Session 4: Functional patterns and narrowing

What you are practising: discriminated unions, narrowing, immutability, and map/filter/reduce, pulled together in one integrated exercise.

Setup

Accept the assignment, clone your repository, then install the tooling once:

npm install

The workflow

1. Open a terminal in the exercise/ folder.
2. Rename the starter: mv progress.js progress.ts.
3. Work through what the compiler surfaces.
4. Compile with a bare npx tsc. Not npx tsc progress.ts: with a filename argument, tsc ignores tsconfig.json and the strict settings with it.
5. Run the output: node progress.js.

Your task

This is the last TypeScript exercise of the module and it pulls together sessions 1 to 4. The file contains skeleton functions and a test block. All of them are untyped, and a couple contain runtime bugs that only become obvious once types are applied. Run the starter and compare its output against the data; two things are quietly wrong.

1. Rename progress.js to progress.ts.
2. Model the domain with interfaces and/or type aliases:
   • a Learner with id, name, and email
   • a ModuleProgress with a title and a status, discriminated on status: 'not-started', 'in-progress', or 'completed'. A completed module has a completedOn: Date, an in-progress module has a lastActivity: Date, and a not-started module has nothing extra.
   • an Enrolment that links a learner to an array of ModuleProgress
3. Implement these functions with correct types:
   • completedModules(enrolment: Enrolment): ModuleProgress[], returning only completed modules
   • progressSummary(enrolment: Enrolment): string, returning a human-readable summary using a discriminated union switch
   • findLearner(enrolments: Enrolment[], id: string): Enrolment | undefined, using a generic helper if you want
4. Use map, filter, or reduce at least once.
5. Do not mutate any input.
6. Compile with npx tsc and run with node progress.js.

Keep the exported function names (completedModules, progressSummary, findLearner) — the autograder imports them by name.

Stretch goals

• Add an exhaustive helper and use it in progressSummary so that adding a new module status becomes a compile error.
• Add a function that takes an Enrolment and returns a new Enrolment with one module marked as complete, using the four immutable moves from Part 1 of the session.
• Extract the generic findById from session 3 and use it inside findLearner.

Done when

npx tsc completes with no errors, node progress.js shows Amara's completed module in the list and a summary with real dates (no undefined), no input is mutated, and your work is committed and pushed.

How your work is graded

Every push runs two GitHub Actions checks:

• Type check — runs npx tsc on your exercise under the strict settings, once you have renamed the starter to .ts.
• Autograding — runs an automated test suite (Vitest) against your functions and reports a functionality score, plus an automated code-quality review.