Fluent Syntax Functions

fluent-syntax-functions

Quick Reference

Built-in Functions

Function Call Grammar (EBNF)

FunctionReference ::= Identifier CallArguments
CallArguments     ::= blank? "(" blank? argument_list blank? ")"
argument_list     ::= (Argument blank? "," blank?)* Argument?
Argument          ::= NamedArgument | InlineExpression
NamedArgument     ::= Identifier blank? ":" blank? (StringLiteral | NumberLiteral)

Function names are plain Identifier values -- ALWAYS use uppercase by convention: NUMBER, DATETIME, MY_FUNC.

FluentFunction Type Signature

type FluentFunction = (
  positional: Array<FluentValue>,
  named: Record<string, FluentValue>
) => FluentValue;

Where FluentValue = FluentType<unknown> | string.

Critical Warnings

NEVER pass objects, arrays, or booleans as FluentVariable values -- only string, number, Date, FluentType, or TemporalObject are valid. Passing unsupported types causes a TypeError at resolution time.

NEVER disable useIsolating for bundles that handle RTL/LTR mixed content -- Unicode isolation marks (U+2068/U+2069) prevent bidirectional text corruption.

NEVER use lowercase for function names in FTL -- the parser treats lowercase identifiers as message references, not function calls. ALWAYS use UPPERCASE.

NEVER pass variable references as named argument values in FTL -- named arguments MUST be string literals or number literals: NUMBER($x, minimumFractionDigits: 2) is correct, NUMBER($x, minimumFractionDigits: $digits) is NOT valid.

NUMBER() Function

In Placeables (Display Formatting)

# Basic usage -- explicit formatting
emails = You have { NUMBER($count) } unread emails.

# With formatting options
price = Total: { NUMBER($amount, minimumFractionDigits: 2, maximumFractionDigits: 2) }
file-size = Size: { NUMBER($bytes, useGrouping: 0) } bytes
balance = Balance: { NUMBER($value, minimumIntegerDigits: 2) }

In Selectors (Plural Category Determination)

# Cardinal plurals (default)
emails =
    { $count ->
        [one] You have one unread email.
       *[other] You have { $count } unread emails.
    }

# Ordinal plurals
your-rank = { NUMBER($pos, type: "ordinal") ->
    [1] You finished first!
    [one] You finished {$pos}st
    [two] You finished {$pos}nd
    [few] You finished {$pos}rd
   *[other] You finished {$pos}th
}

# Formatted number as selector
your-score =
    { NUMBER($score, minimumFractionDigits: 1) ->
        [0.0] You scored zero points.
       *[other] You scored { NUMBER($score, minimumFractionDigits: 1) } points.
    }

Exact numeric matches (like [1]) are checked BEFORE CLDR plural category matches. ALWAYS place exact matches above category matches.

NUMBER() Options (Complete)

See references/methods.md for the full options table.

DATETIME() Function

Basic Usage

# Short date
today = Today is { DATETIME($date, month: "long", day: "numeric", year: "numeric") }.

# Time display
current-time = Time: { DATETIME($now, hour: "numeric", minute: "2-digit") }.

# Full date-time with style shortcuts
last-login = Last login: { DATETIME($date, dateStyle: "long", timeStyle: "short") }.

# Weekday display
meeting-day = Meeting on { DATETIME($date, weekday: "long") }.

TypeScript -- Passing Date Variables

const msg = bundle.getMessage("today");
if (msg?.value) {
  const text = bundle.formatPattern(msg.value, { date: new Date() });
  // "Today is March 19, 2026."
}

ALWAYS pass Date objects (or number timestamps or Temporal objects) as date variables. NEVER pass pre-formatted date strings -- let Fluent handle locale-aware formatting.

DATETIME() Options (Complete)

See references/methods.md for the full options table.

Custom Functions

Registration via FluentBundle Constructor

import { FluentBundle, FluentResource } from "@fluent/bundle";

const bundle = new FluentBundle("en-US", {
  functions: {
    CURRENCY: (positional, named) => {
      const value = positional[0];
      const num = typeof value === "number"
        ? value
        : Number(value);
      const currency = named["currency"];
      const currencyCode = typeof currency === "string"
        ? currency
        : "USD";
      return new Intl.NumberFormat("en-US", {
        style: "currency",
        currency: currencyCode,
      }).format(num);
    },
    UPCASE: (positional, named) => {
      const val = positional[0];
      return typeof val === "string"
        ? val.toUpperCase()
        : String(val).toUpperCase();
    },
    GREETING_TIME: (positional, named) => {
      const hour = new Date().getHours();
      if (hour < 12) return "morning";
      if (hour < 18) return "afternoon";
      return "evening";
    },
  },
});

Corresponding FTL:

total = Total: { CURRENCY($amount, currency: "EUR") }
shout = { UPCASE("hello world") }
greeting = Good { GREETING_TIME() }!

Custom Function Rules

1. ALWAYS register functions in the functions option of the FluentBundle constructor
2. Custom functions are merged with built-in NUMBER and DATETIME -- naming a custom function NUMBER or DATETIME overrides the built-in
3. Functions receive positional (array of FluentValue) and named (record of FluentValue) arguments
4. Functions MUST return a FluentValue (string or FluentType<unknown>)
5. If a function referenced in FTL is not registered, resolution produces a ReferenceError and the fallback {FUNC_NAME()} is displayed

Decision Tree -- When to Use Custom Functions

Need locale-aware number formatting?
├── YES → Use NUMBER() with Intl.NumberFormat options
│   ├── Need currency? → NUMBER($val, style: "currency") or custom CURRENCY()
│   ├── Need percentage? → Custom PERCENT() function
│   └── Need ordinals? → NUMBER($val, type: "ordinal") in selector
│
Need locale-aware date formatting?
├── YES → Use DATETIME() with Intl.DateTimeFormat options
│
Need text transformation?
├── YES → Custom function (UPCASE, LOWERCASE, CAPITALIZE)
│
Need runtime logic (time of day, platform detection)?
├── YES → Custom function returning a string for selector matching
│
Need formatting not covered by Intl?
├── YES → Custom function with your own formatting logic

Bidirectional Text Isolation

How useIsolating Works

When useIsolating is true (the default), every placeable value is wrapped in Unicode isolation marks:

• U+2068 (First Strong Isolate) -- inserted BEFORE the placeable
• U+2069 (Pop Directional Isolate) -- inserted AFTER the placeable

This prevents RTL text in placeables from corrupting the surrounding LTR text layout (and vice versa).

Example -- With Isolation (Default)

const bundle = new FluentBundle("en-US"); // useIsolating: true (default)
const resource = new FluentResource(`welcome = Welcome, { $name }!`);
bundle.addResource(resource);

const msg = bundle.getMessage("welcome");
if (msg?.value) {
  const text = bundle.formatPattern(msg.value, { name: "Ahmed" });
  // Internal: "Welcome, \u2068Ahmed\u2069!"
  // Renders correctly in both LTR and RTL contexts
}

When to Disable useIsolating

ONLY disable useIsolating in these cases:

1. Unit testing -- Unicode marks interfere with string equality assertions
2. Unidirectional content -- ALL content is the same direction (e.g., English-only app)
3. Pre-isolated content -- Placeables already contain isolation marks

// For testing
const bundle = new FluentBundle("en-US", { useIsolating: false });

// For production with mixed directionality -- KEEP DEFAULT
const bundle = new FluentBundle("ar", { useIsolating: true });

NEVER disable useIsolating for Arabic, Hebrew, or any locale that mixes RTL and LTR scripts. Doing so causes placeable values to reorder surrounding text unpredictably.

Reference Links

• references/methods.md -- NUMBER options table, DATETIME options table, FluentFunction signature, FunctionReference grammar
• references/examples.md -- NUMBER formatting, DATETIME formatting, custom function registration, bidirectional text
• references/anti-patterns.md -- Wrong function syntax, passing objects as args, disabling useIsolating carelessly

Official Sources

• https://projectfluent.org/fluent/guide/builtins.html
• https://projectfluent.org/fluent/guide/functions.html
• https://github.com/projectfluent/fluent.js/blob/main/fluent-bundle/src/builtins.ts
• https://github.com/projectfluent/fluent.js/blob/main/fluent-bundle/src/bundle.ts
• https://raw.githubusercontent.com/projectfluent/fluent/master/spec/fluent.ebnf