Fullstack Developer

## 🧠 Core Philosophy

1. **Production-first mindset** — Every line of code is written as if it's going to production tomorrow
2. **DRY + SOLID principles** — No duplication, single responsibility, clean interfaces
3. **Security by default** — Authentication, input validation, SQL injection prevention, XSS protection always included
4. **Performance aware** — Caching strategies, lazy loading, query optimization, bundle size management
5. **Test-driven when appropriate** — Unit tests, integration tests, E2E coverage
6. **Explain your choices** — Always briefly explain *why* you made an architectural or implementation decision

---

## 🎨 Frontend Excellence

### Frameworks & When to Use


| Framework        | Best For                                          |
| ---------------- | ------------------------------------------------- |
| **Next.js**      | SSR, SEO, full-stack, production apps             |
| **React + Vite** | SPAs, dashboards, internal tools                  |
| **Vue 3 + Nuxt** | Teams preferring composition API, smaller bundles |
| **Vanilla JS**   | Lightweight widgets, no framework overhead needed |

### Component Patterns

```jsx
// ✅ ALWAYS write components like this — typed, accessible, composable
interface ButtonProps {
  variant?: 'primary' | 'secondary' | 'danger';
  size?: 'sm' | 'md' | 'lg';
  loading?: boolean;
  disabled?: boolean;
  onClick?: () => void;
  children: React.ReactNode;
}

export const Button = ({
  variant = 'primary',
  size = 'md',
  loading = false,
  disabled = false,
  onClick,
  children
}: ButtonProps) => {
  return (
    <button
      className={cn(buttonVariants({ variant, size }))}
      disabled={disabled || loading}
      onClick={onClick}
      aria-busy={loading}
    >
      {loading ? <Spinner size="sm" /> : children}
    </button>
  );
};
```

### State Management Strategy

- **Local state** → `useState` / `useReducer`
- **Server state** → `TanStack Query` (React Query)
- **Global UI state** → `Zustand` (lightweight) or `Jotai`
- **Forms** → `React Hook Form` + `Zod` validation
- **Avoid Redux** unless team is already using it and app is large

### CSS Approach (Preferred Order)

1. **Tailwind CSS** — utility-first, fast, consistent
2. **CSS Modules** — scoped styles for complex components
3. **shadcn/ui** — for rapid UI with Tailwind
4. Avoid inline styles (except dynamic values)

---

## ⚙️ Backend Excellence

### API Design (REST)

```
GET    /api/v1/users          → List users (paginated)
POST   /api/v1/users          → Create user
GET    /api/v1/users/:id      → Get single user
PUT    /api/v1/users/:id      → Full update
PATCH  /api/v1/users/:id      → Partial update
DELETE /api/v1/users/:id      → Soft delete (set deleted_at)

Always version your APIs: /api/v1/...
Always return consistent response shape:
{
  "success": true,
  "data": { ... },
  "meta": { "page": 1, "total": 100 },
  "error": null
}
```

### Node.js / Express Best Practices

```typescript
// ✅ Proper error handling middleware
app.use((err: Error, req: Request, res: Response, next: NextFunction) => {
  const status = err instanceof AppError ? err.statusCode : 500;
  logger.error({ err, req: { method: req.method, url: req.url } });
  res.status(status).json({
    success: false,
    data: null,
    error: {
      message: status === 500 ? 'Internal server error' : err.message,
      code: err.name
    }
  });
});

// ✅ Always use async wrapper to avoid unhandled rejections
const asyncHandler = (fn: Function) => (req: Request, res: Response, next: NextFunction) => {
  Promise.resolve(fn(req, res, next)).catch(next);
};
```

### Python / FastAPI Best Practices

```python
from fastapi import FastAPI, HTTPException, Depends, status
from pydantic import BaseModel, validator
from typing import Optional

app = FastAPI(title="My API", version="1.0.0")

class UserCreate(BaseModel):
    email: str
    password: str
    name: str

    @validator('email')
    def email_must_be_valid(cls, v):
        if '@' not in v:
            raise ValueError('Invalid email')
        return v.lower()

@app.post("/users", response_model=UserResponse, status_code=status.HTTP_201_CREATED)
async def create_user(user: UserCreate, db: AsyncSession = Depends(get_db)):
    # Always check for conflicts before creating
    existing = await db.get_user_by_email(user.email)
    if existing:
        raise HTTPException(status_code=409, detail="Email already registered")
    return await db.create_user(user)
```

---

## 🗃️ Database Design

### PostgreSQL Schema Conventions

```sql
-- ✅ Always include these in every table
CREATE TABLE users (
  id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
  updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
  deleted_at  TIMESTAMPTZ,  -- soft delete
  
  -- actual columns
  email       TEXT NOT NULL UNIQUE,
  name        TEXT NOT NULL,
  
  -- indexes
  CONSTRAINT users_email_check CHECK (email ~* '^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}$')
);

CREATE INDEX CONCURRENTLY idx_users_email ON users(email) WHERE deleted_at IS NULL;
CREATE INDEX CONCURRENTLY idx_users_created_at ON users(created_at DESC);
```

### ORM Usage

- **Prisma** (Node.js) — best DX, type-safe, migrations
- **SQLAlchemy** (Python) — most powerful, flexible
- **DrizzleORM** (Node.js) — lightweight, SQL-like syntax

### Query Optimization Rules

1. Always index foreign keys
2. Use `SELECT specific_columns` not `SELECT *`
3. Add `LIMIT` to all list queries
4. Use connection pooling (PgBouncer or built-in pool)
5. Explain analyze slow queries

---

## 🔐 Security Standards

### Authentication (Always implement these)

```typescript
// JWT with refresh tokens
const ACCESS_TOKEN_EXPIRY = '15m';   // Short-lived
const REFRESH_TOKEN_EXPIRY = '7d';   // Long-lived, stored in httpOnly cookie

// Password hashing
import bcrypt from 'bcryptjs';
const SALT_ROUNDS = 12;
const hashedPassword = await bcrypt.hash(password, SALT_ROUNDS);

// Never store plain passwords. Never log passwords. Never return passwords in API responses.
```

### Input Validation (Always)

```typescript
// Zod schema validation
import { z } from 'zod';

const CreateUserSchema = z.object({
  email: z.string().email().toLowerCase(),
  password: z.string().min(8).max(100).regex(/(?=.*[A-Z])(?=.*[0-9])/),
  name: z.string().min(1).max(255).trim()
});

// Validate at the edge — in middleware before it hits your handler
```

### Security Checklist

- [ ]  HTTPS everywhere
- [ ]  Rate limiting on auth endpoints
- [ ]  CORS configured properly
- [ ]  Helmet.js (Node) / security headers
- [ ]  SQL injection prevention (parameterized queries only)
- [ ]  XSS prevention (sanitize user input)
- [ ]  CSRF tokens for state-changing requests
- [ ]  Secrets in environment variables, never in code

---

## 🐳 DevOps & Deployment

### Docker Setup

```dockerfile
# ✅ Production-optimized multi-stage Dockerfile
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production

FROM node:20-alpine AS runner
WORKDIR /app
ENV NODE_ENV=production
COPY --from=builder /app/node_modules ./node_modules
COPY . .
EXPOSE 3000
USER node
CMD ["node", "server.js"]
```

### Docker Compose (Full Stack)

```yaml
version: '3.9'
services:
  app:
    build: .
    ports: ["3000:3000"]
    environment:
      DATABASE_URL: postgresql://user:pass@db:5432/myapp
    depends_on:
      db:
        condition: service_healthy
  
  db:
    image: postgres:16-alpine
    volumes: [postgres_data:/var/lib/postgresql/data]
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user"]
      interval: 5s

volumes:
  postgres_data:
```

### Deployment Platforms


| Platform          | Best For                  |
| ----------------- | ------------------------- |
| **Vercel**        | Next.js, frontend         |
| **Railway**       | Full-stack, quick deploys |
| **Render**        | APIs, workers, databases  |
| **AWS/GCP/Azure** | Enterprise, custom needs  |
| **Fly.io**        | Global edge, Docker apps  |

---

## 🧪 Testing Strategy

```typescript
// Unit test example (Vitest / Jest)
describe('UserService', () => {
  it('should hash password before saving', async () => {
    const user = await userService.create({ email: '[email protected]', password: 'Secret123' });
    expect(user.password).not.toBe('Secret123');
    expect(await bcrypt.compare('Secret123', user.password)).toBe(true);
  });

  it('should throw 409 if email already exists', async () => {
    await userService.create({ email: '[email protected]', password: 'Secret123' });
    await expect(userService.create({ email: '[email protected]', password: 'Secret123' }))
      .rejects.toThrow('Email already registered');
  });
});
```

**Coverage targets:**

- Unit tests: Business logic, utilities, validators → 80%+
- Integration tests: API endpoints, database operations → Key flows
- E2E tests (Playwright): Critical user journeys only

---

## 📦 Project Structure

### Next.js App (Recommended)

```
my-app/
├── src/
│   ├── app/                    # App router pages
│   │   ├── (auth)/login/       # Route groups
│   │   ├── dashboard/
│   │   └── api/                # API routes
│   ├── components/
│   │   ├── ui/                 # Generic UI (Button, Input, Modal)
│   │   └── features/           # Feature-specific components
│   ├── lib/
│   │   ├── db.ts               # Database connection
│   │   ├── auth.ts             # Auth helpers
│   │   └── validations.ts      # Zod schemas
│   ├── hooks/                  # Custom React hooks
│   ├── services/               # Business logic (not React-specific)
│   └── types/                  # TypeScript types
├── prisma/schema.prisma
├── .env.local
└── docker-compose.yml
```

---

## 🔍 Code Review Standards

When reviewing code, always check for:

1. **Security vulnerabilities** (injection, auth bypass, exposed secrets)
2. **N+1 query problems** (missing eager loading / batching)
3. **Missing error handling** (unhandled promises, no try/catch)
4. **Race conditions** (concurrent operations without locks)
5. **Memory leaks** (event listeners not cleaned up, infinite loops)
6. **Missing input validation**
7. **Hardcoded credentials or magic numbers**

---

## 💡 Common Patterns Reference

For detailed implementations, see:

- `references/auth-patterns.md` — JWT, OAuth, session management
- `references/api-patterns.md` — Pagination, filtering, rate limiting
- `references/frontend-patterns.md` — Forms, data fetching, routing

---

## 🏆 Quality Bar

Every output from this skill should feel like it came from a **senior engineer at a top tech company**. That means:

- ✅ TypeScript types always included
- ✅ Error handling is never an afterthought
- ✅ Brief comments on *why*, not *what*
- ✅ Accessible HTML (proper ARIA, semantic tags)
- ✅ Environment variables for all config
- ✅ Never hardcode URLs, secrets, or magic numbers
- ✅ Responsive by default
- ✅ Loading and error states always handled