## Introduction Handling dynamic content in translations is one of the most common challenges in React internationalization. React-i18next provides powerful interpolation features that let you insert variables, format values, and create flexible translations. This guide covers everything you need to know about placeholders in react-i18next, from basic variable substitution to advanced formatting. ## Prerequisites - React project with react-i18next installed - Basic understanding of i18next configuration - Familiarity with React hooks ## Basic Placeholder Syntax React-i18next uses double curly braces `{{variable}}` for interpolation by default: ```json // en.json { "greeting": "Hello, {{name}}!", "notification": "You have {{count}} new messages" } ``` ```jsx // Component.jsx import { useTranslation } from 'react-i18next'; function Greeting() { const { t } = useTranslation(); return (

{t('greeting', { name: 'John' })}

{/* Output: Hello, John! */}

{t('notification', { count: 5 })}

{/* Output: You have 5 new messages */}

); } ``` ## Variable Interpolation Options ### Multiple Variables You can use multiple placeholders in a single translation: ```json { "welcome": "Welcome back, {{firstName}} {{lastName}}! Last login: {{lastLogin}}" } ``` ```jsx t('welcome', { firstName: 'John', lastName: 'Doe', lastLogin: 'Yesterday' }) // Output: Welcome back, John Doe! Last login: Yesterday ``` ### Nested Object Access Access nested object properties using dot notation: ```jsx const user = { name: 'John', address: { city: 'Berlin' } }; // Translation: "{{user.name}} lives in {{user.address.city}}" t('location', { user }) // Output: John lives in Berlin ``` ### Default Values Provide fallback values for missing variables: ```json { "greeting": "Hello, {{name, 'Guest'}}!" } ``` Or in JavaScript: ```jsx t('greeting', { name: userName || 'Guest' }) ``` ## Formatting Values i18next provides built-in formatting options for common use cases. ### Number Formatting ```json { "price": "Price: {{amount, number}}", "currency": "Total: {{amount, currency(USD)}}" } ``` ```jsx t('price', { amount: 1234.56 }) // Output: Price: 1,234.56 (locale-dependent) t('currency', { amount: 99.99 }) // Output: Total: $99.99 ``` ### Date Formatting ```json { "published": "Published: {{date, datetime}}", "relative": "{{date, relativetime}}" } ``` ```jsx t('published', { date: new Date() }) // Output: Published: 1/15/2025, 10:30 AM t('relative', { date: new Date(Date.now() - 86400000) }) // Output: 1 day ago ``` ### Custom Formatters Define custom formatting functions in your i18next configuration: ```jsx // i18n.js import i18n from 'i18next'; i18n.init({ interpolation: { format: function(value, format, lng) { if (format === 'uppercase') return value.toUpperCase(); if (format === 'lowercase') return value.toLowerCase(); if (format === 'truncate') return value.substring(0, 20) + '...'; // Currency formatting if (format.startsWith('currency')) { const currency = format.match(/$(\w+)$/)?.[1] || 'USD'; return new Intl.NumberFormat(lng, { style: 'currency', currency }).format(value); } return value; } } }); ``` ```json { "title": "{{name, uppercase}}", "preview": "{{content, truncate}}" } ``` ## Pluralization with Placeholders Combine placeholders with plural forms: ```json { "items": "{{count}} item", "items_plural": "{{count}} items", "items_zero": "No items" } ``` ```jsx t('items', { count: 0 }) // No items t('items', { count: 1 }) // 1 item t('items', { count: 5 }) // 5 items ``` ### ICU Format Support For more complex pluralization, use ICU message format: ```json { "items": "{count, plural, =0 {No items} one {# item} other {# items}}" } ``` ## HTML in Translations ### Using the Trans Component For translations with HTML elements, use the `Trans` component: ```json { "terms": "By signing up, you agree to our Terms of Service" } ``` ```jsx import { Trans } from 'react-i18next'; function Terms() { return ( }} /> ); } // Output: By signing up, you agree to our Terms of Service ``` ### Combining Variables and HTML ```json { "welcome": "Hello {{name}}, you have {{count}} notifications" } ``` ```jsx , highlight: }} /> ``` ## Best Practices ### 1. Use Descriptive Variable Names ```json // Good { "greeting": "Hello, {{userName}}!" } // Avoid { "greeting": "Hello, {{0}}!" } ``` ### 2. Keep Placeholders Consistent Choose one style and stick with it across your project: | Style | Example | Notes | |-------|---------|-------| | `{{var}}` | `{{name}}` | i18next default | | `{var}` | `{name}` | Common in other frameworks | | `%{var}` | `%{name}` | Ruby-style | Configure your preferred style: ```jsx i18n.init({ interpolation: { prefix: '{', suffix: '}' } }); ``` ### 3. Document Context for Translators ```json { "items_count": "{{count}} items", "_items_count_context": "Number of items in shopping cart. Count is always a positive integer." } ``` ### 4. Handle Missing Values Gracefully ```jsx // Set default values in config i18n.init({ interpolation: { defaultVariables: { appName: 'MyApp', supportEmail: 'support@example.com' } } }); ``` ## Common Issues and Solutions ### Issue 1: Placeholders Not Replaced **Problem:** `{{name}}` appears in output instead of the value. **Solutions:** 1. Check if the key matches exactly (case-sensitive) 2. Verify you're passing the object correctly: `t('key', { name: value })` 3. Ensure i18next is properly initialized ### Issue 2: XSS Vulnerability **Problem:** User input rendered as HTML. **Solution:** i18next escapes values by default. Never disable escaping for user input: ```jsx // Safe (default) t('message', { content: userInput }) // Dangerous - only use for trusted content t('message', { content: userInput, interpolation: { escapeValue: false } }) ``` ### Issue 3: Numbers Not Formatted **Problem:** Numbers display without locale formatting. **Solution:** Use the number format: ```json { "amount": "{{value, number}}" } ``` Or configure automatic number formatting: ```jsx i18n.init({ interpolation: { format: (value, format, lng) => { if (typeof value === 'number') { return new Intl.NumberFormat(lng).format(value); } return value; } } }); ``` ## Automating Translations with Placeholders When translating files with placeholders, it's crucial that the placeholders remain intact. Tools like appstoretranslate.com automatically detect and preserve: - `{{variable}}` - i18next style - `{variable}` - ICU style - `%s`, `%d`, `%@` - Printf style - Named and positional placeholders This ensures your translations work correctly without manual placeholder fixing. ## Conclusion Mastering placeholders in react-i18next is essential for creating dynamic, multilingual React applications. Key takeaways: 1. Use `{{variable}}` syntax for interpolation 2. Leverage built-in formatters for numbers and dates 3. Use the `Trans` component for HTML in translations 4. Keep placeholder names descriptive and consistent Ready to translate your React app? [Try appstoretranslate.com](https://appstoretranslate.com) to automatically translate your i18n JSON files while preserving all placeholders and formatting. ## Related Resources - [React i18next Trans Component](/blog/react-i18next-trans-component) - [Vue i18n in JavaScript](/blog/vue-i18n-javascript) - [i18next Official Documentation](https://www.i18next.com/translation-function/interpolation)