MQTT.js is a lightweight MQTT client library for JavaScript and Node.js applications. It allows developers to connect browsers, web apps, and server-side applications to MQTT brokers. MQTT.js supports both MQTT 3.x and 5.0, providing flexibility for different IoT solutions. Its asynchronous, event-driven design fits well into JavaScript environments. Developers often use it for real-time dashboards, IoT gateways, and web-based monitoring tools. With its simplicity and wide adoption, MQTT.js is a key library for bringing MQTT to the web.