FormatJS & React Intl - Standardized I18n

Why FormatJS?

FormatJS (best known for its React binding, react-intl) takes a standards-first approach to internationalization. Instead of shipping a custom translation engine, it leverages the JavaScript Intl API built into modern browsers and polyfills.

ICU Message Syntax: It uses the industry-standard syntax for translations, handling complex plurals, gender selections, and selects natively. This is the same format used by Java, PHP, and C++.
Extraction Tooling: Unlike libraries that scan for keys at runtime, FormatJS treats your source code as the source of truth. Its CLI extracts default messages from your components into JSON files for translators.
Type Safety: With modern tooling, it allows for strong typing of message IDs and values, preventing missing translations or incorrect parameter usage.
Modular Architecture: You can use the core libraries without React, or use specific formatters (RelativeTime, Number, Date) independently to keep bundle sizes low.
Formatted Components: Provides a suite of React components (FormattedDate, FormattedNumber, FormattedMessage) that handle localization rendering declaratively.

Code Snippet

A component demonstrating the power of ICU syntax for handling pluralization and component interpolation.

import React from 'react';
import { FormattedMessage, useIntl } from 'react-intl';

export function Notifications({ user, unreadCount }) {
  const intl = useIntl();

  return (
    <div role="status">
      {/* 1. Imperative formatting for attributes */}
      <img 
        src={user.avatar} 
        alt={intl.formatMessage(
          { id: 'user.avatar_alt', defaultMessage: "{name}'s avatar" },
          { name: user.name }
        )} 
      />

      {/* 2. Declarative formatting with complex ICU syntax */}
      <p>
        <FormattedMessage
          id="notifications.summary"
          defaultMessage="{count, plural, 
            =0 {You have no new messages}
            one {You have # new message}
            other {You have # new messages}
          }"
          values={{ count: unreadCount }}
        />
      </p>

      {/* 3. Rich text interpolation */}
      <FormattedMessage
        id="terms.agreement"
        defaultMessage="I agree to the <a>Terms of Service</a>"
        values={{
          a: (chunks) => <a href="/terms">{chunks}</a>
        }}
      />
    </div>
  );
}

In this example, the complex pluralization logic exists within the message string itself, not the code. Translators can adjust these rules (e.g., languages with 4 plural forms like Russian) without developers changing a line of code.

Pros and Cons

No library is perfect; understanding the trade-offs is key to selecting the right tool.

Pros

Standardization: By strictly following the ICU Message format, your translations are portable to other backends or systems.
Linguistic Correctness: It handles the most complex edge cases (ordinal numbers, currency formatting, relative time) correctly across all locales using native APIs.
Extraction Workflow: The philosophy of "write default messages in components, extract later" prevents key drift, where translation keys exist in JSON but are no longer used in code.

Cons

Verbosity: The API (<FormattedMessage>, intl.formatMessage) is significantly more verbose than the simple t('key') found in other libraries.
Build Step Required: To fully utilize its power (and performance), you need to set up the CLI to extract and compile messages, which adds complexity to the build pipeline.
Learning Curve: Developers must learn the ICU syntax ({key, select, ...}) which is powerful but less intuitive than simple string replacement.

Comparison with Other I18n Libraries

The table below outlines the positioning differences between FormatJS and other popular I18n libraries to help you make an informed decision:

Library	Design Philosophy	Best For	Pain Points
FormatJS	Standards compliance Strict adherence to browser `Intl` APIs and ICU Message Syntax.	Enterprise/FinTech Apps requiring strict currency/date formatting and complex grammar support.	Boilerplate Requires significantly more code and configuration than competitors.
react-i18next	Developer Experience Prioritizes ease of use, plugins, and runtime flexibility.	Rapid Development Teams that want to get up and running quickly with minimal config.	Proprietary Format Uses a JSON structure that isn't as standardized as ICU.
LinguiJS	Compiler-based Compiles messages to standard JS to minimize runtime overhead.	Performance Apps that want the benefits of ICU syntax with a smaller bundle size.	Tooling Reliance Heavily dependent on Babel macros and CLI compilation.

Ecosystem & Extensions

FormatJS creates a comprehensive environment for managing translations:

@formatjs/cli: The official command-line tool to extract messages from your React components and compile them into JSON.
eslint-plugin-formatjs: A crucial tool that enforces message ID uniqueness and validates ICU syntax validity during development.
babel-plugin-react-intl: A plugin to remove defaultMessage attributes from production builds to save bandwidth, leaving only the message IDs.

FormatJS (React Intl)

TL;DR