Express is a routing and middleware web framework that has minimal functionality of its own: An Express application is essentially a series of middleware function calls.
Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.
An Express application can use the following types of middleware:
- Application-level middleware
- Router-level middleware
- Error-handling middleware
- Built-in middleware
- Third-party middleware
Using Express Middleware. expressjs.com/en/guide/using-middleware.html.