SimpleFinanceDash/.planning/phases/04-chart-polish-and-bug-fixes/04-01-PLAN.md

phase, plan, type, wave, depends_on, files_modified, autonomous, requirements, must_haves

phase	plan	type	wave	depends_on	files_modified	autonomous	requirements	must_haves
04-chart-polish-and-bug-fixes	01	tdd	1		frontend/src/lib/format.ts frontend/src/lib/format.test.ts	true	FIX-01	truths artifacts key_links formatCurrency no longer hardcodes 'de-DE' — the default locale is 'en' formatCurrency accepts an optional third locale parameter Calling formatCurrency(1234.56, 'EUR', 'de') produces German-formatted output Calling formatCurrency(1234.56, 'USD', 'en') produces English-formatted output Calling formatCurrency(1234.56, 'EUR') without locale uses 'en' default, not 'de-DE' path provides contains frontend/src/lib/format.ts Locale-aware formatCurrency function locale path provides min_lines frontend/src/lib/format.test.ts Unit tests for formatCurrency locale behavior 20 from to via pattern frontend/src/lib/format.ts Intl.NumberFormat locale parameter passed as first arg Intl.NumberFormat(locale

Fix the hardcoded `'de-DE'` locale in `formatCurrency` by adding an optional `locale` parameter with `'en'` as the default. This is the foundation for IXTN-04 (chart tooltips) and ensures all currency formatting respects the user's locale preference.

Purpose: FIX-01 — English-locale users currently see German number formatting everywhere Output: Updated format.ts with locale parameter, comprehensive unit tests

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@.planning/PROJECT.md @.planning/ROADMAP.md @.planning/phases/04-chart-polish-and-bug-fixes/04-RESEARCH.md @frontend/src/lib/format.ts Locale-aware formatCurrency frontend/src/lib/format.ts, frontend/src/lib/format.test.ts - formatCurrency(1234.56, 'EUR', 'en') returns a string containing "1,234.56" (English grouping) - formatCurrency(1234.56, 'EUR', 'de') returns a string containing "1.234,56" (German grouping) - formatCurrency(1234.56, 'USD', 'en') returns a string containing "$" and "1,234.56" - formatCurrency(1234.56, 'EUR') with NO locale arg uses 'en' default — does NOT produce German formatting - formatCurrency(0, 'EUR', 'en') returns a string containing "0.00" - formatCurrency(-500, 'EUR', 'en') returns a string containing "-" and "500.00" - formatCurrency(1234.56, 'EUR', '') falls back gracefully (does not throw RangeError) Update `frontend/src/lib/format.ts`:

```ts
export function formatCurrency(
  amount: number,
  currency: string = 'EUR',
  locale: string = 'en'
): string {
  return new Intl.NumberFormat(locale || 'en', {
    style: 'currency',
    currency,
  }).format(amount)
}
```

Key points:
- Third parameter `locale` defaults to `'en'` (replacing hardcoded `'de-DE'`)
- Defensive `locale || 'en'` guards against empty string (which throws RangeError)
- All existing call sites pass only 2 args — they will now get English formatting instead of German
- This is intentional and correct per FIX-01

cd frontend && bun vitest run src/lib/format.test.ts

<success_criteria>

• format.test.ts has at least 6 test cases covering locale variations, defaults, and edge cases
• All tests pass green
• formatCurrency signature has 3 parameters: amount, currency, locale
• No hardcoded 'de-DE' remains in format.ts </success_criteria>

After completion, create `.planning/phases/04-chart-polish-and-bug-fixes/04-01-SUMMARY.md`