Formatting

Starting with i18next>=21.3.0 you can use the built-in formatting functions based on the Intl API.
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)}}"
}
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
Overriding the language to use
The language can be overridden by passing it in t.options
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' } } });
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
// 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
For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat​
Currency
// 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
For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat​
DateTime
// 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
For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat​
RelativeTime
// 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
For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/RelativeTimeFormat​
List
// JSON
{
  "intlList": "A list of {{val, list}}"
}
​
i18next.t('intlList', { val: ['locize', 'i18next', 'awesomeness'] });
// --> A list of locize, i18next, and awesomeness
For options see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/ListFormat​
Legacy format function i18next<21.3.0
You can add formatting using moment.js and numeral.js or the intl api.
As a sample using momentjs to format dates.
keys
{
    "key": "The current date is {{date, MM/DD/YYYY}}",
    "key2": "{{text, uppercase}} just uppercased"
}
Init i18next with a format function:
i18next.init({
    interpolation: {
        format: function(value, format, lng) {
            if (format === 'uppercase') return value.toUpperCase();
            if(value instanceof Date) return moment(value).format(format);
            return value;
        }
    }
});
sample
i18next.t('key', { date: new Date() });
// -> "The current date is 07/13/2016"
​
i18next.t('key2', { text: 'can you hear me' });
// => "CAN YOU HEAR ME just uppercased"
Keep the language on moment in sync with i18next by listening to the change language event:
i18next.on('languageChanged', function(lng) {
  moment.locale(lng);
});
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
format
noop function
Passing this function is considered LEGACY in i18next>=21.3.0
​
format function function format(value, format, lng, edit) {}
formatSeparator
','
used to separate format from interpolation value
While there are a lot of options going with the defaults should get you covered.
option	default	description
alwaysFormat	false	used to always call the format function for all interpolated values
format	noop function	Passing this function is considered LEGACY in i18next>=21.3.0 format function `function format(value, format, lng, edit) {}`
formatSeparator	','	used to separate format from interpolation value
Last modified 3mo ago