Socket.IO is a JavaScript library that enables real-time, bidirectional, event-based communication between clients and servers.

In traditional HTTP communication, the client sends a request and the server sends a response. The connection is generally not maintained as an application-level communication channel for future server-initiated messages.

Socket.IO changes this interaction model. Once a client connects, both the client and server can independently send events to each other.

For example, imagine building a chat application:

1. User A sends a message.
2. The browser emits a send-message event.
3. The Node.js server receives the event.
4. The server broadcasts a receive-message event.
5. Other connected users immediately receive the message.

You do not need to repeatedly refresh the page or poll the server for new messages.

Where is Socket.IO used?

• Real-time chat applications.
• Live notifications.
• Multiplayer games.
• Collaborative editors.
• Live dashboards.
• Delivery tracking.
• Online presence indicators.
• Real-time customer support.

Socket.IO is built on top of Engine.IO and provides features such as automatic reconnection, event acknowledgements, rooms, namespaces, and broadcasting.