J3.x:Discover on which client your extension code is running
From Joomla! Documentation
Joomla!
≥ 3.7
version
This article gives an overview of the JFactory::getApplication()->isClient() method introduced in Joomla! 3.7.
It replaces the JFactory::getApplication()->isSite() and JFactory::getApplication()->isAdmin() methods.
Frontend
JFactory::getApplication()->isClient('site')
Returns true if executed in the Joomla! website frontend.
Administrator / Backend
JFactory::getApplication()->isClient('administrator')
Returns true if executed in the Joomla! website backend / administrator area.
Administrator / Backend
JFactory::getApplication()->isClient('installation')
Returns true if executed in the Joomla! installation.