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name: currency-formatting description: "Use when reviewing utility functions, component props, or template strings that format numbers, prices, or dates to check whether Intl APIs are used correctly." metadata: category: i18n priority: medium difficulty: beginner estimatedTime: "20" source: frontendchecklist.io url: https://frontendchecklist.io/rules/i18n/currency-formatting

Use Intl APIs for currency, number, and date formatting

Number and currency formatting rules differ significantly across locales — a value formatted as "$1,234.56" in the US is written "1.234,56 quot; in Germany and "¥1,235" in Japan. Hardcoded formatting causes incorrect display for international users and is expensive to maintain manually as you add locales.

Quick Reference

• Never hardcode currency symbols or thousand separators like $ or comma
• Use Intl.NumberFormat with the currency style for monetary values
• Use Intl.DateTimeFormat for locale-aware date and time display
• Pass the user locale explicitly rather than relying on the browser default
• Use Intl.Collator and locale fallback chains for sorting and searching

Check

Identify all places in this codebase where numbers, currencies, or dates are formatted manually instead of using Intl.NumberFormat or Intl.DateTimeFormat. Also flag locale-sensitive sorting that uses plain string comparison instead of Intl.Collator and any formatter that lacks a fallback locale.

Fix

Replace manual number and currency formatting with Intl.NumberFormat and replace manual date formatting with Intl.DateTimeFormat, passing the user locale explicitly. Use Intl.Collator for locale-aware sorting and resolve a supported fallback locale when the requested locale is unavailable.

Explain

Explain why locale-aware formatting with Intl APIs is required for international applications, what goes wrong when formatting is hardcoded, and why sorting/searching should also use locale-aware comparison.

Code Review

Review utility functions and components that render numbers, prices, percentages, or dates. Flag any hardcoded currency symbols, separators, or date format strings, and show Intl-based alternatives.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/i18n/currency-formatting