unit-testing-framework — Claude Code skill: unit tests, AAA style

---
name: unit-testing-framework
description: >
  Write comprehensive unit tests with high coverage using testing frameworks
  like Jest, pytest, JUnit, or RSpec. Use when writing tests for functions,
  classes, components, or establishing testing standards.
---

# Unit Testing Framework

## Table of Contents

- [Overview](#overview)
- [When to Use](#when-to-use)
- [Quick Start](#quick-start)
- [Reference Guides](#reference-guides)
- [Best Practices](#best-practices)

## Overview

Write effective unit tests that are fast, isolated, readable, and maintainable following industry best practices and AAA (Arrange-Act-Assert) pattern.

## When to Use

- Writing tests for new code
- Improving test coverage
- Establishing testing standards
- Refactoring with test safety
- Implementing TDD (Test-Driven Development)
- Creating test utilities and mocks

## Quick Start

Minimal working example:

```javascript
// Jest/JavaScript example
describe("UserService", () => {
  describe("createUser", () => {
    it("should create user with valid data", async () => {
      // Arrange - Set up test data and dependencies
      const userData = {
        email: "[email protected]",
        firstName: "John",
        lastName: "Doe",
      };
      const mockDatabase = createMockDatabase();
      const service = new UserService(mockDatabase);

      // Act - Execute the function being tested
      const result = await service.createUser(userData);

      // Assert - Verify the outcome
      expect(result.id).toBeDefined();
      expect(result.email).toBe("[email protected]");
      expect(mockDatabase.save).toHaveBeenCalledWith(
        expect.objectContaining(userData),
      );
    });
  });
});
```

## Reference Guides

Detailed implementations in the `references/` directory:

| Guide | Contents |
|---|---|
| [Test Structure (AAA Pattern)](references/test-structure-aaa-pattern.md) | Test Structure (AAA Pattern) |
| [Test Cases by Language](references/test-cases-by-language.md) | Test Cases by Language |
| [Mocking & Test Doubles](references/mocking-test-doubles.md) | Mocking & Test Doubles |
| [Testing Async Code](references/testing-async-code.md) | Testing Async Code, Test Coverage |
| [Testing Edge Cases](references/testing-edge-cases.md) | Testing Edge Cases |
| [Example: Complete Test Suite](references/example-complete-test-suite.md) | import { UserService } from "./user-service"; |

## Best Practices

### ✅ DO

- Write tests before or alongside code (TDD)
- Test one thing per test
- Use descriptive test names
- Follow AAA pattern
- Test edge cases and error conditions
- Keep tests isolated and independent
- Use setup/teardown appropriately
- Mock external dependencies
- Aim for high coverage on critical paths
- Make tests fast (< 10ms each)
- Use parameterized tests for similar cases
- Test public interfaces, not implementation

### ❌ DON'T

- Test implementation details
- Write tests that depend on each other
- Ignore failing tests
- Test third-party library code
- Use real databases/APIs in unit tests
- Make tests too complex
- Skip edge cases
- Forget to clean up resources
- Test everything (focus on business logic)
- Write flaky tests