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

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---
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*