BackendUser::isLoggedIn() 

\nn\t3::BackendUser()->isLoggedIn($request = NULL); 

Checks whether a BE user is logged in. Example: Only show certain content in the frontend if the user is logged in in the backend. Previously: $GLOBALS['TSFE']->beUserLogin

// Check after complete initialization of the front/backend
\nn\t3::BackendUser()->isLoggedIn();

// Check using the JWT, e.g. in an eID script before authentication
\nn\t3::BackendUser()->isLoggedIn( $request );
Copied!
@param ServerRequest $request
@return bool

Source Code 

public function isLoggedIn( $request = null )
{
	if ($request) {
		$cookieName = $this->getCookieName();
		$jwt = $request->getCookieParams()[$cookieName] ?? false;
		$identifier = false;
		if ($jwt) {
			try {
				$params = $request->getAttribute('normalizedParams') ?? NormalizedParams::createFromRequest($request);
				$cookieScope = $this->getCookieScope( $params );
				$identifier = \TYPO3\CMS\Core\Session\UserSession::resolveIdentifierFromJwt($jwt, $cookieScope);
			} catch( \Exception $e ) {}
		}
		if ($identifier) return true;
	}
	$context = GeneralUtility::makeInstance(Context::class);
	return $context->getPropertyFromAspect('backend.user', 'isLoggedIn');
}
Copied!