Add Express app with username and array-validation middleware

- Add off-the-shelf-middleware.js: create Express app and listen on PORT (env fallback to 3000).
- Add username middleware (reads X-username header into req.username).
- Add validateArray middleware (ensures JSON body is an array of strings; returns 400 on invalid input).
- Add POST "/" route that composes a response using the middlewares and starts the server.

Author: Iswanna

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

@@ -0,0 +1,60 @@
+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");
+});