thermiteau/mav-bp-unit-testing

Unit Testing Standards

Ensure unit tests are meaningful, fast, and maintainable. Tests verify behaviour, not implementation.

Principles

1. Test behaviour, not implementation — tests verify what code does, not how it does it internally
2. One reason to fail — each test tests one logical concept; when it fails, the cause is obvious
3. Isolate the unit — external dependencies (APIs, databases, file system) are replaced with test doubles
4. Fast and deterministic — unit tests run in milliseconds, never depend on timing, network, or shared state
5. Tests are documentation — test names and structure describe expected behaviour clearly enough to understand the contract without reading the implementation

Test Design

What to Test

• Public API / exported functions — the contract other code depends on
• Edge cases — boundary values, empty inputs, null/undefined, error paths
• Business logic — calculations, transformations, state transitions, validation rules
• Error handling — thrown exceptions, error return values, fallback behaviour

What NOT to Test

• Private/internal implementation details — if you need to test it, extract it to its own unit
• Framework behaviour — don't test that your ORM saves to a database, or that a router routes requests
• Trivial code — simple getters/setters, pass-through functions, type definitions
• External services directly — use test doubles; integration tests cover real service interaction

Test Structure (Arrange-Act-Assert)

Every test follows three phases:

1. Arrange — set up test data and dependencies
2. Act — call the function under test
3. Assert — verify the result

One act per test. If you need multiple acts, you need multiple tests.

Test Isolation & Mocking Discipline

Test Double Types

| Type | Purpose | When to use | | ---- | ------- | ----------- | | Stub | Returns canned data | Replace a dependency that provides input to the unit | | Mock | Verifies interactions | Verify the unit calls a dependency correctly (use sparingly) | | Fake | Simplified implementation | Replace complex infrastructure (in-memory DB, fake HTTP server) | | Spy | Records calls to real implementation | Verify a side effect while keeping real behaviour |

Mocking Rules

• Mock at the boundary — mock external dependencies (APIs, DB, file system), not internal modules
• Prefer stubs over mocks — test what the code returns, not what it calls
• Never mock what you own — if you control the code, test it directly or refactor the design
• One mock per test is ideal — if a test needs many mocks, the unit has too many dependencies
• Always restore — clean up mocks/spies in afterEach to prevent test pollution

Test Data

• Use factory functions or builders for test data, not raw object literals repeated across tests
• Keep test data minimal — only set fields relevant to the test
• Avoid shared mutable state between tests

Test Naming & Organisation

Naming Convention

• Test names describe the behaviour: should return empty array when no items match filter
• Group by unit (function/class) using describe blocks
• Structure: describe('functionName', () => { it('should [behaviour] when [condition]') })

File Organisation

• Test files live alongside or mirror source files (determined by project skill)
• One test file per source module
• Shared test utilities in a dedicated test helpers directory

Test Hygiene

• No logic in tests — no conditionals, loops, or try/catch in test bodies. Tests are linear.
• No test interdependence — tests must pass in any order, in isolation
• Delete flaky tests — a flaky test is worse than no test. Fix the flakiness or remove it.
• Keep tests fast — if a unit test takes >100ms, something is wrong (likely hitting real I/O)
• Coverage is a signal, not a goal — high coverage with weak assertions is worse than moderate coverage with strong assertions. Don't write tests just to hit a number.

Project Implementation Lookup

Before applying these standards, load the project-specific testing implementation:

digraph lookup {
    "docs/maverick/skills/unit-testing/SKILL.md exists?" [shape=diamond];
    "Read and use alongside these standards" [shape=box];
    "Invoke upskill" [shape=box];
    "Read generated skill" [shape=box];

    "docs/maverick/skills/unit-testing/SKILL.md exists?" -> "Read and use alongside these standards" [label="yes"];
    "docs/maverick/skills/unit-testing/SKILL.md exists?" -> "Invoke upskill" [label="no"];
    "Invoke upskill" -> "Read generated skill";
    "Read generated skill" -> "Read and use alongside these standards";
}

1. Check for docs/maverick/skills/unit-testing/SKILL.md
2. If missing, invoke the upskill skill with:
   • topic: unit-testing
   • scan hints:
     • dependencies: vitest, jest, mocha, pytest, unittest, junit, rspec, go test, testing
     • grep: describe\(|it\(|test\(|expect\(|assert|@Test|func Test
     • files: **/*.test.*, **/*.spec.*, **/test_*.*
3. Read the project skill and apply these best practices in the context of the project's specific technology

Detecting Unit Testing Issues in Code Review

| Pattern | Issue | Fix | | ------- | ----- | --- | | Test with no assertions | False confidence | Add meaningful assertions | | Mocking internal modules | Brittle tests | Mock at boundaries only | | Test name describes implementation | Couples to internals | Describe behaviour instead | | Shared mutable state between tests | Test pollution | Use beforeEach/factory functions | | Test catches and swallows errors | Hidden failures | Let errors propagate or assert on them | | >3 mocks in one test | Unit has too many dependencies | Refactor the unit's design | | Test duplicates implementation logic | Tautological test | Assert on outputs, not reimplemented logic | | Commented-out tests | Dead tests hiding failures | Delete or fix |