Looks like you can detect the previous state and compare it to the next state. You can't detect that the app is closing vs going into the background, from what I can find online, but you can detect if it was inactive (closed), or in the background.

Example from React Native Docs

import React, {Component} from 'react'
import {AppState, Text} from 'react-native'

class AppStateExample extends Component {

  state = {
    appState: AppState.currentState
  }

  componentDidMount() {
    AppState.addEventListener('change', this._handleAppStateChange);
  }

  componentWillUnmount() {
    AppState.removeEventListener('change', this._handleAppStateChange);
  }

  _handleAppStateChange = (nextAppState) => {
    if (this.state.appState.match(/inactive|background/) && nextAppState === 'active') {
      console.log('App has come to the foreground!')
    }
    this.setState({appState: nextAppState});
  }

  render() {
    return (
      <Text>Current state is: {this.state.appState}</Text>
    );
  }

}

With @react-native-community/hooks you can use the useAppState hook to check the app state.

When you close the app, it gets a state of unknown once you open it back. When is in background, it says background' or 'inactive'. So when it's unknown` you know the app is opening from being closed, not minimized or in the background.

I suggest to use web socket and it will help you to solve your problem, as following :

react native app

import React from 'react';
const io = require('socket.io-client/dist/socket.io');

const App = ()=>{
    React.useEffect(()=>{
        // connect to web socket
        window.appSocket = io.connect(`<server-origin>/?token=<string>&userAppId=<string>`);
        window.appSocket.on('connect',()=>{
            // do what you line
        })
    },[])

    ...
}
export default App; 

express server side

const express   = require('express');
const server    = express();
const http      = require('http').Server(server);
const io        = require('socket.io')(http);
io.on('connection',(socket)=>{ // this callback function for each web socket connection
    let { token , userAppId } = socket.handshake.query;
    if( userAppId ){ 
        console.log(`${userAppId} online`)
        socket.on('disconnect',(resean)=>{
            console.log(`${userAppId} shut down`)
        })
    }
});

for more details, you can check socket.io

Just ran into this same issue. The answer that's accepted does not actually do what is asked in the OP.

A hacky solution would be to set a flag on the first screen your application opens when opened fresh and to NOT SHOW THAT SCREEN when the app has just been backgrounded.

Not super elegant and I can't show example code, but it fixes my particular issue.

Looks like there is no solution to this as of now . The 'inactive' and 'background' states cannot be used to find kill state also. I think there is a reason we donot know when app is killed as it is done by iOS - it decides based on history and other factors so as to kill the app. Even if we close the app , the OS doesnt immediately kill the app it just goes to 'background' mode.

My current workaround is to run the code when the app is launched instead. It's practically the other side of the same coin. Instead of trying to run as the app is being closed, run that code as the app is being launched next time.

// App.js
export default App() {
  useEffect(() => {
    // Run desired code here <----
    console.log("App is launching")
  }, [])

  return (
    <View>
      <Text>Hello world</Text>
    </View>
  )
}