我很好地實施了 MQTT，但我遇到了方法問題unsubscribe。mqttCon.publish() , mqttCon.subscribe(), mqttCon.on()...除此以外，所有其他功能 ( ) 都運行良好。我已經檢查了拼寫，甚至完成了ctrl +click它，它把我帶到了庫實現，這意味著它是正確的方法并且引用得很好，但我一直收到下面的錯誤。我該如何解決？這是一行： mqttCon.unsubscribe(topic)TypeError: mqttCon.unsubscribe is not a function    at noopHandler (/home/dev/project-dir/src/mqtt/processMessage.js:5:13)    at module.exports (/home/dev/project-dir/src/mqtt/processMessage.js:10:20)    at MqttClient.client.on (/home/dev/project-dir/src/mqtt/mqttCon.js:16:13)    at MqttClient.emit (events.js:189:13)    at MqttClient._handlePublish (/home/dev/project-dir/node_modules/mqtt/lib/client.js:1271:12)    at MqttClient._handlePacket (/home/dev/project-dir/node_modules/mqtt/lib/client.js:410:12)    at work (/home/dev/project-dir/node_modules/mqtt/lib/client.js:321:12)    at Writable.writable._write (/home/dev/project-dir/node_modules/mqtt/lib/client.js:335:5)    at doWrite (/home/dev/project-dir/node_modules/mqtt/node_modules/readable-stream/lib/_stream_writable.js:428:64)    at writeOrBuffer (/home/dev/project-dir/node_modules/mqtt/node_modules/readable-stream/lib/_stream_writable.js:417:5)注意：我使用的是 ES6(Emacscript 6+) javascript 而不是Typescript。節點12.18.1和 npm6.14.6這是我的連接代碼mqttCon.js：const mqtt = require('mqtt')const processMessage = require('./processMessage')const logger = require('../logConf')const options = {    host: '',    port: '',    username:'',    password: '',    protocol: ''};const client = mqtt.connect(options)client.on("connect", function () {    console.log("MQTT connected  with status: " + client.connected);    if (client.connected) {        client.on('message', (topic, message) => {            processMessage(topic, String(message))        })    }})