React Native useEffect rerenders with initial states

Question

In the code below, when the app loads initially "Location changed offline." is logged every time the location updates. When online is set to true with a TouchableOpacity, the message logged to the console looks like this:

 LOG  true
 LOG  attempting to update location...
 LOG  Location updated in real-time!
 LOG  false
 LOG  Location changed offline!
 LOG  true
 LOG  attempting to update location...
 LOG  true
 LOG  attempting to update location...
 LOG  Location updated in real-time!

For some reason it's randomly changing the state of online back to false, thus causing the "Location changed offline." to be logged? What could be causing this?

  const [online, setOnline] = useState(false);

  useEffect(() => {
    Geolocation.watchPosition(
      position => {
        console.log(online);
        if (online) {
          console.log('attempting to update location...');
          const payload = {
            lat: position.coords.latitude,
            lng: position.coords.longitude,
            id: 1,
          };

          axios
            .post('http://localhost:3000/location/update', payload)
            .then(res => {
              if (res.status === 200) {
                console.log('Location updated in real-time!');
                return;
              }
            })
            .catch(err => console.log(err.response));
        } else {
          console.log('Location changed offline!');
        }
      },
      err => console.log(err.response)
    );
  }, [online]);

Answer 1

The problem here is that you add a new watcher every time online changes that captures the current value forever.

Adding subscriptions (like Geolocation.watchPosition() ) within an effect hook should always be removed in the cleanup function.

useEffect(() => {
  const watchId = Geolocation.watchPosition(async (position) => {
    console.log("online?", online);
    if (online) {
      console.log("attempting to update location...");
      const payload = {
        lat: position.coords.latitude,
        lng: position.coords.longitude,
        id: 1,
      };

      try {
        await axios.post("http://localhost:3000/location/update", payload);

        // no need to check the response status
        console.log("Location updated in real-time!");
      } catch (err) {
        console.warn(err.toJSON()); // Axios helper, much nicer to look at
      }
    } else {
      console.log("Location changed offline!");
    }
  }, console.error);

  // return a cleanup function
  return () => {
    Geolocation.clearWatch(watchId);
  };
}, [online]);

See https://reactnative.dev/docs/0.63/geolocation#clearwatch