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Use the MQTT.js library with Node.js or in the browser to publish and subscribe using MQTT.

// require npm package
const mqtt = require("mqtt");

// declare client
const client = mqtt.connect("mqtt://public:public@public.cloud.shiftr.io", {
  clientId: "javascript",
});

// register "connect" callback
client.on("connect", function () {
  console.log("connected!");

  // subscribe to topic
  client.subscribe("hello");

  // publish message every second
  setInterval(function () {
    client.publish("hello", "world");
  }, 1000);
});

// register "message" callback
client.on("message", function (topic, message) {
  console.log(topic + ": " + message.toString());
});

For more information see the JavaScript manual in the documentation.