testing-unit

name	testing-unit
license	MIT
compatibility	Claude Code 2.1.220+.
description	Unit testing patterns for isolated business logic tests — AAA pattern, parametrized tests (test.each, @pytest.mark.parametrize), fixture scoping (function/module/session), mocking with MSW/VCR at network level, and test data management with factories (FactoryBoy, faker-js). Use when writing unit tests, setting up mocks, structuring test data, optimizing test speed, choosing fixture scope, or reducing test boilerplate. Covers Vitest, Jest, pytest.
tags	["testing","unit","mocking","msw","vcr","fixtures","factories","vitest-4","aroundEach"]
context	fork
agent	test-generator
version	2.1.0
author	OrchestKit
user-invocable	false
disable-model-invocation	false
complexity	medium
persuasion-type	reference
targets	[{"library":"vitest","version":">=4.1.0"}]
metadata	{"category":"document-asset-creation"}
allowed-tools	["Read","Glob","Grep","WebFetch","WebSearch"]
path_patterns	["*.test.*","*.spec.*","**/vitest.config.*","**/jest.config.*"]

Unit Testing Patterns

Focused patterns for writing isolated, fast, maintainable unit tests. Covers test structure (AAA), parametrization, fixture management, HTTP mocking (MSW/VCR), and test data generation with factories.

Each category has individual rule files in rules/ loaded on-demand, plus reference material, checklists, and scaffolding scripts.

Core Principles (ALWAYS apply)

1. AAA structure: Every test MUST follow Arrange-Act-Assert. Use // Arrange, // Act, // Assert comments for clarity.
2. Parametrize, don't duplicate: Use test.each (TypeScript) or @pytest.mark.parametrize (Python) when testing multiple inputs. Never copy-paste the same test body with different values.
3. Fixture scoping matters: Use scope="function" (default) for mutable data. Use scope="module" or scope="session" ONLY for expensive read-only resources (DB engines, ML models). Mutable data with shared scope causes flaky tests.
4. Speed target: Each unit test should run under 100ms. If it's slower, you're likely hitting I/O — mock it.
5. Mock at the network level: Use MSW (TypeScript) or VCR.py (Python) to intercept HTTP at the network layer. Never mock fetch/axios/requests directly.

Quick Reference

Category	Rules	Impact	When to Use
Unit Test Structure	3	CRITICAL	Writing any unit test
HTTP Mocking	2	HIGH	Mocking API calls in frontend/backend tests
Test Data Management	3	MEDIUM	Setting up test data, factories, fixtures

Total: 8 rules across 3 categories, 4 references, 3 checklists, 1 example set, 3 scripts

Unit Test Structure

Core patterns for structuring isolated unit tests with clear phases and efficient execution.

Rule	File