js di CI

<script>
var food=document.getElementById('food').value;
var car=document.getElementById('car').value;
if(food=='')
{
return false;
}
if(car=='')
{
return false;
}
xmlhttp.open('get', '<?php echo site_url('beranda/show');?>?food='+food+'&car='+car, true); //error
</script>
//-Controller-
//Beranda.php
class Beranda extends CI_Controller {

public function show(){
$get = $this->input->get();
print_r($get);
//$data['food']=$this->show->fetch_food($food);
//$data['car']=$this->show->fetch_car($car);
//$this->load->view('modal',$data);
}

}