Format Guide

This guide aims to provide an introduction to ICU MessageFormat syntax, along with minimal code examples that are directly executable in a node.js environment. For production use, we recommend using Usage and the Messages runtime accessor class.

String Lookup

The simplest case of MessageFormat involves no formatting, just a string passthrough. This may sound silly, but often it's nice to always use the same message formatting system when doing translations, and not everything requires variables.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en');
const message = mf.compile('This is a message.');

message(); // "This is a message."

Note: if a message does require data to be passed in, an error is thrown if you do not.


The most common way to use MessageFormat is for simple variable replacement. MessageFormat may look odd at first, but it's actually fairly simple. One way to think about the { and } is that every level of them bring you into and out-of "literal" and "code" mode.

By default (like in the previous example), you are just writing a literal. Then the first level of brackets brings you into one of several data-driven situations. The most simple is variable replacement.

Simply putting a variable name in between { and } will place that variable there in the output.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en');
const varMessage = mf.compile('His name is {NAME}.');

varMessage({ NAME: 'Jed' }); // 'His name is Jed.'


SelectFormat is a lot like a switch statement for your messages. Often it's used to select gender in a string. The format of the statement is {varname, select, this{...} that{...} other{...}}, where varname matches a key in the data you give to the resulting function, and 'this' and 'that' are some of the string-equivalent values that it may have. The other key is required, and is selected if no other case matches.

Note: Comparison is made using the JavaScript == operator, so if a key is left out of the input data, the case undefined{...} would match that.

const MessageFormat = require('messageformat');
const mf = new MesssageFormat('en');
const selectMessage = mf.compile(
  `{ GENDER, select,
       male {He}
       female {She}
       other {They}
   } liked this.`

selectMessage({ GENDER: 'male' }); // 'He liked this.'
selectMessage({ GENDER: 'female' }); // 'She liked this.'
selectMessage({}); // 'They liked this.'


PluralFormat is a similar mechanism to SelectFormat, but specific to numerical values. The key that is chosen is generated from the specified input variable by a locale-specific plural function.

The numeric input is mapped to a plural category, some subset of zero, one, two, few, many, and other depending on the locale and the type of plural. English, for instance, uses one and other for cardinal plurals (e.g. "one result", "many results") and one, two, few, and other for ordinal plurals (e.g. "1st result", "2nd result", etc). For information on which keys are used by your locale, please refer to the CLDR table of plural rules.

For some languages, The number of printed digits is significant (e.g. "1 meter", "1.0 meters"); to account for that you may pass in the value as a stringified representation of the number.

Matches for exact values are available with the = prefix, e.g. =0 and =1.

The keyword for cardinal plurals is plural, and for ordinal plurals is selectordinal.

Within a plural statement, # will be replaced by the variable value.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en');
const messages = mf.compile({
  results: `There { COUNT, plural,
                    =0 {are no results}
                    one {is one result}
                    other {are # results}
  position: `You are { POS, selectordinal,
                       one {#st}
                       two {#nd}
                       few {#rd}
                       other {#th}
                     } in the queue.`

messages.results({ COUNT: 0 }); // 'There are no results.'
messages.results({ COUNT: 1 }); // 'There is one result.'
messages.results({ COUNT: 100 }); // 'There are 100 results.'
messages.position({ POS: 1 }); // 'You are 1st in the queue.'
messages.position({ POS: 33 }); // 'You are 33rd in the queue.'

Plural Offset

To generate sentences such as "You and 4 others added this to their profiles.", PluralFormat supports adding an offset to the variable value before determining its plural category. Literal/exact matches are tested before applying the offset.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en');
const offsetMessage = mf.compile(
  `You { ADDS, plural, offset:1
         =0 {did not add this}
         =1 {added this}
         one {and one other person added this}
         other {and # others added this}

offsetMessage({ ADDS: 0 }); // 'You did not add this.'
offsetMessage({ ADDS: 1 }); // 'You added this.'
offsetMessage({ ADDS: 2 }); // 'You and one other person added this.'
offsetMessage({ ADDS: 3 }); // 'You and 2 others added this.'


MessageFormat includes date, duration, number, and time formatting functions in the style of ICU's simpleArg syntax. They are implemented using the Intl object defined by ECMA-402.

Note: Intl is not defined in Node by default until 0.12 and is not available in all browsers (in particular, IE <=10 and Safari <=9.1), so you may need to use a polyfill.


Supported parameters are short, default, long , or full.

const MessageFormat = require('messageformat');
const mf = new MessageFormat(['en', 'fi']);
const messages = mf.compile({
  en: {
    today: 'Today is {T, date}',
    unix: 'Unix time started on {T, date, full}',
    uptime: '{sys} became operational on {d0, date, short}'
  fi: {
    today: 'Tänään on {T, date}'
});{ T: }); // 'Today is Mar 30, 2018'{ T: }); // 'Tänään on 30. maalisk. 2018'
messages.en.unix({ T: 0 });
// 'Unix time started on Thursday, January 1, 1970'
messages.en.uptime({ sys: 'HAL 9000', d0: '12 January 1999' });
// 'HAL 9000 became operational on 1/12/1999'


Represent a duration in seconds as a string.

const MessageFormat = require('messageformat');
const mf = new MessageFormat();
const messages = mf.compile({
  since: 'It has been {D, duration}',
  countdown: 'Countdown: {D, duration}'

messages.since({ D: 123 }); // 'It has been 2:03'
messages.countdown({ D: -151200.42 }); // 'Countdown: -42:00:00.420'


Supported parameters are integer, percent , or currency. To specify a non-default currency, use currency:CODE.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en');
mf.currency = 'EUR'; // needs to be set before first compile() call
const messages = mf.compile({
  almost: '{N} is almost {N, number, integer}',
  complete: '{P, number, percent} complete',
  currency: {
    eur: 'The total is {V, number, currency}.',
    gbp: 'The total is {V, number, currency:GBP}.'

messages.almost({ N: 3.14 }); // '3.14 is almost 3'
messages.complete({ P: 0.99 }); // '99% complete'
messages.currency.eur({ V: 5.5 }); // 'The total is €5.50.'
messages.currency.gbp({ V: 5.5 }); // 'The total is £5.50.'


Supported parameters are short, default, long , or full.

const MessageFormat = require('messageformat')
const mf = new MessageFormat(['en', 'fi'])
const messages = mf.compile({
  now: {
    en: 'The time is now {T, time}',
    fi: 'Kello on nyt {T, time}'
  eagle: 'The Eagle landed at {T, time, full} on {T, date, full}'
}{ T: })  // 'The time is now 11:26:35 PM'{ T: })  // 'Kello on nyt 23.26.35'
messages.eagle({ T: '1969-07-20 20:17:40 UTC' })
  // 'The Eagle landed at 10:17:40 PM GMT+2 on Sunday, July 20, 1969'

Custom Formatters

In MessageFormat source, a formatter function is called with the syntax {var, name, arg}, where var is a variable, name is the formatter name (by default, either date, duration, number or time; spellout and ordinal are not supported by default), and arg is an optional string argument.

In JavaScript, a formatter is a function called with three parameters:

  • The value of the variable; this can be of any user-defined type
  • The current locale code
  • The trimmed arg string value, or null if not set

As formatter functions may be used in a precompiled context, they should not refer to any variables that are not defined by the function parameters or within the function body. To add your own formatters, either add them to the static MessageFormat.formatters object, or use MessageFormat#addFormatters to add them to a MessageFormat instance.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en-GB');
  upcase: function(v) {
    return v.toUpperCase();
  locale: function(v, lc) {
    return lc;
  prop: function(v, lc, p) {
    return v[p];
const messages = mf.compile({
  describe: 'This is {VAR, upcase}.',
  locale: 'The current locale is {_, locale}.',
  answer: 'Answer: {obj, prop, a}'

messages.describe({ VAR: 'big' }); // 'This is BIG.'
messages.locale({}); // 'The current locale is en-GB.'
messages.answer({ obj: { q: 3, a: 42 } }); // 'Answer: 42'


All types of messageformat statements may be nested within each other, to unlimited depth:

{ SEL1, select,
  other {
    { PLUR1, plural,
      one {1}
      other {
        { SEL2, select, other {Deep in the heart.} }


The characters { and } must be escaped with 'quotes' to be included in the output as literal characters. Within plural statements, # must also be similarly escaped. The utility function MessageFormat.escape may help with this.

const MessageFormat = require('messageformat');
const mf = new MessageFormat('en');
const messages = mf.compile({
  esc: "'{' {S, plural, other{# is a '#'}} '}'",
  var: MessageFormat.escape('Use {var} for variables')

messages.esc({ S: 5 }); // '{ 5 is a # }'
messages.var({ var: 5 }); // 'Use {var} for variables'