Middleware

Middleware functions in Express are functions that run before the function associated with an endpoint. Some common examples of this include logging a request or checking for authentication before running the main function of an endpoint. In these cases, the logging and authentication functions will be common among all the endpoints that use them. By using middleware, we can reuse code that is common across endpoints.

With Express, we can have middleware functions that run for all endpoints by using app.use(). For example, if we wanted to create a function that logs a request to the console before running the main route, we could write a logger middleware:

var logger = function (req, res, next) {
  // Request is logged
  console.log(req);
  // Call the special next function which passes the request to next function
  next();
}

To have the logger run with all endpoints, we tell our app to use it with the following:

app.use...