Validation and Contract Checkpoint #3

Code Example

import express from "express";

const app = express();
app.use(express.json());

app.get("/health", (_req, res) => {
  res.status(200).json({
    status: "ok",
    service: "backend-api-mastery",
    objective: "improve observability coverage",
  });
});

app.post("/orders/preview", (req, res) => {
  const { items = [] } = req.body as { items?: Array<{ qty: number; price: number }> };
  const total = items.reduce((sum, item) => sum + item.qty * item.price, 0);
  return res.status(200).json({ scenario: "notifications orchestration API", total });
});

app.listen(3000, () => console.log("Run:", "npm run dev"));

Commands & References

• npm run dev
• node server.js
• app.get('/route', handler)

Lab Steps

1. Prepare environment using: npm run dev
2. Implement endpoint or middleware for this scenario.
3. Test positive/negative cases and inspect response contracts.
4. Document one reliability or security improvement.

Exercises

• Add one edge-case validation and return clear error details.
• Improve one endpoint to be idempotent and test retries.
• Write one test case proving the expected API behavior.