How to detect when a React Native app is closed (not suspended)?

Question

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>
    );
  }

}

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