Currency formatting, by locale

Try it

Swift and JavaScript both delegate currency formatting to ICU, the same locale data Apple's Foundation framework uses — so the result below matches what NumberFormatter produces on iOS and macOS for the same locale and currency in the vast majority of cases.

let formatter = NumberFormatter()
formatter.locale = Locale(identifier: "it-IT")
formatter.numberStyle = .currency
formatter.currencyCode = "EUR"

formatter.string(from: 1234.5) // "1234,50 €"

new Intl.NumberFormat("it-IT", {
  style: "currency",
  currency: "EUR",
  currencyDisplay: "symbol",
  currencySign: "standard",
}).format(1234.5); // "1234,50 €"

Common mistakes

• Using the US format ($1,234.50) as the template for every locale and currency.
• Hardcoding the symbol glued to the number ("€1.234,50") instead of letting the locale decide the symbol's position — German puts it after the amount: "1.234,50 €".
• Using a period as the decimal separator when the locale expects a comma (most of continental Europe), or vice versa.
• Treating a negative amount as just a prepended minus sign, ignoring locale and accounting conventions.
• Assuming currency and locale are the same thing — the same currency formats differently across the locales that use it, see below.

Local currency, by locale

Each locale's own currency, formatted with numberStyle = .currency.

Same currency, different locale

1234.5 USD, formatted for locales that don't natively use it. The currency is the same — the formatting rules come entirely from the locale.

Notice the Italian row: 1234,50 USD, not $1.234,50. Two separate locale rules are at play. First, $ is ambiguous — CAD, AUD and MXN use it too — so CLDR (the locale database behind both Intl.NumberFormat and Apple's NumberFormatter) has Italian fall back to the ISO code for the default symbol style, rather than a symbol a reader could mistake for their own currency; asking for currencyDisplay: "narrowSymbol" still returns $. Second, Italian only groups thousands from five integer digits up — 1234,50 stays ungrouped, but 12.345,60 gets its separator. Neither behavior is a bug in the formatter; both come straight from the locale's CLDR data.

A note on Apple's currency styles

Apple exposes currency formatting as four distinct NumberFormatter.Style cases, not as independent options:

• .currency — symbol, standard sign
• .currencyAccounting — symbol, accounting sign (parentheses for negatives)
• .currencyISOCode — ISO code instead of symbol
• .currencyPlural — spelled-out currency name

JavaScript's Intl.NumberFormat models the same idea with two independent options, currencyDisplay and currencySign — which is why the tool above can express combinations, like an accounting ISO code, that Apple's enum doesn't have a single case for.