---
phase: 07-weight-unit-selection
plan: 01
subsystem: ui
tags: [weight-conversion, formatters, react-hooks, tdd]

# Dependency graph
requires: []
provides:
  - "WeightUnit type export for all weight display components"
  - "Parameterized formatWeight(grams, unit) with g/oz/lb/kg support"
  - "useWeightUnit() hook wrapping settings API for typed unit access"
affects: [07-02-PLAN]

# Tech tracking
tech-stack:
  added: []
  patterns: [unit-conversion-via-formatters, settings-backed-hooks]

key-files:
  created:
    - src/client/hooks/useWeightUnit.ts
    - tests/lib/formatters.test.ts
  modified:
    - src/client/lib/formatters.ts

key-decisions:
  - "Conversion precision: g=0dp, oz=1dp, lb=2dp, kg=2dp matching common usage"
  - "useWeightUnit validates stored value against known units to protect against corrupt data"

patterns-established:
  - "Weight formatting: always call formatWeight(grams, unit) with WeightUnit parameter"
  - "Settings-backed hooks: wrap useSetting with typed validation for domain-specific config"

requirements-completed: [UNIT-02, UNIT-03]

# Metrics
duration: 2min
completed: 2026-03-16
---

# Phase 7 Plan 01: Weight Unit Core Summary

**Parameterized formatWeight with g/oz/lb/kg conversion and useWeightUnit settings hook, backed by 21 TDD tests**

## Performance

- **Duration:** 2 min
- **Started:** 2026-03-16T11:14:19Z
- **Completed:** 2026-03-16T11:16:30Z
- **Tasks:** 2
- **Files modified:** 3

## Accomplishments
- TDD-developed formatWeight function supporting 4 weight units (g, oz, lb, kg) with appropriate precision
- WeightUnit type exported for consumption by all display components in Plan 02
- useWeightUnit convenience hook with validation and "g" default, ready for component integration
- Full backward compatibility preserved -- formatWeight(grams) still returns "Xg" as before

## Task Commits

Each task was committed atomically:

1. **Task 1 (RED): TDD formatWeight tests** - `431c179` (test)
2. **Task 1 (GREEN): Implement formatWeight with unit parameter** - `6cac0a3` (feat)
3. **Task 2: Create useWeightUnit convenience hook** - `ada3791` (feat)

_TDD task had 2 commits (test -> feat). No refactor needed -- code was already minimal._

## Files Created/Modified
- `src/client/lib/formatters.ts` - Added WeightUnit type, conversion constants, switch-based unit formatting
- `src/client/hooks/useWeightUnit.ts` - Convenience hook wrapping useSetting("weightUnit") with typed validation
- `tests/lib/formatters.test.ts` - 21 tests covering all units, null/undefined, backward compat, edge cases

## Decisions Made
- Conversion precision follows common usage: grams rounded (0dp), ounces 1dp, pounds 2dp, kilograms 2dp
- useWeightUnit validates stored value against a whitelist of known units, protecting against corrupt settings data
- Conversion constants (GRAMS_PER_OZ=28.3495, GRAMS_PER_LB=453.592, GRAMS_PER_KG=1000) kept as module-level consts, not exported

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 1 - Bug] Fixed import order in useWeightUnit.ts**
- **Found during:** Task 2 (useWeightUnit hook creation)
- **Issue:** Biome lint required imports sorted alphabetically (type imports before value imports)
- **Fix:** Reordered imports to put `import type { WeightUnit }` before `import { useSetting }`
- **Files modified:** src/client/hooks/useWeightUnit.ts
- **Verification:** `bun run lint` passes clean
- **Committed in:** ada3791 (Task 2 commit)

---

**Total deviations:** 1 auto-fixed (1 bug - lint order)
**Impact on plan:** Trivial formatting fix. No scope creep.

## Issues Encountered
None

## User Setup Required
None - no external service configuration required.

## Next Phase Readiness
- WeightUnit type and formatWeight function ready for Plan 02 to wire into all weight-displaying components
- useWeightUnit hook ready for components to consume the user's preferred unit from settings
- All 108 existing tests pass (full suite regression check confirmed)

## Self-Check: PASSED

All files exist, all commits found, all exports verified.

---
*Phase: 07-weight-unit-selection*
*Completed: 2026-03-16*
