App.js

/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 *
 * @format
 * @flow strict-local
 */
 
import React, { useState } from 'react';
import { View, Text } from 'react-native';
import { magnetometer } from 'react-native-sensors';
var RNFS = require('react-native-fs');
import RNFetchBlob from 'react-native-fetch-blob'

 var path = RNFetchBlob.fs.dirs.DownloadDir + '/readings.txt';

const App = () => {
  const [value, setValue] = useState(0);
  const [axisVal, setAxisValue] = useState("");
 
  const subscription = magnetometer.subscribe(({ x, y, z, timestamp }) => {
 
    var dateTime = new Date(timestamp);
    dateTime.toISOString(); 
    let a = "x: " + x + "\ny: " + y + "\nz: " + z + "\ntimestamp: " + timestamp + "\n time: " + dateTime+"\n\n";
    setAxisValue(a);
    let teslaVal = parseFloat(Math.sqrt((x * x) + (y * y) + (z * z)));
    setValue(teslaVal);


      // write the file
      // RNFS.writeFile(path, a, 'utf8')
      RNFS.appendFile(path, a, 'utf8')
      // RNFS.writeFile(path, 'Lorem ipsum dolor sit amet', 'utf8')

      .then((success) => {
      console.log('path :'+path+' , '+a +'WRITTEN!');
      })
      .catch((err) => {
      console.log(err.message);
      });


  });
  subscription;
 
  return (
    <View style={{ justifyContent: 'center', alignItems: 'center', margin: 10 }}>
 
      <Text style={{ fontSize: 30, marginTop: 20 }}>{"microTesla: " + value}</Text>
      <Text style={{ fontSize: 20, marginTop: 30 }}>{"Cordinate value:\n " + axisVal}</Text>
 
    </View>
  );
};
 
export default App;