---
phase: 04-chart-polish-and-bug-fixes
plan: 01
subsystem: ui
tags: [intl, locale, currency, formatting, typescript, vitest]

# Dependency graph
requires: []
provides:
  - "Locale-aware formatCurrency(amount, currency, locale='en') with 'en' default"
  - "Unit tests covering English, German, USD, zero, negative, and empty-string edge cases"
affects:
  - 04-02-chart-tooltips
  - any component calling formatCurrency

# Tech tracking
tech-stack:
  added: []
  patterns:
    - "Intl.NumberFormat locale parameter passed from call site — no hardcoded locale in utility functions"

key-files:
  created:
    - frontend/src/lib/format.test.ts
  modified:
    - frontend/src/lib/format.ts

key-decisions:
  - "formatCurrency third parameter defaults to 'en', replacing hardcoded 'de-DE' — all existing 2-arg call sites now produce English formatting (FIX-01)"
  - "Defensive locale || 'en' guard prevents RangeError when empty string is passed"

patterns-established:
  - "TDD pattern: write failing test first (RED commit), then implement (GREEN commit) — verified by vitest run between steps"

requirements-completed: [FIX-01]

# Metrics
duration: 5min
completed: 2026-03-12
---

# Phase 4 Plan 01: Locale-Aware formatCurrency Summary

**`Intl.NumberFormat` locale parameter added to `formatCurrency` with `'en'` default, replacing hardcoded `'de-DE'` — English users now see `1,234.56` instead of `1.234,56`**

## Performance

- **Duration:** 5 min
- **Started:** 2026-03-12T08:23:13Z
- **Completed:** 2026-03-12T08:28:00Z
- **Tasks:** 2 (TDD RED + TDD GREEN)
- **Files modified:** 2

## Accomplishments

- Added optional `locale` parameter to `formatCurrency` with `'en'` as the default
- Removed the hardcoded `'de-DE'` locale that was forcing German number formatting for all users
- Defensive `locale || 'en'` guard prevents `RangeError` on empty string input
- 8 unit tests cover English formatting, German formatting, USD, zero, negative amounts, and edge cases
- All 51 frontend tests pass with no regressions

## Task Commits

Each task was committed atomically:

1. **TDD RED: Failing tests for locale-aware formatCurrency** - `6ffce76` (test)
2. **TDD GREEN: Implement locale parameter** - `eb1bb8a` (feat)

_Note: TDD tasks have two commits (test → feat). No refactor needed._

## Files Created/Modified

- `frontend/src/lib/format.test.ts` - 8 unit tests for locale behavior, defaults, and edge cases
- `frontend/src/lib/format.ts` - Updated function signature with `locale: string = 'en'` parameter

## Decisions Made

- Third parameter defaults to `'en'` (not `'en-US'` or `'de-DE'`) — bare `'en'` is a valid BCP 47 tag and produces comma-grouped, period-decimal output consistent with user expectations for English locale
- `locale || 'en'` fallback is intentional — empty string is not a valid BCP 47 locale and `Intl.NumberFormat('')` throws `RangeError` in some environments

## Deviations from Plan

None - plan executed exactly as written.

## Issues Encountered

None. Pre-existing `act(...)` warnings in unrelated test files (`InlineEditCell.test.tsx`, `CategoriesPage.test.tsx`) were observed but are out of scope per deviation rules — logged to deferred items.

## User Setup Required

None - no external service configuration required.

## Next Phase Readiness

- `formatCurrency(amount, currency, locale)` is ready for use in chart tooltips (04-02)
- All call sites currently pass 2 args and will automatically receive English formatting
- Callers can pass user's `preferred_locale` from API response as the third arg

## Self-Check: PASSED

- `frontend/src/lib/format.ts` — FOUND
- `frontend/src/lib/format.test.ts` — FOUND
- `04-01-SUMMARY.md` — FOUND
- Commit `6ffce76` (test RED) — FOUND
- Commit `eb1bb8a` (feat GREEN) — FOUND

---
*Phase: 04-chart-polish-and-bug-fixes*
*Completed: 2026-03-12*