Code Smells Review

Quick Reference — Top 15 Code Smells

1. Function >50 lines — should be decomposed
2. File >300 lines — should be split into modules
3. >3 parameters on a function — use an options object
4. Nesting >3 levels deep — use early returns or extract functions
5. Magic numbers/strings — hardcoded values without named constants
6. Duplicated code blocks — same 5+ lines in multiple places
7. Boolean parameters — doThing(true, false, true) is unreadable
8. Dead code — unreachable branches, unused variables, commented-out blocks
9. Inconsistent naming — mixing camelCase and snake_case
10. Comments explaining WHAT — code should be self-documenting (WHY comments are good)
11. Long ternary chains in JSX — use early returns or a lookup object
12. useEffect doing too much — split into focused effects
13. Component >5 props — consider composition or compound components
14. Array index as key — key={i} causes bugs on reorder
15. Inline styles mixed with Tailwind — pick one approach

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Function-Level Smells

Functions Over 50 Lines

A function doing 50+ things is doing too many things. Split by responsibility.

Exception: React components that are essentially just JSX layout can be longer, but the logic portion should still be <50 lines. If a component has 30 lines of hooks/logic + 100 lines of JSX, the logic portion needs review.

Too Many Parameters

// ❌ Unreadable — what do these booleans mean?
createUser('John', 'john@test.com', true, false, 'admin', 1000)

// ✅ Self-documenting with options object
createUser({
  name: 'John',
  email: 'john@test.com',
  verified: true,
  newsletter: false,
  role: 'admin',
  creditLimit: 1000
})

Deep Nesting (>3 levels)

// ❌ Arrow of doom
function processOrder(order) {
  if (order) {
    if (order.items.length > 0) {
      if (order.payment) {
        if (order.payment.verified) {
          // finally doing the actual work, 4 levels deep
        }
      }
    }
  }
}

// ✅ Guard clauses / early returns
function processOrder(order) {
  if (!order) return { error: 'No order' }
  if (order.items.length === 0) return { error: 'Empty order' }
  if (!order.payment) return { error: 'No payment' }
  if (!order.payment.verified) return { error: 'Unverified payment' }
  
  // Actual work at top level — clean and readable
}

Magic Numbers and Strings

// ❌ What is 86400000? What is 'active'?
if (Date.now() - user.lastLogin > 86400000) {
  await updateStatus(user.id, 'active')
}

// ✅ Named constants
const ONE_DAY_MS = 24 * 60 * 60 * 1000
const UserStatus = { ACTIVE: 'active', INACTIVE: 'inactive' } as const

if (Date.now() - user.lastLogin > ONE_DAY_MS) {
  await updateStatus(user.id, UserStatus.ACTIVE)
}

Boolean Parameters

// ❌ What does `true` mean here?
fetchUsers(true, false)

// ✅ Named options
fetchUsers({ includeInactive: true, withOrders: false })

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React-Specific Smells

Long Ternary Chains in JSX

// ❌ Deeply nested ternaries — impossible to read
return (
  <div>
    {loading ? (
      <Spinner />
    ) : error ? (
      <Error message={error} />
    ) : data.length === 0 ? (
      <Empty />
    ) : (
      <DataTable data={data} />
    )}
  </div>
)

// ✅ Early returns — each state is clear
if (loading) return <Spinner />
if (error) return <Error message={error} />
if (data.length === 0) return <Empty />
return <DataTable data={data} />

useEffect Doing Too Much

// ❌ One effect doing 3 unrelated things
useEffect(() => {
  fetchUser()
  setupWebSocket()
  trackPageView()
}, [])

// ✅ Separate concerns — each can have proper cleanup
useEffect(() => { fetchUser() }, [userId])
useEffect(() => {
  const ws = setupWebSocket()
  return () => ws.close()
}, [])
useEffect(() => { trackPageView() }, [pathname])

Component Props Explosion

// ❌ Too many props — hard to use, hard to maintain
<UserCard
  name={user.name}
  email={user.email}
  avatar={user.avatar}
  role={user.role}
  lastLogin={user.lastLogin}
  isOnline={user.isOnline}
  onEdit={handleEdit}
  onDelete={handleDelete}
  showActions={true}
/>

// ✅ Pass the whole object + compose
<UserCard user={user} onEdit={handleEdit} onDelete={handleDelete} />

// Or use compound components for complex cases
<UserCard user={user}>
  <UserCard.Actions>
    <EditButton onClick={handleEdit} />
    <DeleteButton onClick={handleDelete} />
  </UserCard.Actions>
</UserCard>

Array Index as Key

// ❌ Causes bugs when items are reordered, deleted, or inserted
{items.map((item, i) => <ListItem key={i} {...item} />)}

// ✅ Stable unique identifier
{items.map(item => <ListItem key={item.id} {...item} />)}

Why it matters: React uses keys to determine which elements changed. When using index as key and an item is deleted from the middle, React re-renders all subsequent items unnecessarily and may preserve wrong state.

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When NOT to Flag

• Prototypes/MVPs: Don't demand perfection on throwaway code. Flag only P0 issues.
• Generated code: Don't review auto-generated types, migration SQL, or config for style.
• Test files: Slightly verbose code in tests is acceptable for readability. Don't flag test files for function length or naming conventions.
• Configuration files: next.config.ts, tailwind.config.ts, etc. are inherently messy.
• One-time scripts: Migration scripts, seed scripts — focus on correctness, not elegance.

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Naming Conventions (from Airbnb guide)

Element	Convention	Example
Variables, functions	camelCase	getUserById, isActive
Components	PascalCase	UserCard, DashboardPage
Constants	UPPER_SNAKE_CASE	MAX_RETRIES, API_BASE_URL
Types/Interfaces	PascalCase	UserProfile, ApiResponse
Files (components)	PascalCase or kebab-case	UserCard.tsx or user-card.tsx
Files (utilities)	camelCase or kebab-case	formatDate.ts or format-date.ts
Database columns	snake_case	created_at, user_id
Environment variables	UPPER_SNAKE_CASE	NEXT_PUBLIC_API_URL

Flag: Mixing conventions within the same codebase (camelCase variables alongside snake_case variables).

Sources

• Clean Code JavaScript (Ryan McDermott) — adapted from Robert C. Martin
• Airbnb JavaScript Style Guide
• React documentation on composition vs inheritance
• Next.js App Router patterns

Changelog

• 2026-03-21: Initial skill — code smells for code review