Testing Mastery — Unified Testing Skill

Write tests that document intent, catch regressions, and run fast. Choose the right strategy for the right situation.

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Decision Tree: Which Testing Strategy?

Is this a new feature?
├─ YES → Use TDD (see references/tdd-cycle.md)
│        Write failing test → Minimal code → Refactor
└─ NO
   ├─ Is this a bug fix?
   │  └─ YES → Write regression test first, then fix
   ├─ Is this a critical user flow (login, checkout)?
   │  └─ YES → E2E test (see references/e2e-playwright.md)
   └─ Is this business logic / data transformation?
      └─ YES → Unit + Integration tests (see references/unit-integration.md)

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Testing Pyramid

        /\          E2E (Few, ~10%)
       /  \         Critical user flows only
      /----\
     /      \       Integration (Some, ~20%)
    /--------\      API, DB, service contracts
   /          \
  /------------\    Unit (Many, ~70%)
                    Functions, classes, utilities

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Core Principles

Principle	Rule
AAA	Arrange → Act → Assert
Fast	Unit < 100ms, Integration < 1s
Isolated	No test depends on another
Behavior	Test WHAT, not HOW
Minimal	One assertion per test (ideally)

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Quick Reference

I need to...	Use	Reference
Build feature test-first	TDD (RED-GREEN-REFACTOR)	tdd-cycle.md
Write unit/integration tests	Mocking, data strategies, patterns	unit-integration.md
Test critical user flows in browser	E2E with Playwright	e2e-playwright.md

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Anti-Patterns (Universal)

❌ Don't	✅ Do
Test implementation details	Test observable behavior
Write tests after shipping	Write tests before/during
Duplicate test code	Use factories & fixtures
Complex test setup	Simplify or split
Ignore flaky tests	Fix root cause
Skip cleanup	Reset state in teardown
Multiple asserts per test	One behavior per test

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🔧 Runtime Scripts

Script	Purpose	Command
scripts/test_runner.py	Unified test execution	python scripts/test_runner.py <project_path>
scripts/playwright_runner.py	Browser E2E testing	python scripts/playwright_runner.py <url>
	With screenshot	python scripts/playwright_runner.py <url> --screenshot
	Accessibility check	python scripts/playwright_runner.py <url> --a11y

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Remember: The test is the specification. If you can't write a test for it, you don't understand the requirement.