How to implement AUTO submit form for searching via JavaScript?

I have created form which automatically search text (this text is recognized from recorded voice). After recording user's voice JS (below) recognize voice as text, then call form by id="searchform". This form then call function, which search word in database of words. JS code:

 final_transcript = capitalize(final_transcript);   
    var queryTextField = document.getElementById("search_query");
    queryTextField.value = final_transcript; 
//automatic submit button search form is if form
        document.getElementById('searchform').submit();

JS code call function "media/search" which is implemented in this HTML form:

<li><form class="input-group navbar-form" id="searchform" action="<?php echo base_url();?>media/search" method="post"></li>
    <input type="text" class="form-control" placeholder="Vyhladat titulky..." id="search_query" name="string" />
    <button type="submit"  name="btn_search"></button>
    </form>

"Media/search" function, which search recognized text:

//Search function
        function search($string = null) {
                //function can search string sent in url or in form
                $data['string'] = isset($_POST['string']) ? $_POST['string'] : $string;
                //For not null form
                if(! empty($data['string'])) {
                        //Searching in database
                        $gid = (! access(3) && ! access(4)) ? $this->userinfo('group') : null;
                        $data['list'] = $this->media_model->search($data['string'], $gid);
                }
                //Data are set to show
                $this->template->view("media/search", $data);
        }

Auto submit seems works because the page with results is shown after record of voice, but no results of searching are shown. I think that searching function works, but with no value, so it seems that it search "no word". I need help, I am new in JS. This code was created in my school and I need to change it to work automatically