share mqtt client object between files - node.js

Question

I connect to MQTT this way:
//mqtt.js
const mqtt = require('mqtt');
var options = {
//needed options
};
var client = mqtt.connect('mqtt://someURL', options);
client.on('connect', () => {
console.log('Connected to MQTT server');
});
I want to export the client object this way:
//mqtt.js
module.exports = client;
So that I can import it in other files and make use of it this way:
//anotherFile.js
const client = require('./mqtt');
client.publish(...)
However, we all know that this will not work! How can I achieve this ?
Update
I tried promise and get a very strange behavior. When I use the promise in the same file (mqtt.js) like the code below, everything is OK:
//mqtt.js
const mqtt = require('mqtt');
var mqttPromise = new Promise(function (resolve, reject) {
var options = {
//needed options
};
var client = mqtt.connect('mqtt://someURL', options);
client.on('connect', () => {
client.subscribe('#', (err) => {
if (!err) {
console.log('Connected to MQTT server');
resolve(client);
} else {
console.log('Error: ' + err);
reject(err);
}
});
});
});
mqttPromise.then(function (client) {
//do sth with client
}, function (err) {
console.log('Error: ' + err);
});
But when I export the promise and use it in another file, like this:
//mqtt.js
//same code to create the promise
module.exports = mqttPromise;
//anotherFile.js
const mqttPromise = require('./mqtt');
mqttPromise.then(function (client) {
//do sth with client
}, function (err) {
console.log('Error: ' + err);
});
I get this error:
TypeError: mqttPromise.then is not a function

Answer 1

You can probably achieve your goal creating 2 files, one for handling mqtt methods and another to manage the connection object.
Here's the file for the mqtt handler:
//mqttHandler.js
const mqtt = require('mqtt');
class MqttHandler {
constructor() {
this.mqttClient = null;
this.host = 'YOUR_HOST';
this.username = 'YOUR_USER';
this.password = 'YOUR_PASSWORD';
}
connect() {
this.mqttClient = mqtt.connect(this.host, {port: 1883});
// Mqtt error calback
this.mqttClient.on('error', (err) => {
console.log(err);
this.mqttClient.end();
});
// Connection callback
this.mqttClient.on('connect', () => {
console.log(`mqtt client connected`);
});
this.mqttClient.on('close', () => {
console.log(`mqtt client disconnected`);
});
}
// // Sends a mqtt message to topic: mytopic
sendMessage(message, topic) {
this.mqttClient.publish(topic, JSON.stringify(message));
}
}
module.exports = MqttHandler;
Now lets use the exported module to create a mqtt client connection on another file:
//mqttClient.js
var mqttHandler = require('./mqttHandler');
var mqttClient = new mqttHandler();
mqttClient.connect();
module.exports = mqttClient;
With this exported module you can now call your client connection object and use the methods created in the mqttHandler.js file in another file :
//main.js
var mqttClient = require('./mqttClient');
mqttClient.sendMessage('<your_topic>','<message>');
Although there may be a better method to perform your task, this one worked pretty well for me...
Hope it helps!

Answer 2

cusMqtt.js
const mqtt = require("mqtt");
function prgMqtt() {
const options = {
port: 1883,
host: "mqtt://xxxxxxx.com",
clientId: "mqttjs_" + Math.random().toString(16).substr(2, 8),
username: "xxxxxx",
password: "xxxxxx",
keepalive: 60,
reconnectPeriod: 1000,
protocolId: "MQIsdp",
protocolVersion: 3,
clean: true,
encoding: "utf8",
};
prgMqtt.client = mqtt.connect("mqtt://xxxxxxxx.com", options);
prgMqtt.client.on("connect", () => {
prgMqtt.client.subscribe("Room/Fan");
console.log("connected MQTT");
});
prgMqtt.client.on("message", (topic, message) => {
console.log("message is " + message);
console.log("topic is " + topic);
// client.end();
});
}
exports.prgMqtt = prgMqtt;
index.js/main program call
const { prgMqtt } = require("./startup/cusMqtt");
prgMqtt();
another .js
const { prgMqtt } = require("../startup/cusMqtt");
router.get("/:id", async (req, res) => {
prgMqtt.client.publish("Room/Reply", "Replied Message");
});