Add two Express apps with custom and built-in middlewares

Author: AFatmaa

.gitignore

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+# Dependencies
+node_modules/

off-the-shelf-middleware/express.js

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+import express from "express";
+
+const app = express();
+
+// Built-in Express middleware to parse JSON automatically
+app.use(express.json());
+
+// Custom middleware: Get username from header
+function usernameExtractor(req, res, next) {
+  const username = req.get("X-Username");
+  req.username = username ? username : null;
+  next();
+}
+
+app.use(usernameExtractor);
+
+app.post("/", (req, res) => {
+  const username = req.username; // from header
+  const subjects = req.body; // parsed JSON from express.json()
+
+  if (!Array.isArray(subjects)) {
+    return res.status(400).send("Error: Body must be a JSON array.");
+  }
+
+  const allStrings = subjects.every((item) => typeof item === "string");
+  if (!allStrings) {
+    return res
+      .status(400)
+      .send("Error: Array must contain only string elements.");
+  }
+
+  const authMessage = username
+    ? `You are authenticated as ${username}.`
+    : "You are not authenticated.";
+
+  const subjectCount = subjects.length;
+  const subjectList = subjects.join(", ");
+
+  const infoMessage =
+    subjectCount === 0
+      ? "You have requested information about 0 subjects."
+      : `You have requested information about ${subjectCount} subject${
+          subjectCount > 1 ? "s" : ""
+        }: ${subjectList}.`;
+
+  res.send(`${authMessage}\n\n${infoMessage}`);
+});
+
+// Start the server
+const PORT = 3001;
+app.listen(PORT, () => {
+  console.log(`Server running on http://localhost:${PORT}`);
+});