react state fallback

Use fallback state instead of initialState

Use undefined as initial state and nullish coalescing (??) to fall back to parent or server values. State represents user intent only—undefined means "user hasn't chosen yet." This enables reactive fallbacks that update when the source changes, not just on initial render.

Incorrect (syncs state, loses reactivity):

type Props = { fallbackEnabled: boolean }

function Toggle({ fallbackEnabled }: Props) {
  const [enabled, setEnabled] = useState(defaultEnabled)
  // If fallbackEnabled changes, state is stale
  // State mixes user intent with default value

  return 
}

Correct (state is user intent, reactive fallback):

type Props = { fallbackEnabled: boolean }

function Toggle({ fallbackEnabled }: Props) {
  const [_enabled, setEnabled] = useState<boolean | undefined>(undefined)
  const enabled = _enabled ?? defaultEnabled
  // undefined = user hasn't touched it, falls back to prop
  // If defaultEnabled changes, component reflects it
  // Once user interacts, their choice persists

  return 
}

With server data:

function ProfileForm({ data }: { data: User }) {
  const [_theme, setTheme] = useState<string | undefined>(undefined)
  const theme = _theme ?? data.theme
  // Shows server value until user overrides
  // Server refetch updates the fallback automatically

  return 
}