Component Refactoring

Refactor high-complexity React components with structured patterns.

Complexity Threshold: Components with complexity > 50 or lineCount > 300 should be refactored before testing.

Complexity Score Interpretation

Core Refactoring Patterns

Pattern 1: Extract Custom Hooks

When: Component has complex state management, multiple useEffects, or business logic mixed with UI.

// ❌ Before: Logic mixed in component
const Component = () => {
  const [data, setData] = useState(null)
  const [loading, setLoading] = useState(false)
  const [error, setError] = useState(null)
  
  useEffect(() => {
    setLoading(true)
    fetchData().then(setData).catch(setError).finally(() => setLoading(false))
  }, [])
  
  return <div>{/* ... */}</div>
}

// ✅ After: Logic extracted to hook
const useDataFetching = () => {
  const [data, setData] = useState(null)
  const [loading, setLoading] = useState(false)
  const [error, setError] = useState(null)
  
  useEffect(() => {
    setLoading(true)
    fetchData().then(setData).catch(setError).finally(() => setLoading(false))
  }, [])
  
  return { data, loading, error }
}

const Component = () => {
  const { data, loading, error } = useDataFetching()
  return <div>{/* ... */}</div>
}

Pattern 2: Split Large Components

When: Component has multiple distinct sections or responsibilities.

// ❌ Before: One large component
const Dashboard = () => {
  return (
    <div>
      {/* 100 lines of header */}
      {/* 150 lines of sidebar */}
      {/* 200 lines of content */}
    </div>
  )
}

// ✅ After: Split into focused components
const Dashboard = () => {
  return (
    <div>
      <DashboardHeader />
      <DashboardSidebar />
      <DashboardContent />
    </div>
  )
}

Pattern 3: Extract Render Functions

When: Complex conditional rendering or list rendering.

// ❌ Before: Inline complex rendering
const List = ({ items }) => {
  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>
          {item.type === 'a' ? (
            <div>{/* 50 lines */}</div>
          ) : item.type === 'b' ? (
            <div>{/* 40 lines */}</div>
          ) : (
            <div>{/* 30 lines */}</div>
          )}
        </li>
      ))}
    </ul>
  )
}

// ✅ After: Extract to separate components
const ListItemA = ({ item }) => <div>{/* ... */}</div>
const ListItemB = ({ item }) => <div>{/* ... */}</div>
const ListItemDefault = ({ item }) => <div>{/* ... */}</div>

const ListItem = ({ item }) => {
  const components = { a: ListItemA, b: ListItemB }
  const Component = components[item.type] || ListItemDefault
  return <Component item={item} />
}

Pattern 4: Consolidate State

When: Multiple related useState calls.

// ❌ Before: Related state scattered
const [name, setName] = useState('')
const [email, setEmail] = useState('')
const [phone, setPhone] = useState('')

// ✅ After: Consolidated state
const [formValues, setFormValues] = useState({
  name: '',
  email: '',
  phone: ''
})

// Or use useReducer for complex state
const [state, dispatch] = useReducer(formReducer, initialState)

Refactoring Workflow

1. Measure Current Complexity

   • Count lines of code
   • Count useState/useEffect calls
   • Identify nested conditionals

2. Identify Extraction Candidates

   • Related state that move together
   • Reusable UI sections
   • Business logic that can be isolated

3. Apply Patterns (one at a time)

   • Extract hooks for state/logic
   • Split components for UI sections
   • Consolidate related state

4. Verify After Each Change

   • Tests still pass
   • Behavior unchanged
   • Complexity reduced

Common Mistakes to Avoid

❌ Over-Engineering

// ❌ Too many tiny hooks
const useButtonText = () => useState('Click')
const useButtonDisabled = () => useState(false)

// ✅ Cohesive hook with related state
const useButtonState = () => {
  const [text, setText] = useState('Click')
  const [disabled, setDisabled] = useState(false)
  return { text, setText, disabled, setDisabled }
}

❌ Breaking Existing Patterns

• Follow existing directory structures
• Maintain naming conventions
• Preserve export patterns

❌ Premature Abstraction

• Only extract when there's clear benefit
• Don't create abstractions for single-use code
• Keep refactored code in the same domain area

Success Criteria

After refactoring:

• Complexity score < 50
• Line count < 300
• Max function complexity < 30
• All tests pass
• No behavior changes