West Midlands | 26 March SDC | Iswat Bello | Sprint 3 | Middleware exercises

.gitignore

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+node_modules/
+package-lock.json
+.env
+.DS_Store
+*.log
+dist/
+build/
+.vscode/

middleware-exercise/README.md

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+# Middleware Exercise: Custom vs. Off-the-Shelf
+
+This project demonstrates the implementation of Middleware in Express.js. It explores two ways of handling request data: manually parsing raw data streams and using built-in Express tools.
+
+## 🚀 How to Run
+
+1. **Install dependencies:**
+   ```bash
+   npm install
+   ```
+
+2. **Run the Custom Middleware version:**
+   ```bash
+   node "two custom-written middlewares.js"
+   ```
+
+3. **Run the Off-the-Shelf version:**
+   ```bash
+   node off-the-shelf-middleware.js
+   ```
+
+*Note: The server defaults to port `3000`. To use a specific port, use: `PORT=4000 node [filename].js`*
+
+## 🛠 Key Concepts Learned
+
+### 1. The Middleware Pattern
+Middleware functions sit between the request and the final route handler. In this project:
+- **Username Middleware:** Extracts identity from headers.
+- **Validation Middleware:** Ensures the incoming data matches the expected format before the route logic runs.
+
+### 2. Manual Parsing vs. `express.json()`
+- **Custom Parsing:** Uses `req.on("data")` and `req.on("end")` to catch raw bytes, translate them via `String.fromCharCode`, and parse them.
+- **Built-in Parsing:** Uses `express.json()`, which requires the client to send the `Content-Type: application/json` header.
+
+### 3. Environment Variables & Safety
+- **Port Fallback:** Used `process.env.PORT` with a fallback to `3000`.
+- **Parsing Numbers:** Used `parseInt(..., 10)` to ensure the port string from the environment is converted to a base-10 integer.
+- **ES Modules:** Set `"type": "module"` in `package.json` to allow the use of modern `import/export` syntax.
+
+### 4. Process Management
+Learned how to identify and terminate "zombie" processes hanging on a port using:
+```bash
+sudo lsof -iTCP:3000 -sTCP:LISTEN -Pn
+sudo kill -9 <PID>
+```
+
+## 🧪 Testing with `curl`
+
+To test the application, use the following commands in your terminal:
+
+**Valid Request (with Username):**
+```bash
+curl -X POST -H "Content-Type: application/json" -H "X-Username: Ahmed" -d '["Bees", "Birds"]' http://localhost:3000/
+```
+
+**Valid Request (Guest):**
+```bash
+curl -X POST -H "Content-Type: application/json" -d '["Lizards"]' http://localhost:3000/
+```
+
+**Invalid Data (should return 400):**
+```bash
+curl -X POST -H "Content-Type: application/json" -d 'notjson' http://localhost:3000/
+```
+

middleware-exercise/off-the-shelf-middleware.js

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+import express from "express";
+
+const app = express();
+
+const PORT = process.env.PORT ? parseInt(process.env.PORT, 10) : 3000;
+
+const usernameMiddleware = (req, res, next) => {
+  const headerValue = req.get("X-username");
+
+  if (headerValue) {
+    req.username = headerValue;
+  } else {
+    req.username = null;
+  }
+
+  next();
+};
+
+app.use(express.json());
+
+const validateArray = (req, res, next) => {
+  if (
+    req.body &&
+    Array.isArray(req.body) &&
+    req.body.every((item) => typeof item === "string")
+  ) {
+    next();
+  } else {
+    res.status(400).send("Error message");
+  }
+};
+
+// Hey Express, whenever a POST request arrives at '/', please call this person first (usernameMiddleware), then this person (arrayMiddleware), then finally do the route logic.
+app.post("/", usernameMiddleware, validateArray, (req, res) => {
+  let authPart;
+  if (req.username) {
+    authPart = `You are authenticated as ${req.username}`;
+  } else {
+    authPart = "You are not authenticated";
+  }
+  const MessageCount = req.body.length;
+
+  const messageJoined = req.body.join(",");
+
+  const word = MessageCount === 1 ? "subject" : "subjects";
+
+  if (MessageCount > 0) {
+    res.send(
+      `${authPart}\n\nYou have requested information about ${MessageCount} ${word}: ${messageJoined}.`,
+    );
+  } else {
+    res.send(
+      `${authPart}\n\nYou have requested information about ${MessageCount} ${word}.`,
+    );
+  }
+});
+
+app.listen(PORT, () => {
+  console.log("Type your message here");
+});

middleware-exercise/package.json

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+{
+  "name": "middleware-exercise",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "type": "module",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "express": "^5.2.1"
+  }
+}

middleware-exercise/two-custom-written-middlewares.js

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+import express from "express";
+
+const app = express();
+
+const PORT = process.env.PORT ? parseInt(process.env.PORT, 10) : 3000;
+
+const usernameMiddleware = (req, res, next) => {
+  const headerValue = req.get("X-username");
+
+  if (headerValue) {
+    req.username = headerValue;
+  } else {
+    req.username = null;
+  }
+
+  next();
+};
+
+const arrayMiddleware = (req, res, next) => {
+  const bodyBytes = [];
+
+  // Every time a piece of data arrives, we put it in our list
+  req.on("data", (chunk) => {
+    bodyBytes.push(...chunk);
+  });
+
+  // When the whole message has arrived, we process it
+  req.on("end", () => {
+    const bodyString = String.fromCharCode(...bodyBytes);
+
+    let bodyObject;
+    try {
+      bodyObject = JSON.parse(bodyString);
+    } catch (error) {
+      res.status(400).send("Invalid JSON");
+      return;
+    }
+
+    if (
+      Array.isArray(bodyObject) &&
+      bodyObject.every((item) => typeof item === "string")
+    ) {
+      req.body = bodyObject;
+      next();
+    } else {
+      res
+        .status(400)
+        .send(
+          "Invalid request body. Expected a JSON array containing only strings.",
+        );
+    }
+  });
+};
+
+// Hey Express, whenever a POST request arrives at '/', please call this person first (usernameMiddleware), then this person (arrayMiddleware), then finally do the route logic.
+app.post("/", usernameMiddleware, arrayMiddleware, (req, res) => {
+  let authPart;
+  if (req.username) {
+    authPart = `You are authenticated as ${req.username}`;
+  } else {
+    authPart = "You are not authenticated";
+  }
+  const MessageCount = req.body.length;
+
+  const messageJoined = req.body.join(",");
+
+  const word = MessageCount === 1 ? "subject" : "subjects";
+
+  if (MessageCount > 0) {
+    res.send(
+      `${authPart}\n\nYou have requested information about ${MessageCount} ${word}: ${messageJoined}.`,
+    );
+  } else {
+    res.send(
+      `${authPart}\n\nYou have requested information about ${MessageCount} ${word}.`,
+    );
+  }
+});
+
+app.listen(PORT, () => {
+  console.log("Type your message here");
+});