Push notifications if app is closed (React-Native)

I'm developing React-native app (iOS & Android).
Server connected through web-socket. Server sends notifications about some specific events. App suppose to notify user (using alerts). What if I need to notify user even if app is running in background or closed - how can i do that? Push notifications with this library will help in cases when app is in background, right? But it will not work if app is closed? So, how can i notify user if app is in background or closed?