Express.js

While Node.js provides the environment to run JavaScript on the server, building a web application with its native `http` module can be verbose and complex. Express.js is a de-facto standard web application framework for Node.js that simplifies this process immensely. It's designed to be minimal and unopinionated, giving you the flexibility to structure your application as you see fit. Express provides a thin layer of fundamental web application features, without obscuring the Node.js features that you know and love. Its core functionalities revolve around routing and middleware. Routing allows you to define how your application responds to client requests to specific endpoints (URIs) and HTTP methods. For example, you can easily define what should happen when a user makes a GET request to `/users` or a POST request to `/login`. Express also provides powerful middleware capabilities. A middleware function can execute any code, make changes to the request (`req`) and response (`res`) objects, or trigger the next middleware function in the stack. This makes it easy to modularize your application's logic. For instance, you can use middleware to handle authentication, log requests, parse incoming data, or handle errors. Its simplicity, performance, and robust feature set have made Express the most popular choice for building backend services, REST APIs, and full-stack web applications with Node.js.