Formatting

Starting with i18next>=21.3.0 you can use the built-in formatting functions based on the Intl API.

Starting with i18next v26.0.0, the legacy interpolation.format function will be removed. The built-in Formatter is now always used. For custom formatting, use i18next.services.formatter.add() or .addCached() as described below, or provide a custom Formatter module via .use().

You may need to polyfill the Intl API:

• Intl.PluralRules

• Intl.RelativeTimeFormat

• Intl.ListFormat

• Intl.DisplayNames

• Intl.NumberFormat (ES2020)

• Intl.Locale

• Intl.getCanonicalLocales

• Intl.DateTimeFormat (ES2020)

🎓 Check out this topic in the i18next crash course video.

Basic usage

The translation string has the following signature:

{
  "key": "Some format {{value, formatname}}",
  "keyWithOptions": "Some format {{value, formatname(option1Name: option1Value; option2Name: option2Value)}}"
}

Use a "semicolon" delimited list of options.

JavaScript

Passing options to the formatting:

1. In the translation string using {{value, formatname(options1: options1Value)}}

2. Using the root level options when calling t('key', { option1: option1Value })

3. Using the per value options like: t('key', { formatParams: { value: { option1: option1Value } })

Samples

// JSON
{
  "intlNumber": "Some {{val, number}}",
  "intlNumberWithOptions": "Some {{val, number(minimumFractionDigits: 2)}}"
}

i18next.t('intlNumber', { val: 1000 });
// --> Some 1,000
i18next.t('intlNumber', { val: 1000.1, minimumFractionDigits: 3 });
// --> Some 1,000.100
i18next.t('intlNumber', { val: 1000.1, formatParams: { val: { minimumFractionDigits: 3 } } });
// --> Some 1,000.100
i18next.t('intlNumberWithOptions', { val: 2000 });
// --> Some 2,000.00
i18next.t('intlNumberWithOptions', { val: 2000, minimumFractionDigits: 3 });
// --> Some 2,000.000

TypeScript

Overriding the language to use

The language can be overridden by passing it in t.options

JavaScript

i18next.t('intlNumber', { val: 1000.1, lng: 'de' }); 
// or: i18next.t('intlNumber', { val: 1000.1, locale: 'de' });
i18next.t('intlNumber', { val: 1000.1, formatParams: { val: { lng: 'de' } } });
// or: i18next.t('intlNumber', { val: 1000.1, formatParams: { val: { locale: 'de' } } });

TypeScript

Adding custom format function

It's rather simple to add own function:

// after i18next.init(options);
i18next.services.formatter.add('lowercase', (value, lng, options) => {
  return value.toLowerCase();
});
i18next.services.formatter.add('underscore', (value, lng, options) => {
  return value.replace(/\s+/g, '_');
});

Make sure you add your custom format function AFTER the i18next.init() call.

There's also an addCached version for optimized performance:

i18next.services.formatter.addCached('specialformat', (lng, options) => {
  const formatter = new Intl.NumberFormat(lng, options);
  return (val) => formatter.format(val);
});

Using multiple formatters

{
  "key": "Some format {{value, formatter1, formatter2}}"
}

Built-in formats

Number

JavaScript

// JSON
{
  "intlNumber": "Some {{val, number}}",
  "intlNumberWithOptions": "Some {{val, number(minimumFractionDigits: 2)}}"
}

i18next.t('intlNumber', { val: 1000 });
// --> Some 1,000
i18next.t('intlNumber', { val: 1000.1, minimumFractionDigits: 3 });
// --> Some 1,000.100
i18next.t('intlNumber', { val: 1000.1, formatParams: { val: { minimumFractionDigits: 3 } } });
// --> Some 1,000.100
i18next.t('intlNumberWithOptions', { val: 2000 });
// --> Some 2,000.00
i18next.t('intlNumberWithOptions', { val: 2000, minimumFractionDigits: 3 });
// --> Some 2,000.000

TypeScript

For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat

Currency

JavaScript

// JSON
{
  "intlCurrencyWithOptionsSimplified": "The value is {{val, currency(USD)}}",
  "intlCurrencyWithOptions": "The value is {{val, currency(currency: USD)}}",
  "twoIntlCurrencyWithUniqueFormatOptions": "The value is {{localValue, currency}} or {{altValue, currency}}",
}

i18next.t('intlCurrencyWithOptionsSimplified', { val: 2000 });
// --> The value is $2,000.00
i18next.t('intlCurrencyWithOptions', { val: 2300 });
// --> The value is $2,300.00
i18next.t('twoIntlCurrencyWithUniqueFormatOptions',
          {
            localValue: 12345.67,
            altValue: 16543.21,
            formatParams: {
              localValue: { currency: 'USD', locale: 'en-US' },
              altValue: { currency: 'CAD', locale: 'fr-CA' },
            },
          },);
// --> The value is $12,345.67 or 16 543,21 $ CA

TypeScript

For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat

DateTime

JavaScript

// JSON
{
  "intlDateTime": "On the {{val, datetime}}",
}

i18next.t('intlDateTime', { val: new Date(Date.UTC(2012, 11, 20, 3, 0, 0)) });
// --> On the 12/20/2012
i18next.t('intlDateTime',
          {
            val: new Date(Date.UTC(2012, 11, 20, 3, 0, 0)),
            formatParams: {
              val: { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' },
            },
          });
// --> On the Thursday, December 20, 2012

TypeScript

For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat

RelativeTime

JavaScript

// JSON
{
  "intlRelativeTime": "Lorem {{val, relativetime}}",
  "intlRelativeTimeWithOptions": "Lorem {{val, relativetime(quarter)}}",
  "intlRelativeTimeWithOptionsExplicit": "Lorem {{val, relativetime(range: quarter; style: narrow;)}}",
}

i18next.t('intlRelativeTime', { val: 3 });
// --> Lorem in 3 days
i18next.t('intlRelativeTimeWithOptions', { val: -3 });
// --> Lorem 3 quarters ago
i18next.t('intlRelativeTimeWithOptionsExplicit', { val: -3 });
// --> Lorem 3 qtrs. ago
i18next.t('intlRelativeTimeWithOptionsExplicit', { val: -3, style: 'long' });
// --> Lorem 3 quarters ago

TypeScript

For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/RelativeTimeFormat

List

JavaScript

// JSON
{
  "intlList": "A list of {{val, list}}"
}

i18next.t('intlList', { val: ['locize', 'i18next', 'awesomeness'] });
// --> A list of locize, i18next, and awesomeness

TypeScript

For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/ListFormat

Additional options

Prefix/Suffix for interpolation and other options can be overridden in init option:

sample

i18next.init({
    interpolation: { ... }
});

option	default	description
alwaysFormat	false	used to always call the format function for all interpolated values
formatSeparator	','	used to separate format from interpolation value

While there are a lot of options going with the defaults should get you covered.

Custom formatter

Do you want to create some sort of default formatter, that does not need a specific format to be passed via i18n resources? i18next gets you covered also with that...

Create a custom formatter plugin, and pass it to i18next:

JavaScript

const myFormatter = {
  type: 'formatter',
  init(services, backendOptions, i18nextOptions) {}, // of you need some init stuff to be done...
  format(value, format, lng, options) {
    // do whatever you like here... and return the formatted string
    if (!format && value instanceof Date) {
      return value.toUTCString()
    }
  },
  add(name, fc) {
    // handle adding a new format, if you need it
  },
  addCached(name, fc) {
    // handle adding a new cached format, if you need it
  }
}

i18next
  .use(myFormatter)
  .init({
    fallbackLng: "en",
    interpolation: {
      alwaysFormat: true // set alwaysFormat to true
    },
    resources: {
      en: {
        translation: {
          testKeyWithoutFormat: 'On the "{val}"',
        }
      }
    }
  });
  
i18next.t("testKeyWithoutFormat", { val: new Date() }) // On the "Mon, 07 Jul 2025 11:44:50 GMT"

TypeScript