# Tdd

> Guide through the Red-Green-Refactor cycle for test-driven development

- Skill: `jellydn/tdd` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add jellydn/tdd`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jellydn/tdd/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: MIT
- Author: jellydn (https://skillmd.com/u/jellydn)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/jellydn/tdd

---


# Test-Driven Development (TDD)

Guides you through the complete TDD workflow with Red-Green-Refactor cycle.

## Usage

`/tdd <ACTION> [ARGUMENTS]`

## Actions

- **start <FEATURE>** - Initialize TDD session for a feature
- **red <TEST_NAME>** - Create failing test (Red phase)
- **green** - Run tests and implement code (Green phase)
- **refactor** - Guide refactoring process (Refactor phase)
- **cycle <FEATURE>** - Run complete Red-Green-Refactor cycle
- **watch** - Start test watcher for continuous feedback
- **status** - Show current test status and next steps

## TDD Principles

### The Red-Green-Refactor Cycle

1. **Red**: Write a failing test that defines desired behavior
2. **Green**: Write minimal code to make the test pass
3. **Refactor**: Improve code quality while keeping tests green

### Best Practices

- Write tests first - Tests define the interface and behavior
- Small steps - Make tiny, incremental changes
- Fast feedback - Run tests frequently for immediate validation
- Clean code - Refactor regularly to maintain quality
- One concept per test - Keep tests focused and atomic
- AAA Pattern - Structure tests as Arrange, Act, Assert
- Black-box testing - Test only public methods and behavior, not implementation details

## Process

### For "start <FEATURE>":

1. Initialize TDD session for the feature
2. Create or identify target source file
3. Create corresponding test file if it doesn't exist
4. Explain the feature requirements and acceptance criteria

### For "red <TEST_NAME>":

1. Create a failing test that describes the desired behavior
2. Ensure test fails for the right reason (not syntax errors)
3. Run tests to confirm red state
4. Explain what the test is validating

### For "green":

1. Implement minimal code to make failing tests pass
2. Focus on making tests pass, not perfect code
3. Run tests to confirm green state
4. Write just enough code to pass the test

### For "refactor":

1. Improve code quality while maintaining green tests
2. Remove duplication and improve design
3. Run tests continuously during refactoring
4. Make one refactoring change at a time

## Test Template

A test template is available at `$SKILL_PATH/templates/test-template.md`:

```typescript
import { describe, it, expect } from "vitest";
import { functionName } from "./module";

describe("functionName", () => {
	it("should return formatted output when given valid input", () => {
		// Arrange - Setup test scenario
		const input = "test input";
		const expectedOutput = "expected output";

		// Act - Execute the unit under test
		const result = functionName(input);

		// Assert - Verify expected outcome
		expect(result).toBe(expectedOutput);
	});
});
```

## Common Commands

- Run tests: `npm test` or `pnpm test`
- Watch mode: `npm test --watch`
- With coverage: `npm test --coverage`