Socket.IO allows us to define two kinds of middleware functions in the server side:

• Namespace middleware: Registers a function that gets executed for every new connected Socket and has the following signature:

      namespace.use((socket, next) => { ... }) 

• Socket middleware: Registers a function that gets executed for every incoming Packet and has the following signature:

      socket.use((packet, next) => { ... }) 

It works similarly to how ExpressJS middleware functions do. We can add new properties to the socket or packet objects. Then, we can call next to pass the control to the next middleware in the chain. If next is not called, then the socket won't be connected, or the packet received.