Frontend Builder

Build maintainable, performant React and Next.js frontends.

Core Principles

1. Component Composition

Break UI into small, reusable, single-purpose components

2. State Proximity

Keep state as close to where it's used as possible

3. Performance by Default

Optimize rendering, code splitting, and asset loading

4. Developer Experience

Clear naming, consistent patterns, helpful errors

Framework Selection

React (Vite) vs. Next.js

Use React + Vite when:

• Client-side only application
• No SEO requirements
• Simple deployment (static hosting)
• Faster initial setup

Use Next.js when:

• SEO important (marketing sites, blogs, e-commerce)
• Server-side rendering needed
• API routes required
• File-based routing preferred
• Image optimization critical

Recommended for most projects: Next.js (App Router)

---

Component Architecture

Component Types

1. Page Components (Route entry points):

// app/users/page.tsx (Next.js App Router)
export default function UsersPage() {
  return (
    <div>
      <Header />
      <UserList />
      <Footer />
    </div>
  )
}

2. Feature Components (Business logic):

// components/features/UserList.tsx
export function UserList() {
  const { data, isLoading } = useUsers()

  if (isLoading) return <LoadingSpinner />

  return (
    <div>
      {data.map(user => <UserCard key={user.id} user={user} />)}
    </div>
  )
}

3. UI Components (Reusable, no business logic):

// components/ui/button.tsx
export function Button({ children, variant = 'primary', ...props }) {
  return (
    <button
      className={cn(buttonVariants[variant])}
      {...props}
    >
      {children}
    </button>
  )
}

Component Best Practices

// ✅ Good: Small, focused, typed
interface UserProfileProps {
  user: User
  onEdit?: () => void
}

export function UserProfile({ user, onEdit }: UserProfileProps) {
  return (
    <div className="flex gap-4">
      <Avatar src={user.avatar} alt={user.name} />
      <UserDetails user={user} />
      {onEdit && <Button onClick={onEdit}>Edit</Button>}
    </div>
  )
}

// ❌ Bad: Giant, untyped, unclear
export function UserProfile(props) {
  // 500 lines of JSX, multiple responsibilities
  return <div>...</div>
}

---

State Management

Decision Tree

How many components need this state?
│
├─ One component → useState
├─ Parent + children → Props or useState + props
├─ Siblings → Lift to common parent
├─ Widely used (theme, auth) → Context API
└─ Complex app state → Zustand or Redux

Local State (useState)

// For component-level state
function Counter() {
  const [count, setCount] = useState(0)
  const [isOpen, setIsOpen] = useState(false)

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>{count}</button>
    </div>
  )
}

Context API

// For app-wide state (theme, auth, user)
const UserContext = createContext<UserContextType | undefined>(undefined)

export function UserProvider({ children }: { children: ReactNode }) {
  const [user, setUser] = useState<User | null>(null)

  return (
    <UserContext.Provider value={{ user, setUser }}>
      {children}
    </UserContext.Provider>
  )
}

export function useUser() {
  const context = useContext(UserContext)
  if (!context) throw new Error('useUser must be within UserProvider')
  return context
}

Zustand (Recommended for Complex State)

import { create } from 'zustand'

interface CounterStore {
  count: number
  increment: () => void
  decrement: () => void
  reset: () => void
}

export const useCounterStore = create<CounterStore>((set) => ({
  count: 0,
  increment: () => set((state) => ({ count: state.count + 1 })),
  decrement: () => set((state) => ({ count: state.count - 1 })),
  reset: () => set({ count: 0 })
}))

// Usage
function Counter() {
  const { count, increment } = useCounterStore()
  return <button onClick={increment}>{count}</button>
}

---

Data Fetching

React Query (Recommended)

import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'

// Query (GET)
function Users() {
  const { data, isLoading, error } = useQuery({
    queryKey: ['users'],
    queryFn: fetchUsers,
    staleTime: 5 * 60 * 1000 // 5 minutes
  })

  if (isLoading) return <LoadingSpinner />
  if (error) return <ErrorMessage error={error} />

  return <UserList users={data} />
}

// Mutation (POST, PUT, DELETE)
function CreateUser() {
  const queryClient = useQueryClient()

  const mutation = useMutation({
    mutationFn: createUser,
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ['users'] })
    }
  })

  return (
    <button onClick={() => mutation.mutate({ name: 'John' })}>
      Create User
    </button>
  )
}

Next.js Server Components (App Router)

// app/users/page.tsx
// Server Component - fetches on server
export default async function UsersPage() {
  const users = await fetchUsers() // Runs on server

  return <UserList users={users} />
}

// Client Component - for interactivity
'use client'

export function UserList({ users }: { users: User[] }) {
  const [selected, setSelected] = useState<string | null>(null)

  return (
    <div>
      {users.map(user => (
        <UserCard
          key={user.id}
          user={user}
          onClick={() => setSelected(user.id)}
        />
      ))}
    </div>
  )
}

---

Form Handling

React Hook Form (Recommended)

import { useForm } from 'react-hook-form'
import { zodResolver } from '@hookform/resolvers/zod'
import { z } from 'zod'

const loginSchema = z.object({
  email: z.string().email('Invalid email address'),
  password: z.string().min(8, 'Password must be at least 8 characters')
})

type LoginForm = z.infer<typeof loginSchema>

function LoginForm() {
  const {
    register,
    handleSubmit,
    formState: { errors, isSubmitting }
  } = useForm<LoginForm>({
    resolver: zodResolver(loginSchema)
  })

  const onSubmit = async (data: LoginForm) => {
    await login(data)
  }

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <div>
        <input
          {...register('email')}
          type="email"
          placeholder="Email"
          className="border p-2"
        />
        {errors.email && (
          <span className="text-red-500">{errors.email.message}</span>
        )}
      </div>

      <div>
        <input
          {...register('password')}
          type="password"
          placeholder="Password"
          className="border p-2"
        />
        {errors.password && (
          <span className="text-red-500">{errors.password.message}</span>
        )}
      </div>

      <button type="submit" disabled={isSubmitting}>
        {isSubmitting ? 'Logging in...' : 'Login'}
      </button>
    </form>
  )
}

---

Styling

Tailwind CSS (Recommended)

// Install: @shadcn/ui for component library
function Button({ variant = 'primary', children, ...props }) {
  return (
    <button
      className={cn(
        'px-4 py-2 rounded font-medium transition-colors',
        {
          'bg-blue-500 text-white hover:bg-blue-600': variant === 'primary',
          'bg-gray-200 text-gray-900 hover:bg-gray-300': variant === 'secondary',
          'bg-red-500 text-white hover:bg-red-600': variant === 'danger'
        }
      )}
      {...props}
    >
      {children}
    </button>
  )
}

CSS Modules (Alternative)

// Button.module.css
.button {
  padding: 0.5rem 1rem;
  border-radius: 0.25rem;
}

.primary {
  background-color: blue;
  color: white;
}

// Button.tsx
import styles from './Button.module.css'

export function Button({ variant = 'primary', children }) {
  return (
    <button className={`${styles.button} ${styles[variant]}`}>
      {children}
    </button>
  )
}

---

Performance Optimization

React Optimization

import { memo, useMemo, useCallback } from 'react'

// 1. Memoize expensive calculations
function DataTable({ data }) {
  const sortedData = useMemo(
    () => data.sort((a, b) => a.name.localeCompare(b.name)),
    [data]
  )

  return <Table data={sortedData} />
}

// 2. Memoize callbacks
function Parent() {
  const handleClick = useCallback(() => {
    console.log('Clicked')
  }, [])

  return <ExpensiveChild onClick={handleClick} />
}

// 3. Memoize components
const ExpensiveChild = memo(function ExpensiveChild({ onClick }) {
  return <button onClick={onClick}>Click</button>
})

Next.js Optimization

// 1. Image optimization
import Image from 'next/image'

<Image
  src="/photo.jpg"
  alt="Photo"
  width={500}
  height={300}
  priority // Above the fold
/>

// 2. Font optimization
import { Inter } from 'next/font/google'

const inter = Inter({ subsets: ['latin'] })

export default function RootLayout({ children }) {
  return (
    <html className={inter.className}>
      <body>{children}</body>
    </html>
  )
}

// 3. Dynamic imports (code splitting)
import dynamic from 'next/dynamic'

const HeavyComponent = dynamic(() => import('./HeavyComponent'), {
  loading: () => <LoadingSpinner />
})

---

Error Handling

Error Boundary

'use client'

import { Component, ReactNode } from 'react'

interface Props {
  children: ReactNode
  fallback?: ReactNode
}

interface State {
  hasError: boolean
  error?: Error
}

export class ErrorBoundary extends Component<Props, State> {
  constructor(props: Props) {
    super(props)
    this.state = { hasError: false }
  }

  static getDerivedStateFromError(error: Error): State {
    return { hasError: true, error }
  }

  render() {
    if (this.state.hasError) {
      return this.props.fallback || (
        <div className="p-4 bg-red-50 border border-red-200">
          <h2 className="text-red-800">Something went wrong</h2>
          <p className="text-red-600">{this.state.error?.message}</p>
        </div>
      )
    }

    return this.props.children
  }
}

Next.js Error Handling

// app/error.tsx
'use client'

export default function Error({
  error,
  reset
}: {
  error: Error
  reset: () => void
}) {
  return (
    <div>
      <h2>Something went wrong!</h2>
      <button onClick={reset}>Try again</button>
    </div>
  )
}

---

Folder Structure

Next.js App Router

app/
├── (auth)/              # Route group (auth pages)
│   ├── login/
│   └── signup/
├── (dashboard)/         # Route group (dashboard)
│   ├── layout.tsx
│   ├── page.tsx
│   └── settings/
├── api/                 # API routes
│   └── users/
│       └── route.ts
└── layout.tsx           # Root layout

components/
├── ui/                  # shadcn/ui components
│   ├── button.tsx
│   ├── input.tsx
│   └── dialog.tsx
├── features/            # Feature components
│   ├── UserList.tsx
│   └── UserProfile.tsx
└── layouts/             # Layout components
    ├── Header.tsx
    └── Footer.tsx

lib/
├── utils.ts             # Utility functions
├── api.ts               # API client
└── validation.ts        # Zod schemas

hooks/
├── useUser.ts
└── useDebounce.ts

stores/
└── userStore.ts         # Zustand stores

---

TypeScript Best Practices

// 1. Type component props
interface ButtonProps {
  variant?: 'primary' | 'secondary' | 'danger'
  children: ReactNode
  onClick?: () => void
}

export function Button({ variant = 'primary', children, onClick }: ButtonProps) {
  return <button onClick={onClick}>{children}</button>
}

// 2. Type API responses
interface User {
  id: string
  name: string
  email: string
}

async function fetchUsers(): Promise<User[]> {
  const res = await fetch('/api/users')
  return res.json()
}

// 3. Type state
const [user, setUser] = useState<User | null>(null)
const [isLoading, setIsLoading] = useState<boolean>(false)

---

Summary

Great frontends:

• ✅ Use Next.js for most projects (SEO, performance, DX)
• ✅ Break UI into small, typed components
• ✅ Choose appropriate state management (useState → Context → Zustand)
• ✅ Use React Query for server state
• ✅ Style with Tailwind CSS + shadcn/ui
• ✅ Optimize with memoization and code splitting
• ✅ Handle errors gracefully with Error Boundaries
• ✅ Follow consistent folder structure

---

Related Resources

Related Skills:

• api-designer - For designing backend APIs to consume
• ux-designer - For creating UX designs to implement
• deployment-advisor - For hosting Next.js/React apps

Related Patterns:

• META/DECISION-FRAMEWORK.md - Frontend framework selection
• STANDARDS/architecture-patterns/component-patterns.md - Component design patterns (when created)

Related Playbooks:

• PLAYBOOKS/setup-nextjs-project.md - Next.js project setup (when created)
• PLAYBOOKS/optimize-frontend-performance.md - Performance optimization (when created)