Use when reviewing utility functions, component props, or template strings that format numbers, prices, or dates to check whether Intl APIs are used correctly.
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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 $" 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.
---
For full implementation details, code examples, and framework-specific guidance,
see `references/rule.md`.
Rule page: https://frontendchecklist.io/rules/i18n/currency-formatting
