Problem using the method isAjax()

Hello, I am trying to make Ajax requisitions and when I try check $this->request->isAjax() == true is always returning false but if I print inside the condition of isPost is working. my view: item/index.html.

<script type="text/javascript">
var http;
if (window.XMLHttpRequest) { 
    http = new XMLHttpRequest();
} else if (window.ActiveXObject) {
    http = new ActiveXObject("Microsoft.XMLHTTP");
}
http.open('POST', 'https://127.0.0.1/item/search', true);
http.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
http.onreadystatechange = function() {
    if(http.readyState == 4 && http.status == 200) {
        alert(http.responseText);
    }
}
http.send('user=test');
</script>

ItemController.php

public function searchAction(){
            $this->view->disable(); 
            $request = new Request(); //  I  make this becaus I was triying $request->isAjax() 
            $response = new Response();
            $response->setHeader("Content-type", "application/json;charset=utf-8");
            if ($this->request->isPost() == true) {
                if ($this->request->isAjax() == true) {
                    //here is print nothing
                    $user = $this->request->getPost('user');
                    $sucess = array ('this' => $user);
                    echo json_encode($sucess);
                }   
                //here prints
            }
    }