/react-native

Hoist Intl Formatter Creation

Don't create Intl.DateTimeFormat, Intl.NumberFormat, or Intl.RelativeTimeFormat inside render or loops. These are expensive to instantiate. Hoist to module scope when the locale/options are static.

Incorrect (new formatter every render):

function Price({ amount }: { amount: number }) {
  const formatter = new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD',
  })
  return <Text>{formatter.format(amount)}</Text>
}

Correct (hoisted to module scope):

const currencyFormatter = new Intl.NumberFormat('en-US', {
  style: 'currency',
  currency: 'USD',
})

function Price({ amount }: { amount: number }) {
  return <Text>{currencyFormatter.format(amount)}</Text>
}

For dynamic locales, memoize:

const dateFormatter = useMemo(
  () => new Intl.DateTimeFormat(locale, { dateStyle: 'medium' }),
  [locale]
)

Common formatters to hoist:

// Module-level formatters
const dateFormatter = new Intl.DateTimeFormat('en-US', { dateStyle: 'medium' })
const timeFormatter = new Intl.DateTimeFormat('en-US', { timeStyle: 'short' })
const percentFormatter = new Intl.NumberFormat('en-US', { style: 'percent' })
const relativeFormatter = new Intl.RelativeTimeFormat('en-US', {
  numeric: 'auto',
})

Creating Intl objects is significantly more expensive than RegExp or plain objects—each instantiation parses locale data and builds internal lookup tables.