JavaScript Intl vs Localize: Functional Comparison

This document compares the functional coverage of the JavaScript Intl API and the Localize Elixir library. Both are built on the Unicode CLDR data set, but they expose different subsets of that data and offer different capabilities.

Summary

JavaScript Intl provides a focused set of formatting and comparison APIs for use in browsers and server-side JS. Localize provides a broader set of internationalization functions including everything Intl offers plus message formatting, unit conversion, number parsing, rule-based number formatting, territory and currency metadata, calendar locale data, Gettext integration, and more.

Feature-by-Feature Comparison

Number Formatting

Date and Time Formatting

Duration Formatting

List Formatting

Display Names

Plural Rules

Collation (String Comparison and Sorting)

Text Segmentation

Message Formatting

Units of Measurement

Currency Data

Territory Data

Locale Management

Text Utilities

Capabilities Unique to JS Intl

• formatToParts() — All JS Intl formatters can return structured arrays of {type, value} objects representing the individual parts of a formatted string. This is useful for custom rendering (for example, styling the currency symbol differently from the number). Localize does not expose format parts.

• Intl.Segmenter segment metadata — The JS segmenter returns isWordLike for each word segment, distinguishing content words from punctuation and whitespace. The unicode_string library (used by Intl.Segmenter in this project) does not provide this metadata.

• Intl.PluralRules.selectRange() — Determines the plural category for a range of numbers (for example, "1–3 items"). Localize does not provide range plural selection.

Capabilities Unique to Localize

• ICU MessageFormat 2 — Localize.Message.format/3 parses and evaluates MessageFormat 2 strings with variable placeholders, plural rules, select expressions, and custom formatting functions. JS has no built-in message formatting (developers typically use external libraries like intl-messageformat).

• Rule-based number formatting (RBNF) — Localize.Number.Rbnf.to_string/3 formats numbers as spellout text ("one hundred twenty-three"), ordinal text ("twenty-third"), Roman numerals ("CXXIII"), and other algorithmic number systems per locale.

• Number parsing — Localize.Number.Parser.parse/2 parses locale-formatted number strings back into numbers, handling locale-specific decimal separators and grouping characters. JS Intl has no parsing counterpart.

• Unit conversion and arithmetic — Localize.Unit.convert/2 converts between compatible units (kilometers to miles, Celsius to Fahrenheit). Localize.Unit.Math provides add, subtract, multiply, divide, and compare operations on unit values. JS Intl can format units but cannot convert between them.

• Territory hierarchy and metadata — Localize.Territory provides parent/child relationships between territories (for example, "US" is contained in "Northern America" which is contained in "Americas"), territory info (population, GDP, languages), and Unicode flag emoji.

• Calendar locale data — Localize.Calendar exposes month names, day names, era names, quarter names, and day period names in all styles (wide, abbreviated, narrow) for all supported calendars and locales. It also provides first-day-of-week and weekend configuration per territory.

• Duration calculation — Localize.Duration.new/2 computes the calendar duration between two dates or times. JS Intl.DurationFormat can only format a duration object that you provide — it cannot calculate one.

• Locale display names — Localize.Locale.LocaleDisplay.display_name/2 produces human-readable locale descriptions like "English (United States)" or "français (France)". JS Intl has no equivalent.

• Subdivision names — Localize.Territory.subdivision_name/2 provides localized names for country subdivisions (states, provinces, regions). JS Intl does not support subdivisions.

• Custom currencies — Localize.Currency.new/2 allows defining private-use currency codes for applications that need currencies not in ISO 4217.

• Gettext integration — Localize.Gettext.Interpolation provides a Gettext interpolation backend that uses CLDR formatting for interpolated values.

• Text utilities — Localize.quote/2 wraps text in locale-appropriate quotation marks (for example, « » for French, „ " for German). Localize.ellipsis/2 adds locale-appropriate ellipsis characters.

• List intersperse — Localize.List.intersperse/2 returns a list with separator strings inserted between elements (rather than a concatenated string), which is useful for building safe HTML output where elements may contain markup.