SWEXP Module 04 — Professional TypeScript Engineering · Starter Workspace

This repo is your work-along workspace for Module 04. The lessons live in the LMS; here you do the labs and the capstone: open an exercise, read its README.md, implement the // TODOs in its src/, run the tests, and submit.

The tests are the spec. Each exercise's tests/ describes exactly what your code must do — make them pass without weakening the types (no any, no as, no @ts-ignore). No answer keys are shipped.

Quick start

npm install            # one time (already done in your LMS code-server workspace)
npm test               # run every exercise's behaviour tests
npm run test:types     # add the type-level checks (expectTypeOf)
npm run check          # strict type-check — "the compiler is your first reviewer"
npm run grade          # your score + per-exercise breakdown (what CI reports)

Run a single exercise while you work on it:

npx vitest run labs/lab-04-unions-narrowing      # or any folder below
npx vitest watch labs/lab-04-unions-narrowing    # re-run on save

Exercises

Exercise	Folder	You implement
Lab 00 — Setup	labs/lab-00-setup	first function; confirm the toolchain
Lab 01 — Static typing	labs/lab-01-static-typing	typed lineTotal / orderTotal
Lab 02 — Domain modelling	labs/lab-02-domain-model	Order union (illegal states unrepresentable)
Lab 03 — Generics & utility types	labs/lab-03-generics-utility	derive types from Order; one generic wrapper
Lab 04 — Unions & narrowing	labs/lab-04-unions-narrowing	formatId / applyDiscount / previewNote
Lab 05 — Guards & discriminated unions	labs/lab-05-guards-discriminated	exhaustive handle; isOrder guard
Lab 06 — Strict tsconfig	labs/lab-06-tsconfig-strict	fix 3 bugs strict mode catches
Capstone — Migration	assignments/capstone	integrate it all into one strict module

Each folder is self-contained: a README.md (the brief), src/ (starter code with // TODOs), and tests/ (the spec). Reference cheatsheets are in resources/.

How grading & submission work

• Every exercise contributes tests — behaviour (*.test.ts) and, for the modelling labs, type-level assertions (*.test-d.ts). npm run grade reports a per-exercise score plus a strict type-check gate.
• Submit by committing your changes and pushing (or opening a pull request). The Autograde GitHub Action runs the same grader, posts your score to the run summary, and comments it on any PR.
• You're done when the score is 100% and the type-check is clean.

The rules of this module

• Make illegal states unrepresentable — model so bad values can't be constructed.
• The compiler is your first reviewer — a claim that doesn't type-check isn't true.
• any is never the fix. Neither is as at a trust boundary — use a type guard.