An express application is a series of middleware function calls.

In Express, a middleware is a function with three parameters: req, res & next.

req represents the incoming HTTP request. res represents the outgoing HTTP response. And next hands off the processing of the request and the construction of the response to the next function in the middleware stack.

Normally, a single middleware function does a single task, then sends the request to the next middleware function for further handling.

The order matters in the middleware stack. Functions get processed in the order they appear in the stack provided the previous function called next() and thus passed control to the next middleware.

app.use((req, res, next) => {
  console.log("Request received")
  next()
})

The middleware above serves no particular URL. Therefore, app.use() will call the callback function every time the server receives a request.

We can also define functions and use them as middleware as shown below:

const logging = (req, res, next) => {
  console.log(req)
  next()
}

app.use(logging)