Acceptance Test-Driven Development (ATDD)

Workflow

ATDD extends TDD by adding an outer acceptance test loop:

1. Write an acceptance test that describes the desired behavior from the user's perspective
2. Run it — it should fail (the feature doesn't exist yet)
3. Enter the TDD inner loop:
   • Write a failing unit test (RED)
   • Write minimal code to pass (GREEN)
   • Refactor (REFACTOR)
   • Repeat until the acceptance test passes
4. Verify the acceptance test passes — the feature is complete

Acceptance Test Patterns

Acceptance tests should be named with these patterns:

• *.acceptance.test.ts — acceptance/integration tests
• *.e2e.test.ts — end-to-end tests

Key Principles

• Acceptance tests describe WHAT, not HOW — they test observable behavior
• Unit tests describe HOW — they test internal mechanics
• Write acceptance tests in the language of the user/stakeholder
• One acceptance test per user story or feature
• Multiple unit tests per acceptance test — the inner TDD loop

Example Workflow

Feature: User registration

1. Write acceptance test: user-registration.acceptance.test.ts
   - Test: "User can register with email and password"
   - Test: "Registration fails with duplicate email"
   - Run → FAIL (no registration endpoint)

2. TDD inner loop:
   a. Unit test: validate-email.test.ts → RED → implement → GREEN
   b. Unit test: hash-password.test.ts → RED → implement → GREEN
   c. Unit test: create-user.test.ts → RED → implement → GREEN
   d. Unit test: registration-handler.test.ts → RED → implement → GREEN

3. Run acceptance test → PASS → Feature complete!

Anti-Patterns to Avoid

• ❌ Writing unit tests without an acceptance test that frames the feature
• ❌ Writing acceptance tests that test implementation details
• ❌ Skipping the acceptance test because "it's a small feature"
• ❌ Writing all acceptance tests upfront (write them one feature at a time)