tdd-expert

# Test-Driven Development (TDD) Expert

**Self-contained TDD expertise for ANY user project.**

---

## The TDD Cycle: Red-Green-Refactor

### 1. RED Phase: Write Failing Test

**Goal**: Define expected behavior through a failing test

```typescript
import { describe, it, expect } from 'vitest';
import { Calculator } from './Calculator';

describe('Calculator', () => {
  it('should add two numbers', () => {
    const calculator = new Calculator();
    expect(calculator.add(2, 3)).toBe(5); // WILL FAIL - Calculator doesn't exist
  });
});
```

**RED Checklist**:
- [ ] Test describes ONE specific behavior
- [ ] Test fails for RIGHT reason (not syntax error)
- [ ] Test name is clear
- [ ] Expected behavior obvious

### 2. GREEN Phase: Minimal Implementation

**Goal**: Simplest code that makes test pass

```typescript
// Calculator.ts
export class Calculator {
  add(a: number, b: number): number {
    return a + b; // Minimal implementation
  }
}
```

**GREEN Checklist**:
- [ ] Test passes
- [ ] Code is simplest possible
- [ ] No premature optimization
- [ ] No extra features

### 3. REFACTOR Phase: Improve Design

**Goal**: Improve code quality without changing behavior

```typescript
// Refactor: Support variable arguments
export class Calculator {
  add(...numbers: number[]): number {
    return numbers.reduce((sum, n) => sum + n, 0);
  }
}

// Tests still pass!
```

**REFACTOR Checklist**:
- [ ] All tests still pass
- [ ] Code is more readable
- [ ] Removed duplication
- [ ] Better design patterns

---

## TDD Benefits

**Design Benefits**:
- Forces modular, testable code
- Reveals design problems early
- Encourages SOLID principles
- Promotes simple solutions

**Quality Benefits**:
- 100% test coverage (by definition)
- Tests document behavior
- Regression safety net
- Faster debugging

**Productivity Benefits**:
- Less time debugging
- Confidence to refactor
- Faster iterations
- Clearer requirements

---

## BDD: Behavior-Driven Development

**Extension of TDD wi