Update related list with change in list in CakePHP 3

I have two tables categories and subcategories where categories hasMany subcategories relation.

I have another table products which is related to categories and subcategories.

In add method of products I want to select category from the list and then subcategory associated with particular category selected.

echo $this->Form->input('category_id', ['options' => $categories, 'empty' => true]);
echo $this->Form->input('subcategory_id', ['options' => $subcategories]);
echo $this->Form->input('product_type_id', ['options' => $productTypes]);
echo $this->Form->input('title');

As there is no js helper in CakePHP 3. How could I do this using Ajax.

I'm new to CakePHP and Ajax as well. And currently this show all subcategories in the list.

I get it working like this and its working fine.

view file contain

<?= $this->Form->input('categories', ['options' => $categories, 'empty' => 'Select', 'id' => 'categories']) ?>
<?= $this->Form->input('subcategory_id', ['type' => 'select', 'id' => 'subcategories]) ?>

and myAjax.js file contain

$('#categories').change(function () {
    var dataSet = {category_id: $(this).val()};
    var requestUrl = appBaseUrl+'products/ajax-subcategories';
    $.ajax({
        type: "POST",
        url: requestUrl,
        data: dataSet,
        success: function(result) {
            $('#subcategories').html(result);
        },
        error: function(jqXHR, textStatus, errorThrown) {
            console.log(textStatus, errorThrown);
        }
    });
});

and ajaxSubcategories action inside ProductsController.php

public function ajaxSubcategories()
{
    $this->autoRender = false;
    $this->loadModel('Subcategories');
    $category_id = $this->request->data['category_id'];

    $subcategories = $this->Subcategories->find()
        ->where(['category_id' => $category_id, 'deleted' => false, 'status' => 0]);

    $data = '';
    foreach($subcategories as $subcategory) {
        $data .= '<option value="'.$subcategory->id.'">'.$subcategory->title.'</option>';
    }
    echo $data;
}

Hope this might help someone.