Files
GearBox/.planning/phases/07-weight-unit-selection/07-01-PLAN.md

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---
phase: 07-weight-unit-selection
plan: 01
type: tdd
wave: 1
depends_on: []
files_modified:
- src/client/lib/formatters.ts
- src/client/hooks/useWeightUnit.ts
- tests/lib/formatters.test.ts
autonomous: true
requirements:
- UNIT-02
- UNIT-03
must_haves:
truths:
- "formatWeight converts grams to g, oz, lb, kg with correct precision"
- "formatWeight defaults to grams when no unit is specified (backward compatible)"
- "formatWeight handles null/undefined input for all units"
- "useWeightUnit hook returns a valid WeightUnit from settings, defaulting to 'g'"
artifacts:
- path: "src/client/lib/formatters.ts"
provides: "WeightUnit type export and parameterized formatWeight function"
exports: ["WeightUnit", "formatWeight", "formatPrice"]
contains: "WeightUnit"
- path: "src/client/hooks/useWeightUnit.ts"
provides: "Convenience hook wrapping useSetting for weight unit"
exports: ["useWeightUnit"]
- path: "tests/lib/formatters.test.ts"
provides: "Unit tests for formatWeight with all 4 units and edge cases"
min_lines: 30
key_links:
- from: "src/client/hooks/useWeightUnit.ts"
to: "src/client/hooks/useSettings.ts"
via: "useSetting('weightUnit')"
pattern: "useSetting.*weightUnit"
- from: "src/client/hooks/useWeightUnit.ts"
to: "src/client/lib/formatters.ts"
via: "imports WeightUnit type"
pattern: "import.*WeightUnit.*formatters"
---
<objective>
Create the weight unit conversion core: a parameterized `formatWeight` function with a `WeightUnit` type and a `useWeightUnit` convenience hook, all backed by tests.
Purpose: Establish the conversion contracts (type, function, hook) that Plan 02 will wire into every component. TDD approach ensures the conversion math is correct before any UI work.
Output: Working `formatWeight(grams, unit)` with tests green, `useWeightUnit()` hook ready for consumption.
</objective>
<execution_context>
@/home/jlmak/.claude/get-shit-done/workflows/execute-plan.md
@/home/jlmak/.claude/get-shit-done/templates/summary.md
</execution_context>
<context>
@.planning/PROJECT.md
@.planning/ROADMAP.md
@.planning/STATE.md
@.planning/phases/07-weight-unit-selection/07-RESEARCH.md
@src/client/lib/formatters.ts
@src/client/hooks/useSettings.ts
</context>
<interfaces>
<!-- Key types and contracts the executor needs. Extracted from codebase. -->
From src/client/lib/formatters.ts (current):
```typescript
export function formatWeight(grams: number | null | undefined): string {
if (grams == null) return "--";
return `${Math.round(grams)}g`;
}
export function formatPrice(cents: number | null | undefined): string {
if (cents == null) return "--";
return `$${(cents / 100).toFixed(2)}`;
}
```
From src/client/hooks/useSettings.ts:
```typescript
export function useSetting(key: string) {
return useQuery({
queryKey: ["settings", key],
queryFn: async () => {
try {
const result = await apiGet<Setting>(`/api/settings/${key}`);
return result.value;
} catch (err: any) {
if (err?.status === 404) return null;
throw err;
}
},
});
}
export function useUpdateSetting() {
const queryClient = useQueryClient();
return useMutation({
mutationFn: ({ key, value }: { key: string; value: string }) =>
apiPut<Setting>(`/api/settings/${key}`, { value }),
onSuccess: (_data, variables) => {
queryClient.invalidateQueries({ queryKey: ["settings", variables.key] });
},
});
}
```
</interfaces>
<feature>
<name>formatWeight unit conversion</name>
<files>src/client/lib/formatters.ts, tests/lib/formatters.test.ts</files>
<behavior>
Conversion constants: 1 oz = 28.3495g, 1 lb = 453.592g, 1 kg = 1000g
- formatWeight(100, "g") -> "100g"
- formatWeight(100, "oz") -> "3.5 oz"
- formatWeight(1000, "lb") -> "2.20 lb"
- formatWeight(1500, "kg") -> "1.50 kg"
- formatWeight(null, "oz") -> "--"
- formatWeight(undefined, "kg") -> "--"
- formatWeight(100) -> "100g" (default unit, backward compatible)
- formatWeight(0, "oz") -> "0.0 oz"
- formatWeight(5, "lb") -> "0.01 lb" (small weight precision)
- formatWeight(50000, "kg") -> "50.00 kg" (large weight)
</behavior>
<implementation>
1. Add `WeightUnit` type export: `"g" | "oz" | "lb" | "kg"`
2. Add conversion constants as module-level consts (not exported)
3. Modify `formatWeight` signature to `(grams: number | null | undefined, unit: WeightUnit = "g"): string`
4. Keep the null guard as-is at the top
5. Add switch statement for unit-specific formatting:
- g: `Math.round(grams)` + "g" (0 decimals, current behavior)
- oz: `.toFixed(1)` + " oz" (1 decimal)
- lb: `.toFixed(2)` + " lb" (2 decimals)
- kg: `.toFixed(2)` + " kg" (2 decimals)
6. Do NOT modify `formatPrice` — leave it untouched
</implementation>
</feature>
<tasks>
<task type="auto" tdd="true">
<name>Task 1: TDD formatWeight with unit parameter</name>
<files>src/client/lib/formatters.ts, tests/lib/formatters.test.ts</files>
<behavior>
- formatWeight(100, "g") returns "100g"
- formatWeight(100, "oz") returns "3.5 oz"
- formatWeight(1000, "lb") returns "2.20 lb"
- formatWeight(1500, "kg") returns "1.50 kg"
- formatWeight(null) returns "--" for all units
- formatWeight(undefined, "kg") returns "--"
- formatWeight(100) returns "100g" (backward compatible, no second arg)
- formatWeight(0, "oz") returns "0.0 oz"
</behavior>
<action>
RED: Create `tests/lib/formatters.test.ts`. Import `formatWeight` from `@/client/lib/formatters`. Write tests for:
- All 4 units with a known gram value (e.g., 1000g = "1000g", "35.3 oz", "2.20 lb", "1.00 kg")
- Null and undefined input returning "--" for each unit
- Default parameter (no second arg) producing current "g" behavior
- Zero grams producing "0g", "0.0 oz", "0.00 lb", "0.00 kg"
- Precision edge cases (small values like 5g in lb = "0.01 lb")
Run tests — they should fail because formatWeight does not accept a unit parameter yet.
GREEN: Modify `src/client/lib/formatters.ts`:
- Export `type WeightUnit = "g" | "oz" | "lb" | "kg"`
- Add constants: `GRAMS_PER_OZ = 28.3495`, `GRAMS_PER_LB = 453.592`, `GRAMS_PER_KG = 1000`
- Change signature to `formatWeight(grams: number | null | undefined, unit: WeightUnit = "g")`
- Add switch statement after the null guard for unit-specific conversion and formatting
- Leave `formatPrice` completely untouched
Run tests — all should pass.
REFACTOR: None expected — the code is already minimal.
</action>
<verify>
<automated>bun test tests/lib/formatters.test.ts</automated>
</verify>
<done>formatWeight handles all 4 units with correct precision, null handling, and backward-compatible default. WeightUnit type is exported. All tests pass.</done>
</task>
<task type="auto">
<name>Task 2: Create useWeightUnit convenience hook</name>
<files>src/client/hooks/useWeightUnit.ts</files>
<action>
Create `src/client/hooks/useWeightUnit.ts`:
```typescript
import { useSetting } from "./useSettings";
import type { WeightUnit } from "../lib/formatters";
const VALID_UNITS: WeightUnit[] = ["g", "oz", "lb", "kg"];
export function useWeightUnit(): WeightUnit {
const { data } = useSetting("weightUnit");
if (data && VALID_UNITS.includes(data as WeightUnit)) {
return data as WeightUnit;
}
return "g";
}
```
This hook:
- Wraps `useSetting("weightUnit")` for a typed return value
- Validates the stored value is a known unit (protects against bad data)
- Defaults to "g" when no setting exists (backward compatible — UNIT-03 persistence works via existing settings API)
- Returns `WeightUnit` type so components can pass directly to `formatWeight`
</action>
<verify>
<automated>bun run lint</automated>
</verify>
<done>useWeightUnit hook exists, imports from useSettings and formatters, returns typed WeightUnit with "g" default. Lint passes.</done>
</task>
</tasks>
<verification>
- `bun test tests/lib/formatters.test.ts` passes with all unit conversion tests green
- `bun run lint` passes with no errors
- `src/client/lib/formatters.ts` exports `WeightUnit` type and updated `formatWeight` function
- `src/client/hooks/useWeightUnit.ts` exists and exports `useWeightUnit`
- Existing tests still pass: `bun test` (full suite)
</verification>
<success_criteria>
- formatWeight("g") produces identical output to the old function (backward compatible)
- formatWeight with oz/lb/kg produces correct conversions with appropriate decimal precision
- WeightUnit type is exported for use by Plan 02 components
- useWeightUnit hook is ready for components to consume
- All existing tests remain green (no regressions)
</success_criteria>
<output>
After completion, create `.planning/phases/07-weight-unit-selection/07-01-SUMMARY.md`
</output>