need help with check/uncheck all checkbox [cakephp]

I need help with this. I'm creating a checkbox to select or deselect the country options. Please help to give some clues and explaination. I'll be very grateful. Thanks in advance. 

Below are the code:

Location: View/country/add.ctp file

<script type="text/javascript">

$(document).ready(function () {

    $('#selectall').click(function () {

         var checked = $(this).is(':checked');

        

        $('.selectedId').each(function () {

            var checkBox = $(this);

            console.debug(checkBox);

            if (checked) {

                checkBox.prop('checked', true);                

            }

            else {

                checkBox.prop('checked', false);                

            }

        });

        

    });

});

</script>

    <div>

<?php echo $this->Form->create('Showcase'); ?>

    <fieldset>

        <legend><?php echo __('Add Showcase Product'); ?></legend>

    <?php

        echo $this->Form->input('p_start', array('label' => 'Start Date'));

        echo $this->Form->input('p_end', array('label' => 'End Date'));

        echo $this->Form->input('priority', array('label' => 'Priority'));

        echo $this->Form->input('Select all Country', array("type" => "checkbox", 'id'=>'selectall'));

        echo $this->Form->input("Country", array("options" => $viewData["countryList"], "multiple" => "checkbox", "onclick" => "resetSelectAll();" , "class" => "selectedId" , "name" => "selectedId")); 

?>

    </fieldset>

    

  

<?php

echo $this->Form->end(__('Submit'));

echo $this->Html->link('Back', array('action' => 'listShowcases'));

?>

</div>

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