Mysterious Error AJAX SyntaxError: Unexpected token < in JSON at position 0

I am getting an parse error when I run the AJAX code for autocomplete. The response looks great. But I don’t why the error occurring.

Here is the Script and HTML code

<script type="text/javascript"><!--
$('input[name='filter_parent']').autocomplete({
  'source': function(request, response) {
    $.ajax({
      url: 'index.php?route=catalog/category/autocompletecat&user_token={{ user_token }}&filter_parent=' +  encodeURIComponent(request),
      dataType: 'json',
      success: function(json) {
        response($.map(json, function(item) {
          return {
            label: item['name'],
            value: item['category_id']
          }
        }));
      },
      beforeSend: function(jqXHR) {},
      error: function(jqXHR, textStatus, errorThrown) {console.log(errorThrown);},
      complete: function(jqXHR,status) {}
    });
  },
  'select': function(item) {
    $('input[name='filter_parent']').val('');
  }
});
//--></script>

<div id="filter-category" class="col-md-3 col-md-push-9 col-sm-12 hidden-sm hidden-xs">
    <div class="panel panel-default">
      <div class="panel-heading">
        <h3 class="panel-title"><i class="fa fa-filter"></i> {{ text_filter }}</h3>
      </div>
      <div class="panel-body">
        <div class="form-group">
          <label class="control-label" for="input-parent">{{ entry_parent }}</label>
          <input type="text" name="filter_parent" value="{{ filter_parent }}" placeholder="{{ entry_parent }}" id="input-parent" class="form-control" />
        </div>
        <div class="form-group">
          <label class="control-label" for="input-status">{{ entry_status }}</label>
          <select name="filter_status" id="input-status" class="form-control">
            <option value=""></option>
            {% if filter_status == '1' %}
            <option value="1" selected="selected">{{ text_enabled }}</option>
            {% else %}
            <option value="1">{{ text_enabled }}</option>
            {% endif %}
            {% if filter_status == '0' %}
            <option value="0" selected="selected">{{ text_disabled }}</option>
            {% else %}
            <option value="0">{{ text_disabled }}</option>
            {% endif %}
          </select>
        </div>
        <div class="form-group text-right">
          <button type="button" id="button-filter" class="btn btn-default"><i class="fa fa-filter"></i> {{ button_filter }}</button>
        </div>
      </div>
    </div>
  </div>

Here is the controller with the json response

public function autocompletecat() {
    $json = array();

    $json[] = array(
      'category_id' => 0,
      'name'        => strip_tags(html_entity_decode('---All---', ENT_QUOTES, 'UTF-8'))
    );
    

   if (isset($this->request->get['filter_parent'])) {

      $this->load->model('catalog/category');

      $filter_data = array(
        'filter_parent' => $this->request->get['filter_parent'],
        'sort'        => 'name',
        'order'       => 'ASC',
        'start'       => 0,
        'limit'       => 5
      );


      $results = $this->model_catalog_category->getSuperParentCategories($filter_data);

      foreach ($results as $result) {
        $json[] = array(
          'category_id' => $result['category_id'],
          'name'        => strip_tags(html_entity_decode($result['name'], ENT_QUOTES, 'UTF-8'))
        );
      }
    }

    $sort_order = array();

    foreach ($json as $key => $value) {
      $sort_order[$key] = $value['name'];
    }

    array_multisort($sort_order, SORT_ASC, $json);


    $this->response->addHeader('Content-Type: application/json');
    $this->response->setOutput(json_encode($json));
}

[{"category_id":0,"name":"---All---"}]

I tried with other functions, none of them are working. This is so mysterious, I don’t know what causes this error. Please help me on this. I almost spend the entire day for this.

Thanks in Advance

Answer

Finally found it. Error is causing because of user_token which is not declared in controller.

$data['user_token'] = $this->session->data['user_token'];