Minimize State Variables and Derive Values

Use the fewest state variables possible. If a value can be computed from existing state or props, derive it during render instead of storing it in state. Redundant state causes unnecessary re-renders and can drift out of sync.

Incorrect (redundant state):

function Cart({ items }: { items: Item[] }) {
  const [total, setTotal] = useState(0)
  const [itemCount, setItemCount] = useState(0)

  useEffect(() => {
    setTotal(items.reduce((sum, item) => sum + item.price, 0))
    setItemCount(items.length)
  }, [items])

  return (
    <View>
      <Text>{itemCount} items</Text>
      <Text>Total: ${total}</Text>
    </View>
  )
}

Correct (derived values):

function Cart({ items }: { items: Item[] }) {
  const total = items.reduce((sum, item) => sum + item.price, 0)
  const itemCount = items.length

  return (
    <View>
      <Text>{itemCount} items</Text>
      <Text>Total: ${total}</Text>
    </View>
  )
}

Another example:

// Incorrect: storing both firstName, lastName, AND fullName
const [firstName, setFirstName] = useState('')
const [lastName, setLastName] = useState('')
const [fullName, setFullName] = useState('')

// Correct: derive fullName
const [firstName, setFirstName] = useState('')
const [lastName, setLastName] = useState('')
const fullName = `${firstName} ${lastName}`

State should be the minimal source of truth. Everything else is derived.

Reference: Choosing the State Structure