How do I debug a REST request when I can’t see the output of "dump()" calls?

  cors, symfony, symfony4

I want to debug the Request sent to my Controller method :

use FOSRestBundleControllerAnnotations as Rest;
use SymfonyBundleFrameworkBundleControllerAbstractController;
use SymfonyComponentHttpFoundationRequest;
use SymfonyComponentHttpFoundationResponse;

class IteropController extends AbstractController

     * @RestPost("/api/getiterop", name="api_get_iterop")
     * @RestRequestParam(name="sary", nullable=true)
    public function getIteropForm(Request $req)
        $response = new Response();
        $response->headers->set('Access-Control-Allow-Origin', '*');
        $response->headers->set('Content-Type', 'application/json');
        $response->setContent(json_encode(['prenom' => $req->request->get('prenom') , 'sary' => $req->files->get('sary') ? 'misy' : 'tsisy']));
        return $response;

But at runtime I get (failed)net::ERR_FAILED in the Network tab of the browser development tool, and the CORS error appears on the console log of the browser.

So how to debug the Rest POST?

Source: Symfony Questions