fine, so using true is enough. However, this will simply do the same logic as in your original function and verify that at least one is checked.

3) If you also want only a maximum of two to be checked, then you'll need to apply the maxlength rule.

4) The messages option was missing the rule specification. It will work, but the one custom message would apply to all rules on the same field.

5) If a name attribute contains brackets, you must enclose it within quotes.

DEMO: http://jsfiddle.net/K6Wvk/

$(document).ready(function () {

    $('#formid').validate({ // initialize the plugin
        rules: {
            'test[]': {
                required: true,
                maxlength: 2
            }
        },
        messages: {
            'test[]': {
                required: "You must check at least 1 box",
                maxlength: "Check no more than {0} boxes"
            }
        }
    });

});