middleware-example.mjs

// INFORMATION FILE (don't try to run it! 🙏)

// In an Express.js application, a common middleware stack typically consists
// of multiple middleware functions that process incoming HTTP requests and outgoing responses.
// Here's an example of a 5-layer middleware stack in an Express.js application:

// 1️⃣ - Built-in Middleware (Express.json, Express.urlencoded,
// Express.static, etc.):
// Express comes with built-in middleware functions
// that handle common tasks. For example,
// express.json() parses incoming JSON data,
// express.urlencoded() parses incoming form data,
// and express.static() serves static files.
// These built-in middlewares are often used as the first layer in the stack.

const express = require("express");
const app = express();

app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(express.static("public"));

// 2️⃣ - Custom Application-Level Middleware:
// This layer includes custom middleware functions
// that are specific to your application.
// These can perform tasks such as authentication,
// session management, and request validation.

app.use(customAuthMiddleware);
app.use(sessionMiddleware);
app.use(requestValidationMiddleware);

// 3️⃣ - Route-Level Middleware:
// Middleware can also be applied to specific routes.
// This layer includes middleware functions that
// are executed only for certain routes. For example,
// you might want to log requests for specific routes.

app.use("/admin", adminMiddleware);
app.use("/api", apiMiddleware);

// 4️⃣ - Error Handling Middleware:
// This middleware layer is responsible for
// handling errors that occur during request processing.
// It typically comes after other middleware to catch
// and respond to errors that might occur in the application.

app.use((err, req, res, next) => {
  // Handle errors and send an appropriate response
});

// 5️⃣ - Response Middleware:
// After the core request processing is complete,
// response middleware functions can be used to
// modify the response. For example,
// you might set response headers or apply
// post-processing to the response data.

app.use(responseHeadersMiddleware);
app.use(responseFormattingMiddleware);

// 📝 Notes: The actual middleware layers and their order may vary
// depending on the specific requirements of your application.
// Keep in mind that middleware in Express is executed in the order
// they are added using app.use(), so the order matters.