A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
| Question to ask | Good diagnostic move |
|---|---|
| What value entered? | Inspect the runtime value and type. |
| What rule applied? | Check the construct's language or host semantics. |
| What changed? | Compare the smallest working and failing cases. |
import express from 'express';const app = express();app.use(express.json());app.get('/health', (_req, res) => res.json({ ok: true }));app.listen(3000);
<strong>Core idea:</strong> Backend JavaScript is mostly about boundaries: requests, responses, routing, middleware, validation, and failures crossing those boundaries in predictable ways. In this lesson, the practical target is routing and middleware: you should be able to predict what the runtime will do before changing the code.
<strong>Core idea:</strong> Backend JavaScript is mostly about boundaries: requests, responses, routing, middleware, validation, and failures crossing those boundaries in predictable ways. In this lesson, the practical target is routing and middleware: you should be able to predict what the runtime will do before changing the code.
Mentor note: for routing and middleware, name the invariant you need before reaching for a clever shortcut. Clear state and explicit assumptions make later debugging much cheaper.
Finish the lesson once you have worked through the material. This awards ★ 20 XP.