We have some JavaScript that we were able to use in the Classic Designer to compare email fields but apparently all the JavaScript used in Classic has to be rewritten to be used in the Modern Designer. Is there any guide or examples for doing that?

This is what the old JavaScript looks like:

$(document).ready(function () {
    $('.email, .confirm').on('blur input', function () {
        if ($('.email input').val() != $('.confirm input').val()) {
            $('.Submit').attr("disabled", "disabled");
            if ($('.warningText').length > 0 || $('.confirm input').val() == ''){
                return;
            } else {
                $('<p class="warningText">The email addresses you entered do not match.</p>').insertAfter('.confirm');
            }
        }
        else
            $('.warningText').remove();
        $('.Submit').removeAttr('disabled');
    });
});