Express.js

Express.js is the absolute, unquestioned, and historically ubiquitous titan of the "Node.js Backend" industry. While React, Angular, and Vue run in the user's browser (Frontend), Express.js runs on the actual server (Backend). It was the crucial 'E' in the legendary MEAN/MERN stack (MongoDB, Express, React, Node), proving that JavaScript could be used to build massive server-side APIs. The absolute core superpower of Express is its "Fierce Minimalism and Middleware Architecture." It provides almost nothing out of the box—no database, no strict structure. It simply provides an incredibly fast, highly elegant routing engine to handle HTTP requests (GET, POST). Developers then plug in 'Middleware' functions to handle things like security, logging, or database connections exactly the way they want. Because it completely democratized backend development for frontend JavaScript engineers, offering unparalleled flexibility and a massive npm ecosystem, it remains the absolute default choice for building Node.js REST APIs and lightweight microservices.