Formatting

The i18next built-in formats are available without any registration. They use the culture of the target language and the bundled CLDR 47 data, so the results match the browser Intl APIs used by i18next. Currency symbols and number separators come from the culture data of the runtime, which is ICU on .NET 5 and later. On .NET Framework the Windows culture data is used, so a few symbols can differ from the browser.

{
    "price": "Price: {{value, currency(EUR)}}",
    "amount": "{{value, number(minimumFractionDigits: 2)}}",
    "date": "{{value, datetime(dateStyle: long; timeStyle: short)}}",
    "updated": "Updated {{value, relativetime(numeric: auto)}}",
    "inQuarters": "{{value, relativetime(quarter)}}",
    "people": "{{value, list}}",
    "choice": "{{value, list(type: disjunction)}}",
    "shout": "{{value, list, uppercase}}"
}

i18n.T("price", new { value = 1234.5 });                  // Price: €1,234.50
i18n.T("de", "price", new { value = 1234.5 });            // Price: 1.234,50 €
i18n.T("amount", new { value = 5 });                      // 5.00
i18n.T("updated", new { value = -1 });                    // Updated yesterday
i18n.T("inQuarters", new { value = 2 });                  // in 2 quarters
i18n.T("people", new { value = new[] { "Anna", "Ben", "Carl" } });   // Anna, Ben, and Carl
i18n.T("choice", new { value = new[] { "tea", "coffee" } });         // tea or coffee

Format	Options
number	minimumFractionDigits, maximumFractionDigits, useGrouping
currency(EUR)	currency code (positional or currency), minimumFractionDigits, maximumFractionDigits
datetime	dateStyle, timeStyle (full, long, medium, short), weekday, era, year, month, day, hour, minute, second, fractionalSecondDigits, timeZoneName, hour12, hourCycle
relativetime(day)	unit (positional or range): year, quarter, month, week, day, hour, minute, second; numeric (always, auto); style (long, short, narrow)
list	type (conjunction, disjunction, unit), style (long, short, narrow)

Formats are chained with the format separator ({{value, number, uppercase}}). Besides the i18next formats every .NET format string works ({{value, N2}}, {{value, #,##0.00}}), and date-fns or Moment.js patterns are supported by the bundled formatters. Custom formatters implement IFormatter:

public class ReverseFormatter : IFormatter
{
    public bool CanFormat(object value, string format, string language) => format == "reverse";

    public string Format(object value, string format, string language) => new string(value.ToString().Reverse().ToArray());
}

interpolator.Formatters.Add(new ReverseFormatter());
interpolator.ChainableFormats.Add("reverse"); // allows {{value, reverse, uppercase}}

Format parameters

{
    "price": "{{value, number}}"
}

i18n.T("price", new { value = 5, formatParams = new { value = new { minimumFractionDigits = 2 } } });   // 5.00

Interpolation options

translator.ReturnEmptyString = false;   // empty strings fall back to other languages and default values
interpolator.SkipOnVariables = true;    // default: values containing {{...}} or $t(...) are not processed again
interpolator.AlwaysFormat = true;       // values without a format are passed to the formatters, too

Formatters

Formatters are added to DefaultInterpolator.Formatters (or with AddFormatter when using dependency injection). The first formatter whose CanFormat returns true formats the value, the DefaultFormatter is used when none matches.

Formatter	Formats	Example
DefaultFormatter	Always active. The i18next formats via IntlFormatter, otherwise .NET format strings with the culture of the language	{{value, N2}}
IntlFormatter	number, currency, datetime, relativetime and list like the browser Intl APIs	{{value, currency(EUR)}}
DateFnsFormatter	DateTime and DateTimeOffset with date-fns format tokens, all date-fns locales bundled	{{date, EEEE, do MMMM yyyy}}
MomentJsFormatter	DateTime and DateTimeOffset with Moment.js tokens mapped to .NET patterns	{{date, dddd, MMMM Do}}
LowercaseFormatter	lowercase with the culture of the language	{{value, lowercase}}
UppercaseFormatter	uppercase with the culture of the language	{{value, uppercase}}

interpolator.Formatters.Add(new DateFnsFormatter { WeekStartsOn = 1 });

{
    "dueDate": "Due {{date, PPPP}}",
    "week": "Week {{date, wo}} of {{date, yyyy}}"
}

DateFnsFormatter produces the same output as date-fns 4 for every bundled locale, including ordinals (do), the long localized formats (P, PP, PPpp, ...) and week numbering. WeekStartsOn and FirstWeekContainsDate override the locale defaults.