Important: #109517 - Setup extension merged into backend extension 

For Composer-based installations 

The Composer package typo3/cms-backend now replaces typo3/cms-setup. This means:

• There is no need to require typo3/cms-setup in composer.json anymore. Existing references are resolved automatically because typo3/cms-backend declares that it replaces the package.
• No manual action is required during an upgrade – Composer handles the replacement transparently.
• New projects should not add typo3/cms-setup as a dependency.

For class-based references 

The following public classes have been moved and class aliases are in place for backwards compatibility:

• \TYPO3\CMS\Setup\Event\AddJavaScriptModulesEvent → \TYPO3\CMS\Backend\Event\AddUserSettingsJavaScriptModulesEvent
• \TYPO3\CMS\Setup\Form\Element\AvatarElement → \TYPO3\CMS\Backend\Form\Element\AvatarElement
• \TYPO3\CMS\Setup\UserFunctions\UserSettingsItemsProcFunc → \TYPO3\CMS\Backend\UserFunctions\UserSettingsItemsProcFunc

Extensions using the old class names will continue to work, but should be updated to the new namespaces.

Moved event AddJavaScriptModulesEvent 

A special case is \TYPO3\CMS\Setup\Event\AddJavaScriptModulesEvent . This file has been moved to typo3/sysext/backend/DeprecatedClasses/Setup/Event/AddJavaScriptModulesEvent.php and is added as a specific PSR-4 autoload entry to the Core's composer.json map, so that the legacy event can be dispatched properly. No deprecation message is emitted when dispatching this legacy event.

Alongside, a new event \TYPO3\CMS\Backend\Event\AddUserSettingsJavaScriptModulesEvent has been added with a distinguishing name. Both events are dispatched in TYPO3 v14, with the legacy event being deprecated and removed in TYPO3 v15 (see Deprecation: #109517 - PSR-14 event \TYPO3\CMS\Setup\Event\AddJavaScriptModulesEvent).

Extensions providing compatibility to two versions at once should proceed like this:

<?php
declare(strict_types=1);

namespace MyExtension\Listener;

use TYPO3\CMS\Core\Attribute\AsEventListener;
use TYPO3\CMS\Setup\Event\AddJavaScriptModulesEvent;

final class SetupModuleListener
{
    #[AsEventListener('my-extension/setup-module-listener')]
    public function __invoke(AddJavaScriptModulesEvent $event): void
    {
        $event->addJavaScriptModule('@my-extension/setupModule/some-file.js');
    }
}

<?php
declare(strict_types=1);

namespace MyExtension\Listener;

use TYPO3\CMS\Backend\Event\AddUserSettingsJavaScriptModulesEvent;
use TYPO3\CMS\Core\Attribute\AsEventListener;

final class SetupModuleListener
{
    #[AsEventListener('my-extension/setup-module-listener')]
    public function __invoke(AddUserSettingsJavaScriptModulesEvent $event): void
    {
        $event->addJavaScriptModule('@my-extension/setupModule/some-file.js');
    }
}