Algorithm to validate brazillian phone numbers using jquery-mask-plugin

br-phone-format.js

/**
 * Format phone numbers where an ninth digit is optional
 */
(function(){
    let phoneInputs = [ '[name="phone"]' ];

    let phoneMaskBehavior = function (val) {
        return val.replace(/\D/g, '').length === 11 ? '(00) 00000-0000' : '(00) 0000-00009';
    };

    $.each(phoneInputs, function(index, el) {
        if ($(el).length) {
            $(el).mask(phoneMaskBehavior, {
                onKeyPress: function(val, e, field, options) {
                    field.mask(phoneMaskBehavior.apply({}, arguments), options);
                }
            });
        }
    });
})();