The locales and options parameters customize the behavior of the function and let applications specify the language whose formatting conventions should be used.

In implementations that support the Intl.NumberFormat API, these parameters correspond exactly to the Intl.NumberFormat() constructor's parameters. Implementations without Intl.NumberFormat support are asked to ignore both parameters, making the locale used and the form of the string returned entirely implementation-dependent.

locales Optional

A string with a BCP 47 language tag, or an array of such strings. Corresponds to the locales parameter of the Intl.NumberFormat() constructor.

In implementations without Intl.NumberFormat support, this parameter is ignored and the host's locale is usually used.

options Optional

An object adjusting the output format. Corresponds to the options parameter of the Intl.NumberFormat() constructor.

In implementations without Intl.NumberFormat support, this parameter is ignored.

See the Intl.NumberFormat() constructor for details on these parameters and how to use them.

A string with a language-sensitive representation of the given number.

In implementations with Intl.NumberFormat, this is equivalent to new Intl.NumberFormat(locales, options).format(number).

When formatting large numbers of numbers, it is better to create a Intl.NumberFormat object and use the function provided by its format property.

In basic use without specifying a locale, a formatted string in the default locale and with default options is returned.

const number = 3500;

console.log(number.toLocaleString()); // Displays "3,500" if in U.S. English locale

Not all implementations are required to support ECMA-402 (the Internationalization API). For those that don't, the locales and options arguments must both be ignored. You can check support by testing if illegal language tags are rejected with a RangeError:

function toLocaleStringSupportsLocales() {
  const number = 0;
  try {
    number.toLocaleString('i');
  } catch (e) {
    return e.name === 'RangeError';
  }
  return false;
}

However, prior to ES5.1, implementations were not required to throw a range error exception if toLocaleString is called with illegal arguments. A check that works in all hosts, including those supporting ECMA-262 prior to ed 5.1, is to test for the features specified in ECMA-402 that are required to support regional options for Number.prototype.toLocaleString directly:

function toLocaleStringSupportsOptions() {
  return !!(typeof Intl === 'object' && Intl && typeof Intl.NumberFormat === 'function');
}

This tests for a global Intl object, checks that it's not null and that it has a NumberFormat property that is a function.

This example shows some of the variations in localized number formats. In order to get the format of the language used in the user interface of your application, make sure to specify that language (and possibly some fallback languages) using the locales argument:

const number = 123456.789;

// German uses comma as decimal separator and period for thousands
console.log(number.toLocaleString('de-DE'));
// → 123.456,789

// Arabic in most Arabic speaking countries uses Eastern Arabic digits
console.log(number.toLocaleString('ar-EG'));
// → ١٢٣٤٥٦٫٧٨٩

// India uses thousands/lakh/crore separators
console.log(number.toLocaleString('en-IN'));
// → 1,23,456.789

// the nu extension key requests a numbering system, e.g. Chinese decimal
console.log(number.toLocaleString('zh-Hans-CN-u-nu-hanidec'));
// → 一二三,四五六.七八九

// when requesting a language that may not be supported, such as
// Balinese, include a fallback language, in this case Indonesian
console.log(number.toLocaleString(['ban', 'id']));
// → 123.456,789

The results provided by toLocaleString can be customized using the options parameter:

const number = 123456.789;

// request a currency format
console.log(number.toLocaleString('de-DE', { style: 'currency', currency: 'EUR' }));
// → 123.456,79 €

// the Japanese yen doesn't use a minor unit
console.log(number.toLocaleString('ja-JP', { style: 'currency', currency: 'JPY' }))
// → ￥123,457

// limit to three significant digits
console.log(number.toLocaleString('en-IN', { maximumSignificantDigits: 3 }));
// → 1,23,000

// Use the host default language with options for number formatting
const num = 30000.65;
console.log(num.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}));
// → "30,000.65" where English is the default language, or
// → "30.000,65" where German is the default language, or
// → "30 000,65" where French is the default language

• Number.prototype.toString()