Unit Testing with Jest 🧪

Group coaching session – Part 1: unit testing, Part 2: Test-Driven Development (TDD).

Getting set up

1. Fork this repo (button top-right on GitHub) so you have your own copy.
2. Clone your fork:

   git clone https://github.com/<your-username>/unit-testing-workshop.git
   cd unit-testing-workshop

3. Install Jest (you'll need Node 18 or newer – check with node -v):

   npm install

4. Check it works:

   npm test

   You should see everything pass, with a list of todo tests. Those todos are your job.

Useful commands

Command	What it does
npm test	Run every test
npm run test:watch	Re-run tests every time you save a file
npx jest 03	Run only files matching "03" (e.g. exercise 03)
npm run test:part1	Run only Part 1
npm run test:part2	Run only Part 2
npm run test:coverage	Show which lines of code your tests actually run

How we're working

You'll work in a group – talk it through, share screens, argue about the best test case. But everyone writes the tests in their own fork.

• Rotate who's "driving" (sharing their screen and typing) for each exercise.
• Commit after each exercise: git add . && git commit -m "Exercise 03 tests"
• Push at the end so your work's saved (and makes good portfolio evidence).

Part 1 – Unit testing

Each exercise gets a bit harder. Every test file has instructions at the top, one worked example, and a list of test.todo(...) for you to turn into real tests.

#	Exercise	You'll learn
01	01-first-tests – calculator	The shape of a test: describe, test, expect, toBe
02	02-strings – text formatting	toMatch, toHaveLength, testing against a spec
03	03-edge-cases-and-errors – pricing & grades	Boundaries, toThrow, test.each
04	04-arrays-and-objects – shopping basket	toBe vs toEqual, beforeEach, immutability
05	05-bug-hunt 🐛 – buggy functions	Writing tests from a spec to find bugs, then fixing them
06	06-async – OTJ learner service	Testing Promises with async/await, resolves/rejects
07	07-mocking 🌶️ – OTJ reminder emails	jest.fn(), mocks, dependency injection, controlling time

In the session: aim for 01–04, then jump to 05 (bug hunt) if you've got time. Don't rush – quality over quantity.

Take-away: finish whatever's left after the session, especially 06 (async) and 07 (mocking). Commit and push as you go.

Part 2 – TDD

Open part-2-tdd/CHALLENGE.md. You'll build sign-up form validation tests first.

In the session: aim for Levels 1–4. Levels 5–6 and the stretch goals are yours to take away.

What makes a good unit test?

• Tests one thing – if it fails, you know exactly what broke.
• Has a clear name – reads like a sentence: "rejects an email with no @".
• Arrange → Act → Assert – set up the data, call the function, check the result.
• Fast and independent – no real network, no real emails, no relying on other tests.
• Tests behaviour, not implementation – what the function promises, not how it does it.