How to register a global middleware

In Socket.IO v2, a middleware that was registered for the main namespace would act as a global middleware, applying to all namespaces, since a client would first connect to the main namespace, and then to the custom namespace:

// Socket.IO v2

io.use((socket, next) => {
  // always triggered, even if the client tries to reach the custom namespace
  next();
});

io.of("/my-custom-namespace").use((socket, next) => {
  // triggered after the global middleware
  next();
});

This is not the case any more starting with Socket.IO v3: a middleware attached to the main namespace is only called when a client tries to reach the main namespace:

// Socket.IO v3 and above

io.use((socket, next) => {
  // only triggered when the client tries to reach the main namespace
  next();
});

io.of("/my-custom-namespace").use((socket, next) => {
  // only triggered when the client tries to reach this custom namespace
  next();
});

To create a global middleware in newer versions, you need to attach your middleware to all namespaces:

either manually:

const myGlobalMiddleware = (socket, next) => {
  next();
}

io.use(myGlobalMiddleware);
io.of("/my-custom-namespace").use(myGlobalMiddleware);

or by using the new_namespace event:

const myGlobalMiddleware = (socket, next) => {
  next();
}

io.use(myGlobalMiddleware);

io.on("new_namespace", (namespace) => {
  namespace.use(myGlobalMiddleware);
});

// and then declare the namespaces
io.of("/my-custom-namespace");

caution

The namespaces that are registered before the new_namespace listener won't be affected.

or, when using dynamic namespaces, by registering a middleware on the parent namespace:

const myGlobalMiddleware = (socket, next) => {
  next();
}

io.use(myGlobalMiddleware);

const parentNamespace = io.of(/^\/dynamic-\d+$/);

parentNamespace.use(myGlobalMiddleware);

caution

Existing namespaces always have priority over dynamic namespaces. For example:

io.of("/admin");

io.of(/.*/).use((socket, next) => {
  // won't be called for the main namespace nor for the "/admin" namespace
  next();
});