Example App.js for including mqtt.js into local expo.dev project

App.js

import { StatusBar } from 'expo-status-bar';
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';

// expo install mqtt
// fix refer to mqtt.js issue: https://github.com/mqttjs/MQTT.js/issues/573
const mqtt = require('mqtt/dist/mqtt')

export default function App() {

  const brokerUrl = "...";
  const username = "...";
  const password = "...";
  
  const client = mqtt.connect(brokerUrl,{
    username: username,
    password: password,
  })

  client.on('message',(topic,message,packet) => {
    console.log(message.toString());
  });

  client.subscribe('test_topic');

  return (
    <View style={styles.container}>
      <Text>Open up App.js to start working on your app!</Text>
      <StatusBar style="auto" />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
  },
});