Today, in Node.js, the word middleware is used well beyond the boundaries of the express framework, and indicates a particular pattern whereby a set of processing units, filters, and handlers, under the form of functions are connected to form an asynchronous sequence in order to perform preprocessing and postprocessing of any kind of data. The main advantage of this pattern is flexibility; in fact, this pattern allows us to obtain a plugin infrastructure with incredibly little effort, providing an unobtrusive way for extending a system with new filters and handlers.
