JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://code.jquery.com/jquery-2.1.4.js"></script>
<script src="http://ajax.aspnetcdn.com/ajax/jquery.validate/1.14.0/jquery.validate.js"></script>
<form id="myform">
    <input type="checkbox" name="foo" />
    <br />
    <input type="text" name="bar" />
    <br />
    <br />
    <input type="submit" />
</form>

JavaScript

$(document).ready(function() {

    $('#myform').validate({
        rules: {
            foo: {
                required: true
            },
            bar: {
                maxlength: function(element) {
                    if ($("[name='foo']").is(":checked")) {
                        return 15;
                    } else {
                        return 5;
                    }

                }
            }
        }
    });

});