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Symfony 5+ how to view dump output with AJAX requests

So the idea is to be able to use the Symfony VarDumper component to be able to dump values when sending an AJAX request and view them for debugging while in development mode, but not in production mode.

Create a new directory

To do this you will need to create an event subscriber and listen for the kernel.response event. To do this you create a new directory inside your root directory, I called mine EventSubscribers, the name doesn’t matter you could call yous BlueWaffle and it will work exactly the same.

Create a new file

Inside this new directory create a new subscriber class, I called mine AjaxDumpSubscriber. You can make your subscriber implement

 implements EventSubscriberInterface

I am not sure if that is needed, I saw it somewhere in the docs. They never mentioned if it was needed so I am assuming it is maybe needed.

Then add the method stub for


Here is what the github source code comment says about this method

interface EventSubscriberInterface
     * Returns an array of event names this subscriber wants to listen to.
     * The array keys are event names and the value can be:
     *  * The method name to call (priority defaults to 0)
     *  * An array composed of the method name to call and the priority
     *  * An array of arrays composed of the method names to call and respective
     *    priorities, or 0 if unset
     * For instance:
     *  * ['eventName' => 'methodName']
     *  * ['eventName' => ['methodName', $priority]]
     *  * ['eventName' => [['methodName1', $priority], ['methodName2']]]
     * The code must not depend on runtime state as it will only be called at compile time.
     * All logic depending on runtime state must be put into the individual methods handling the events.
     * @return array<string, string|array{0: string, 1: int}|list<array{0: string, 1?: int}>>
    public static function getSubscribedEvents();

The full class

The entire class should look like this, from what I can tell. This is working code.

namespace App\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class AjaxDumpSubscriber  implements EventSubscriberInterface

     * @return array
    public static function getSubscribedEvents(): array
        return [KernelEvents::RESPONSE];

    public function onKernelResponse(ResponseEvent $event)
        if (!$event->getKernel()->isDebug()) {
        $request = $event->getRequest();

        if (!$request->isXmlHttpRequest()) {
        $response = $event->getResponse();
        //set this one value only if this is in development mode
        $response->headers->set('Symfony-Debug-Toolbar-Replace', 1);

That is it, now you can dump() values like normal… but you can’t view them like normal, not in Symfony 5.4, not for me anyways .

This only partially works, maybe I missed something or I expect something? Most of the time when you use dump() there will be an icon in the profiler bar that indicates debug output was output aka. dump() was used. Usually you can click on it and quickly see the values that were dumped. This feature doesn’t work with  AJAX requests at the moment, from what I can observe.

Instead you need to click the response when it indicates in the profiler then click the Debug tab on the left panel to see your dump() values.


The VarDumper component. -> Docs link you must install this before you can dump anything.

The profiler component -> Docs link you need this to see dumped stuff, even though it doesn’t work properly with the VarDumper

Creating an EventSubscriber -> Docs link

Using event subscribers -> docs link