claudegoodies
Skill

test-driven-development

From addyosmani

Drives development with tests. Use when implementing any logic, fixing any bug, or changing any behavior. Use when you need to prove that code works, when a bug report arrives, or when you're about to modify existing functionality.

Facts

Repository
addyosmani/agent-skills
Status
Actively maintained
Last commit
Source file
skills/test-driven-development/SKILL.md

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The instructions Claude Code reads when this skill runs.

# Test-Driven Development

## Overview

Write a failing test before writing the code that makes it pass. For bug fixes, reproduce the bug with a test before attempting a fix. Tests are proof — "seems right" is not done. A codebase with good tests is an AI agent's superpower; a codebase without tests is a liability.

## When to Use

- Implementing any new logic or behavior
- Fixing any bug (the Prove-It Pattern)
- Modifying existing functionality
- Adding edge case handling
- Any change that could break existing behavior

**When NOT to use:** Pure configuration changes, documentation updates, or static content changes that have no behavioral impact.

**Related:** For browser-based changes, combine TDD with runtime verification using Chrome DevTools MCP — see the Browser Testing section below.

## The TDD Cycle

```
    RED                GREEN              REFACTOR
 Write a test    Write minimal code    Clean up the
 that fails  ──→  to make it pass  ──→  implementation  ──→  (repeat)
      │                  │                    │
      ▼                  ▼                    ▼
   Test FAILS        Test PASSES         Tests still PASS
```

### Step 1: RED — Write a Failing Test

Write the test first. It must fail. A test that passes immediately proves nothing.

```typescript
// RED: This test fails because createTask doesn't exist yet
describe('TaskService', () => {
  it('creat
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