Lingui Best Practices

Lingui Best Practices

DueDateHQ repository override

When this skill is loaded through .claude/skills/lingui-best-practices, read the root CLAUDE.md and AGENTS.md first. Their project constraints supersede generic examples below.

• Do not introduce React useEffect; preserve the repository's existing locale-loading flow.
• Reuse the current Lingui provider, configuration, macro imports, and catalog layout.
• After changing translatable source or catalogs, run pnpm run i18n:check and pnpm run ci.
• Missing translations and uncommitted catalog drift are failures, not warnings to ignore.
• A Lingui change is complete only when extraction reports zh-CN Missing 0, strict compilation succeeds, and all generated messages.po / messages.ts changes are reviewed and staged.
• Never leave an empty msgstr, use untranslated English as a temporary zh-CN value, hand-edit compiled messages.ts, or delete/suppress a message to make CI pass.
• Interpolated user-visible labels must be translated before interpolation. Do not place raw English branches such as {condition ? 'days sooner' : 'days later'} inside <Trans>; translate each branch with t or model the choice with the appropriate Lingui macro.

Lingui is a powerful internationalization (i18n) framework for JavaScript. This skill covers best practices for implementing i18n in React and vanilla JavaScript applications.

Quick Start Workflow

The standard Lingui workflow consists of these steps:

1. Wrap your app in I18nProvider
2. Mark messages for translation using macros (Trans, t, etc.)
3. Extract messages: lingui extract
4. Translate the catalogs
5. Compile catalogs: lingui compile
6. Load and activate locale in your app

Core Packages

Import from these packages:

// React macros (recommended)
import { Trans, Plural, Select, useLingui } from "@lingui/react/macro";

// Core macros for vanilla JS
import { t, msg, plural, select } from "@lingui/core/macro";

// Runtime (rarely used directly)
import { I18nProvider } from "@lingui/react";
import { i18n } from "@lingui/core";

Setup I18nProvider

Wrap your application with I18nProvider:

import { I18nProvider } from "@lingui/react";
import { i18n } from "@lingui/core";
import { messages } from "./locales/en/messages";

i18n.load("en", messages);
i18n.activate("en");

function App() {
  return (
    <I18nProvider i18n={i18n}>
      {/* Your app */}
    </I18nProvider>
  );
}

Translating UI Text

Use Trans for JSX Content

The Trans macro is the primary way to translate JSX:

import { Trans } from "@lingui/react/macro";

// Simple text
<Trans>Hello World</Trans>

// With variables
<Trans>Hello {userName}</Trans>

// With components (rich text)
<Trans>
  Read the <a href="/docs">documentation</a> for more info.
</Trans>

// Extracted as: "Read the <0>documentation</0> for more info."

When to use: For any translatable text in JSX elements.

Use useLingui for Non-JSX

For strings outside JSX (attributes, alerts, function calls):

import { useLingui } from "@lingui/react/macro";

function MyComponent() {
  const { t } = useLingui();

  const handleClick = () => {
    alert(t`Action completed!`);
  };

  return (
    <div>
      <img src="..." alt={t`Image description`} />
      <button me`}</button>
    </div>
  );
}

When to use: Element attributes, alerts, function parameters, any non-JSX string.

Use msg for Lazy Translations

When you need to define messages at module level or in arrays/objects:

import { msg } from "@lingui/core/macro";
import { useLingui } from "@lingui/react";

// Module-level constants
const STATUSES = {
  active: msg`Active`,
  inactive: msg`Inactive`,
  pending: msg`Pending`,
};

function StatusList() {
  const { _ } = useLingui();
  
  return Object.entries(STATUSES).map(([key, message]) => (
    <div key={key}>{_(message)}</div>
  ));
}

When to use: Module-level constants, arrays of messages, conditional message selection.

Pluralization

Use the Plural macro for quantity-dependent messages:

import { Plural } from "@lingui/react/macro";

<Plural 
  value={messageCount}
  have # message"
  other="You have # messages"
/>

The # placeholder is replaced with the actual value.

Exact Matches

Use _N syntax for exact number matches (takes precedence over plural forms):

<Plural
  value={count}
  _0="No messages"
  message"
  other="# messages"
/>

With Variables and Components

Combine with Trans for complex messages:

<Plural
  value={count}
  have # message, ${userName}`}
  other={
    <Trans>
      You have <strong>#</strong> messages, {userName}
    </Trans>
  }
/>

Formatting Dates and Numbers

Use i18n.date() and i18n.number() for locale-aware formatting:

import { useLingui } from "@lingui/react/macro";

function MyComponent() {
  const { i18n } = useLingui();
  const lastLogin = new Date();
  
  return (
    <Trans>
      Last login: {i18n.date(lastLogin)}
    </Trans>
  );
}

These use the browser's Intl API for proper locale formatting.

Message IDs and Context

Explicit IDs

Provide a custom ID for stable message keys:

<Trans id="header.welcome">Welcome to our app</Trans>

Context for Disambiguation

When the same text has different meanings, use context:

<Trans context="direction">right</Trans>
<Trans context="correctness">right</Trans>

These create separate catalog entries.

Comments for Translators

Add context for translators:

<Trans comment="Greeting shown on homepage">Hello World</Trans>

Configuration

Basic lingui.config.js:

import { defineConfig } from "@lingui/cli";

export default defineConfig({
  sourceLocale: "en",
  locales: ["en", "es", "fr", "de"],
  catalogs: [
    {
      path: "<rootDir>/src/locales/{locale}/messages",
      include: ["src"],
      exclude: ["**/node_modules/**"],
    },
  ],
});

For detailed configuration patterns, see configuration.md.

Best Practices

Always Use Macros

Prefer macros over runtime components. Macros are compiled at build time, reducing bundle size:

// ✅ Good - uses macro
import { Trans } from "@lingui/react/macro";

// ❌ Avoid - runtime only
import { Trans } from "@lingui/react";

Keep Messages Simple

Avoid complex expressions in messages - they'll be replaced with placeholders:

// ❌ Bad - loses context
<Trans>Hello {user.name.toUpperCase()}</Trans>
// Extracted as: "Hello {0}"

// ✅ Good - clear variable name
const userName = user.name.toUpperCase();
<Trans>Hello {userName}</Trans>
// Extracted as: "Hello {userName}"

Use Trans for JSX, t for Strings

Choose the right tool:

// ✅ For JSX content
<h1><Trans>Welcome</Trans></h1>

// ✅ For string values
const { t } = useLingui();
<img alt={t`Profile picture`} />

Don't Use Macros at Module Level

Macros need component context - use msg instead:

// ❌ Bad - won't work
import { t } from "@lingui/core/macro";
const LABELS = [t`Red`, t`Green`, t`Blue`];

// ✅ Good - use msg for lazy translation
import { msg } from "@lingui/core/macro";
const LABELS = [msg`Red`, msg`Green`, msg`Blue`];

Use the ESLint Plugin

Install and configure eslint-plugin-lingui to catch common mistakes automatically:

npm install --save-dev eslint-plugin-lingui

// eslint.config.js
import pluginLingui from "eslint-plugin-lingui";

export default [
  pluginLingui.configs["flat/recommended"],
];

Common Patterns

Dynamic Locale Switching

import { i18n } from "@lingui/core";

async function changeLocale(locale) {
  const { messages } = await import(`./locales/${locale}/messages`);
  i18n.load(locale, messages);
  i18n.activate(locale);
}

Loading Catalogs Dynamically

import { useEffect } from "react";
import { i18n } from "@lingui/core";

function loadCatalog(locale) {
  return import(`./locales/${locale}/messages`);
}

function App() {
  useEffect(() => {
    loadCatalog("en").then(catalog => {
      i18n.load("en", catalog.messages);
      i18n.activate("en");
    });
  }, []);
  
  return <I18nProvider i18n={i18n}>{/* ... */}</I18nProvider>;
}

Memoization with useLingui

When using memoization, use the t function from the macro version:

import { useLingui } from "@lingui/react/macro";
import { msg } from "@lingui/core/macro";
import { useMemo } from "react";

const welcomeMessage = msg`Welcome!`;

function MyComponent() {
  const { t } = useLingui(); // Macro version - reference changes with locale
  
  // ✅ Safe - t reference updates with locale
  const message = useMemo(() => t(welcomeMessage), [t]);
  
  return <div>{message}</div>;
}

Troubleshooting

If you encounter issues:

1. Messages not extracted: Check include patterns in lingui.config.js
2. Translations not applied: Ensure catalogs are compiled with lingui compile
3. Runtime errors: Verify I18nProvider wraps your app
4. Type errors: Run lingui compile --typescript for TypeScript projects

For detailed common mistakes and pitfalls, see common-mistakes.md.