...Currently, only newer Webkit-based browsers (Like Chrome and Safari), Firefox and IE11+ provide this new fullscreen feature. Simply pass no argument to the fullScreen method to query the current state. The method returns the current fullscreen element (or true if browser doesn't support this) when fullscreen mode is active, false if not active or null when the browser does not support fullscreen mode at all. So you can use this method also to display a fullscreen button only when the browser supports fullscreen mode.