Testing Philosophy

Universal principles for writing effective tests. Language-agnostic—applies across testing frameworks and languages.

Test Thinking

Before writing tests, commit to a clear approach:

• What is the ONE behavior this test suite must verify? If you can't answer clearly, the production code needs refactoring.
• Behavior or implementation? Tests should survive refactoring. If you're testing how, not what, you're coupling to implementation.
• What failure would make you distrust this code? Test that scenario first.

CRITICAL: You are capable of identifying subtle behavioral contracts that most tests miss. Don't write generic happy-path tests—find the edge cases that matter, the error handling that fails silently, the state transitions that corrupt data.

Core Principle

Test behavior, not implementation.

Tests should verify what code does, not how it does it. Implementation can change; behavior should remain stable.

Test-First Workflow (Canon TDD)

When to TDD:

• ✅ Core domain logic, algorithms, business rules
• ✅ Well-defined requirements
• ✅ Production code (not prototypes)
• ✅ AI-assisted development (tests guard against hallucinations)
• ❌ UI prototyping, exploration, fuzzy requirements

Canon TDD Pattern (Kent Beck 2024):

1. Write test list - enumerate all scenarios (happy, edge, error)
2. Turn one into failing test - focus on interface design
3. Make it pass - minimal implementation
4. Refactor - improve design while green
5. Repeat until list empty

AI-Assisted TDD:

• AI generates test list from requirements
• AI implements code to pass tests (human reviews)
• Tests are specifications in executable form
• Commit tests separately before implementation

NEVER test:

• Private method internals (test through public API)
• Mock call counts unless the count IS the behavior
• Internal state unless state IS the contract
• Framework code (trust the framework)

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What and When to Test

Testing Boundaries

Test at module boundaries (public API):

Unit Tests:

• Pure functions (deterministic input → output)
• Isolated modules (single unit of behavior)
• Business logic (calculations, validations, transformations)

Integration Tests:

• Module interactions (how components work together)
• API contracts (request/response shapes)
• Workflows (multi-step processes)

E2E Tests:

• Critical user journeys (end-to-end flows)
• Happy path + critical errors only
• Not every feature needs E2E

What to Test

✅ Always test:

• Public API (what callers depend on)
• Business logic (critical rules, calculations)
• Error handling (failure modes)
• Edge cases (boundaries, null, empty)

❌ Don't test:

• Private implementation details
• Third-party libraries (already tested)
• Simple getters/setters (unless they have logic)
• Framework code (trust the framework)

TDD: Sometimes, When It Adds Value

Use TDD for:

• Complex logic (algorithms, business rules)
• Well-defined requirements (you know what to build)
• Critical functionality (high-risk code)

Skip TDD for:

• UI prototyping (exploring design)
• Exploratory work (discovering requirements)
• Simple CRUD (straightforward logic)

When in doubt: Write test after if TDD feels like overhead.

Coverage Philosophy: Meaningful > Percentage

Don't chase coverage percentages.

✅ Good coverage:

• Critical paths tested (happy + error cases)
• Edge cases covered (boundary values, null, empty)
• Confidence in refactoring

❌ Bad coverage:

• High % but testing wrong things
• Testing implementation details
• Brittle tests that break on refactor

Remember: Untested code is legacy code. But 100% coverage doesn't guarantee quality.

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Mocking and Test Structure

Mocking Philosophy: Minimize Mocks

Prefer real objects when fast and deterministic.

When to Mock:

ALWAYS mock:

• External APIs, third-party services
• Network calls
• Non-deterministic behavior (time, randomness)

USUALLY mock:

• Databases (or use in-memory/test DB for integration)
• File system (depends on speed needs)

SOMETIMES mock:

• Slow operations (if they slow tests significantly)

NEVER mock:

• Your own domain logic (test it directly)
• Simple data structures

Red flag: >3 mocks in a test suggests coupling to implementation.

Test Structure: AAA (Arrange, Act, Assert)

Clear three-phase structure:

// Arrange: Set up test data, mocks, preconditions
setup test data
configure mocks
establish preconditions

// Act: Execute the behavior being tested
result = performAction()

// Assert: Verify expected outcome
verify result matches expectation

Guidelines:

• Visual separation between phases (blank lines)
• One logical assertion per test (can have multiple assert statements for same behavior)
• Keep Arrange simple (complex setup = simplify production code)

Test Naming: Descriptive Sentences

Pattern: "should [expected behavior] when [condition]"

Examples:

• "should return total when all items valid"
• "should throw error when user not found"
• "calculateTotal with empty cart returns zero"
• "should retry on network failure"

Guidelines:

• Be specific about what's being tested
• State expected behavior clearly
• Don't use "test" prefix (redundant in test files)
• Read like documentation

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Test Quality and Smells

Test Smells (Anti-Patterns)

❌ Too many mocks (>3 mocks)

• Indicates coupling to implementation
• Test becomes brittle, changes with internals

❌ Brittle assertions

• Asserting exact strings when substring would work
• Asserting exact ordering when order doesn't matter
• Over-specifying expected values

❌ Unclear test intent

• Can't tell what's being tested from reading test
• Vague test names
• Hidden test logic in helpers

❌ Testing implementation details

• Testing private methods directly
• Asserting internal state
• Mocking your own classes

❌ Flaky tests

• Pass/fail randomly
• Timing dependencies
• Shared mutable state between tests

❌ Slow tests

• Unit tests >100ms
• Integration tests >1s
• Slows development feedback loop

❌ One giant test

• Testing multiple behaviors in single test
• Hard to understand failures
• Breaks single responsibility for tests

❌ Magic values

• Unexplained constants
• Unclear test data
• No context for why values matter

Test Quality Priorities

Readable > DRY

Tests are documentation. Clarity trumps reuse.

✅ Good test practices:

• Each test understandable in isolation
• Explicit setup visible in test
• Some duplication okay for clarity
• Descriptive variable names (even if verbose)

❌ Over-DRY tests:

• Extract helpers that hide test logic
• Shared setup that obscures what's being tested
• Reuse at expense of clarity

Test length:

• No hard limit
• Unit tests: Usually <50 lines
• Integration tests: Can be longer (setup needed)
• Long test? Ask: Testing too much? Simplify production code?

Edge Cases: Required for Critical Paths

Always test critical functionality:

• Boundary values (0, 1, -1, max, min)
• Empty inputs (empty array, empty string, null)
• Error conditions (invalid input, missing data, failures)

Ask: "What could break? What do users depend on?"

Opportunistic edge cases:

• Nice-to-have features
• Non-critical paths
• When you find bugs (add regression test)

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Quick Reference

Testing Decision Tree

Should I write a test?

1. Is this public API? → Yes, test it
2. Is this critical business logic? → Yes, test it
3. Is this error handling? → Yes, test it
4. Is this private implementation? → No, test through public API
5. Is this a framework feature? → No, trust framework
6. Will this test give confidence? → Yes, write it

Should I use TDD?

1. Requirements clear? → Consider TDD
2. Complex logic? → Consider TDD
3. Exploring solution? → Skip TDD, test after
4. Simple CRUD? → Skip TDD, test after

Should I mock this?

1. External system? → Mock it
2. Non-deterministic? → Mock it
3. My domain logic? → Don't mock, test it

4. 3 mocks already? → Refactor, too coupled

Test Checklist

Before writing test:

• What behavior am I testing?
• What's the happy path?
• What edge cases matter?
• Can I test this without mocks?

After writing test:

• Is test name descriptive?
• Is AAA structure clear?
• Does test test behavior (not implementation)?
• Will test break only if behavior changes?
• Is test fast (<100ms for unit)?

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Philosophy

"Tests are a safety net, not a security blanket."

Good tests give confidence to refactor. Bad tests give false confidence and slow development.

Test the contract, not the implementation:

• Contract: What code promises to do
• Implementation: How code does it

Tests should:

• Verify behavior works
• Document how to use code
• Enable refactoring with confidence
• Fail only when behavior breaks

Tests should NOT:

• Duplicate production code
• Test framework features
• Prevent all refactoring
• Replace thinking about design

Remember: The goal is confidence, not coverage. Write tests that make you confident the code works, not tests that make metrics happy.