senior-frontend

# Senior Frontend

Frontend development patterns, performance optimization, and automation tools for React/Next.js applications.

## When to Use
- Use when scaffolding a new React or Next.js project with TypeScript and Tailwind CSS.
- Use when generating new components or custom hooks.
- Use when analyzing and optimizing bundle sizes for frontend applications.
- Use to implement or review advanced React patterns like Compound Components or Render Props.
- Use to ensure accessibility compliance and implement robust testing strategies.

## Table of Contents

- [Project Scaffolding](#project-scaffolding)
- [Component Generation](#component-generation)
- [Bundle Analysis](#bundle-analysis)
- [React Patterns](#react-patterns)
- [Next.js Optimization](#nextjs-optimization)
- [Accessibility and Testing](#accessibility-and-testing)

---

## Project Scaffolding

Generate a new Next.js or React project with TypeScript, Tailwind CSS, and best practice configurations.

### Workflow: Create New Frontend Project

1. Run the scaffolder with your project name and template:

   ```bash
   python scripts/frontend_scaffolder.py my-app --template nextjs
   ```

2. Add optional features (auth, api, forms, testing, storybook):

   ```bash
   python scripts/frontend_scaffolder.py dashboard --template nextjs --features auth,api
   ```

3. Navigate to the project and install dependencies:

   ```bash
   cd my-app && npm install
   ```

4. Start the development server:
   ```bash
   npm run dev
   ```

### Scaffolder Options

| Option               | Description                                       |
| -------------------- | ------------------------------------------------- |
| `--template nextjs`  | Next.js 14+ with App Router and Server Components |
| `--template react`   | React + Vite with TypeScript                      |
| `--features auth`    | Add NextAuth.js authentication                    |
| `--features api`     | Add React Query + API client                      |
| `--features forms`   | Add React Hook Form + Zod validation              |
| `--features testing` | Add Vitest + Testing Library                      |
| `--dry-run`          | Preview files without creating them               |

### Generated Structure (Next.js)

```
my-app/
├── app/
│   ├── layout.tsx        # Root layout with fonts
│   ├── page.tsx          # Home page
│   ├── globals.css       # Tailwind + CSS variables
│   └── api/health/route.ts
├── components/
│   ├── ui/               # Button, Input, Card
│   └── layout/           # Header, Footer, Sidebar
├── hooks/                # useDebounce, useLocalStorage
├── lib/                  # utils (cn), constants
├── types/                # TypeScript interfaces
├── tailwind.config.ts
├── next.config.js
└── package.json
```

---

## Component Generation

Generate React components with TypeScript, tests, and Storybook stories.

### Workflow: Create a New Component

1. Generate a client component:

   ```bash
   python scripts/component_generator.py Button --dir src/components/ui
   ```

2. Generate a server component:

   ```bash
   python scripts/component_generator.py ProductCard --type server
   ```

3. Generate with test and story files:

   ```bash
   python scripts/component_generator.py UserProfile --with-test --with-story
   ```

4. Generate a custom hook:
   ```bash
   python scripts/component_generator.py FormValidation --type hook
   ```

### Generator Options

| Option          | Description                                  |
| --------------- | -------------------------------------------- |
| `--type client` | Client component with 'use client' (default) |
| `--type server` | Async server component                       |
| `--type hook`   | Custom React hook                            |
| `--with-test`   | Include test file                            |
| `--with-story`  | Include Storybook story                      |
| `--flat`        | Create in output dir without subdirectory    |
| `--dry-run`     | Preview without creating files               |

### Generated Component Example

```tsx
"use client";

import { useState } from "react";
import { cn } from "@/lib/utils";

interface ButtonProps {
  className?: string;
  children?: React.ReactNode;
}

export function Button({ className, children }: ButtonProps) {
  return <div className={cn("", className)}>{children}</div>;
}
```

---

## Bundle Analysis

Analyze package.json and project structure for bundle optimization opportunities.

### Workflow: Optimize Bundle Size

1. Run the analyzer on your project:

   ```bash
   python scripts/bundle_analyzer.py /path/to/project
   ```

2. Review the health score and issues:

   ```
   Bundle Health Score: 75/100 (C)

   HEAVY DEPENDENCIES:
     moment (290KB)
       Alternative: date-fns (12KB) or dayjs (2KB)

     lodash (71KB)
       Alternative: lodash-es with tree-shaking
   ```

3. Apply the recommended fixes by replacing heavy dependencies.

4. Re-run with verbose mode to check import patterns:
   ```bash
   python scripts/bundle_analyzer.py . --verbose
   ```

### Bundle Score Interpretation

| Score  | Grade | Action                         |
| ------ | ----- | ------------------------------ |
| 90-100 | A     | Bundle is well-optimized       |
| 80-89  | B     | Minor optimizations available  |
| 70-79  | C     | Replace heavy dependencies     |
| 60-69  | D     | Multiple issues need attention |
| 0-59   | F     | Critical bundle size problems  |

### Heavy Dependencies Detected

The analyzer identifies these common heavy packages:

| Package       | Size  | Alternative                    |
| ------------- | ----- | ------------------------------ |
| moment        | 290KB | date-fns (12KB) or dayjs (2KB) |
| lodash        | 71KB  | lodash-es with tree-shaking    |
| axios         | 14KB  | Native fetch or ky (3KB)       |
| jquery        | 87KB  | Native DOM APIs                |
| @mui/material | Large | shadcn/ui or Radix UI          |

---

## React Patterns

Reference: `references/react_patterns.md`

### Compound Components

Share state between related components:

```tsx
const Tabs = ({ children }) => {
  const [active, setActive] = useState(0);
  return (
    <TabsContext.Provider value={{ active, setActive }}>
      {children}
    </TabsContext.Provider>
  );
};

Tabs.List = TabList;
Tabs.Panel = TabPanel;

// Usage
<Tabs>
  <Tabs.List>
    <Tabs.Tab>One</Tabs.Tab>
    <Tabs.Tab>Two</Tabs.Tab>
  </Tabs.List>
  <Tabs.Panel>Content 1</Tabs.Panel>
  <Tabs.Panel>Content 2</Tabs.Panel>
</Tabs>;
```

### Custom Hooks

Extract reusable logic:

```tsx
function useDebounce<T>(value: T, delay = 500): T {
  const [debouncedValue, setDebouncedValue] = useState(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebouncedValue(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);

  return debouncedValue;
}

// Usage
const debouncedSearch = useDebounce(searchTerm, 300);
```

### Render Props

Share rendering logic:

```tsx
function DataFetcher({ url, render }) {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    fetch(url)
      .then((r) => r.json())
      .then(setData)
      .finally(() => setLoading(false));
  }, [url]);

  return render({ data, loading });
}

// Usage
<DataFetcher
  url="/api/users"
  render={({ data, loading }) =>
    loading ? <Spinner /> : <UserList users={data} />
  }
/>;
```

---

## Next.js Optimization

Reference: `references/nextjs_optimization_guide.md`

### Server vs Client Components

Use Server Components by default. Add 'use client' only when you need:

- Event handlers (onClick, onChange)
- State (useState, useReducer)
- Effects (useEffect)
- Browser APIs

```tsx
// Server Component (default) - no 'use client'
async function ProductPage({ params }) {
  const product = await getProduct(params.id); // Server-side fetch

  return (
    <div>
      <h1>{product.name}</h1>
      <AddToCartButton productId={product.id} /> {/* Client component */}
    </div>
  );
}

// Client Component
("use client");
function AddToCartButton({ productId }) {
  const [adding, setAdding] = useState(false);
  return <button onClick={() => addToCart(productId)}>Add</button>;
}
```

### Image Optimization

```tsx
import Image from 'next/image';

// Above the fold - load immediately
<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority
/>

// Responsive image with fill
<div className="relative aspect-video">
  <Image
    src="/product.jpg"
    alt="Product"
    fill
    sizes="(max-width: 768px) 100vw, 50vw"
    className="object-cover"
  />
</div>
```

### Data Fetching Patterns

```tsx
// Parallel fetching
async function Dashboard() {
  const [user, stats] = await Promise.all([getUser(), getStats()]);
  return <div>...</div>;
}

// Streaming with Suspense
async function ProductPage({ params }) {
  return (
    <div>
      <ProductDetails id={params.id} />
      <Suspense fallback={<ReviewsSkeleton />}>
        <Reviews productId={params.id} />
      </Suspense>
    </div>
  );
}
```

---

## Accessibility and Testing

Reference: `references/frontend_best_practices.md`

### Accessibility Checklist

1. **Semantic HTML**: Use proper elements (`<button>`, `<nav>`, `<main>`)
2. **Keyboard Navigation**: All interactive elements focusable
3. **ARIA Labels**: Provide labels for icons and complex widgets
4. **Color Contrast**: Minimum 4.5:1 for normal text
5. **Focus Indicators**: Visible focus states

```tsx
// Accessible button
<button
  type="button"
  aria-label="Close dialog"
  onClick={onClose}
  className="focus-visible:ring-2 focus-visible:ring-blue-500"
>
  <XIcon aria-hidden="true" />
</button>

// Skip link for keyboard users
<a href="#main-content" className="sr-only focus:not-sr-only">
  Skip to main content
</a>
```

### Testing Strategy

```tsx
// Component test with React Testing Library
import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";

test("button triggers action on click", async () => {
  const onClick = vi.fn();
  render(<Button onClick={onClick}>Click me</Button>);

  await userEvent.click(screen.getByRole("button"));
  expect(onClick).toHaveBeenCalledTimes(1);
});

// Test accessibility
test("dialog is accessible", async () => {
  render(<Dialog open={true} title="Confirm" />);

  expect(screen.getByRole("dialog")).toBeInTheDocument();
  expect(screen.getByRole("dialog")).toHaveAttribute("aria-labelledby");
});
```

---

## Quick Reference

### Common Next.js Config

```js
// next.config.js
const nextConfig = {
  images: {
    remotePatterns: [{ hostname: "cdn.example.com" }],
    formats: ["image/avif", "image/webp"],
  },
  experimental: {
    optimizePackageImports: ["lucide-react", "@heroicons/react"],
  },
};
```

### Tailwind CSS Utilities

```tsx
// Conditional classes with cn()
import { cn } from "@/lib/utils";

<button
  className={cn(
    "px-4 py-2 rounded",
    variant === "primary" && "bg-blue-500 text-white",
    disabled && "opacity-50 cursor-not-allowed",
  )}
/>;
```

### TypeScript Patterns

```tsx
// Props with children
interface CardProps {
  className?: string;
  children: React.ReactNode;
}

// Generic component
interface ListProps<T> {
  items: T[];
  renderItem: (item: T) => React.ReactNode;
}

function List<T>({ items, renderItem }: ListProps<T>) {
  return <ul>{items.map(renderItem)}</ul>;
}
```

---

## Resources

- React Patterns: `references/react_patterns.md`
- Next.js Optimization: `references/nextjs_optimization_guide.md`
- Best Practices: `references/frontend_best_practices.md`

## Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.