不会找到specefic ID

Question

我需要检查在jQuery的响应中是否找到了ID 但它一直给我警报alert("else");！

gastenboek.js

$(document).ready(function() {
    $("#checkbox_warnings").on("change", function() {
        if (!this.checked) {
            $(this).attr("checked", false);
        } else {
            $("#warning_checkbox").slideUp(function () {
                $(this).remove();
            });
        }
    });

    $("#gastenboek").submit(function(event){
        event.preventDefault();

        var form = $(this);
        var url = "gastenboek_handler.php?action=check_gastenboek_form";
        var posting = $.post(url, form.serialize());

        posting.done(function(data) {

            if ($("#er-is-error").length) {
                var errors = jQuery.parseJSON(data);
                $("#gb-errors").slideDown().empty().append(errors);
                alert("if");
            } else {
                alert("else");
                $("#gb-errors").empty().slideUp();
            }
        });
    });
});

确实存在错误，但它没有slideDown ID #gb-errors
我很确定这一点，因为在控制台中它会给出错误的响应！

gastenboek_handler.php

<?php
include_once "/core/init.php";

if ($_GET['action'] == 'check_gastenboek_form')
{
    if (!empty($_POST['email']) && !preg_match('/^[^@]+@[a-zA-Z0-9._-]+\.[a-zA-Z]+$/', $_POST['email']))
    {
        $errors[] = "Dit is geen geldig e-mail adres!";
    }

    if (empty($_POST['msg']))
    {
        $errors[] = "Het berichten veld mag niet leeg zijn.";
    }
    else if (strlen($_POST['msg']) < 15)
    {
        $errors[] = "Het bericht mag niet minder dan 15 karakters hebben!";
    }
    else if (strlen($_POST['msg']) > 2500)
    {
        $errors[] = "Het bericht mag niet langer dan 2500 karakters hebben!";
    }

    if (!empty($errors))
    {
        foreach($errors as $error)
        {
            $return[] = '<span class="glyphicon glyphicon-warning-sign" style="padding-right:5px; color:red;"></span><a class="errors" style="color:red;">' . $error . '</a><br>';
        }
        echo json_encode($return);
        echo '<div id="er-is-error"></div>';
    }
    else
    {
        gb_post($_POST);
    }
}
?>

希望你们知道一些事情！