Express.js middleware are functions that execute during the lifecycle of a request to the Express server. Each middleware has access to the HTTP request and response for each route (or path) on the server, and they can either terminate the HTTP request or pass it on to another middleware function. This “chaining” of middleware allows you to compartmentalize your code and create reusable pieces of software. Middleware functions can perform tasks like validating data, logging, error-handling, interacting with databases, or anything else that happens between when a server receives a request and when it sends a response.
