refactor: centralize shared date/name helpers in utils.js (#12)

New js/utils.js exports toISO, monthPrefix, getWeekday, formatDateDE and
buildNameMap. algorithm.js, calendar.js, excel.js and history.js now import
these instead of re-implementing ISO construction, weekday math, DD.MM.YYYY
formatting and the id→name map inline. Adds tests/utils.test.mjs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/algorithm.js

@@ -1,13 +1,4 @@
-function toISO(year, month, day) {
-  return year + '-' + String(month).padStart(2, '0') + '-' + String(day).padStart(2, '0');
-}
-
-// Returns 1=Mo, 2=Di, 3=Mi, 4=Do, 5=Fr, 6=Sa, 7=So
-function getWeekday(isoDate) {
-  const d = new Date(isoDate + 'T12:00:00Z');
-  const day = d.getUTCDay(); // 0=Sun
-  return day === 0 ? 7 : day;
-}
+import { toISO, getWeekday } from './utils.js';
 
 function isOnVacation(employee, isoDate) {
   return employee.constraints.vacations.some(v => v.from <= isoDate && isoDate <= v.to);

js/calendar.js

@@ -2,6 +2,7 @@ import { state, saveState } from './app.js';
 import { getWorkdays, generatePlan } from './algorithm.js';
 import { toggleHoliday, toggleClosure, addHistoryEntries, removeHistoryEntry } from './data.js';
 import { exportSchedule, downloadBlob } from './excel.js';
+import { toISO, monthPrefix, getWeekday, buildNameMap } from './utils.js';
 
 const MONTHS = ['Januar','Februar','März','April','Mai','Juni',
   'Juli','August','September','Oktober','November','Dezember'];
@@ -122,7 +123,7 @@ export function renderCalendar() {
     }
     const workdays = getWorkdays(currentYear, currentMonth,
       state.calendar.holidays, state.calendar.companyClosures);
-    const prefix = currentYear + '-' + String(currentMonth).padStart(2, '0');
+    const prefix = monthPrefix(currentYear, currentMonth);
     // Preserve manual picks for this month; only auto-entries get regenerated.
     const manualEntries = state.history.filter(h => h.date.startsWith(prefix) && h.manual);
     const historyWithoutMonth = state.history.filter(h => !h.date.startsWith(prefix));
@@ -148,7 +149,7 @@ export function renderCalendar() {
 }
 
 function loadPlanFromHistory() {
-  const prefix = currentYear + '-' + String(currentMonth).padStart(2,'0');
+  const prefix = monthPrefix(currentYear, currentMonth);
   currentPlan = state.history
     .filter(h => h.date.startsWith(prefix))
     .map(h => ({ ...h }));
@@ -158,7 +159,7 @@ function renderGrid(wrapper, exportBtn) {
   wrapper.replaceChildren();
 
   const daysInMonth = new Date(currentYear, currentMonth, 0).getDate();
-  const empNameById = Object.fromEntries(state.employees.map(e => [e.id, e.name]));
+  const empNameById = buildNameMap(state.employees);
   const updateExportVisibility = () => {
     exportBtn.style.display = currentPlan.length > 0 ? 'inline-block' : 'none';
   };
@@ -166,8 +167,8 @@ function renderGrid(wrapper, exportBtn) {
   // Build one day cell. Self-contained so a change to a single day re-renders
   // just that cell (cell.replaceWith) instead of rebuilding the whole grid.
   function makeDayCell(d) {
-    const iso = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-' + String(d).padStart(2,'0');
-    const wdNum = (() => { const x = new Date(iso + 'T12:00:00Z').getUTCDay(); return x === 0 ? 7 : x; })();
+    const iso = toISO(currentYear, currentMonth, d);
+    const wdNum = getWeekday(iso);
     const isWeekend = wdNum >= 6;
     const isHoliday = state.calendar.holidays.includes(iso);
     const isClosure = state.calendar.companyClosures.includes(iso);
@@ -291,8 +292,8 @@ function renderGrid(wrapper, exportBtn) {
   bodyGrid.className = 'calendar-body-grid';
 
   // Offset empty cells before first day
-  const firstISO = currentYear + '-' + String(currentMonth).padStart(2,'0') + '-01';
-  const firstWD = (() => { const d = new Date(firstISO + 'T12:00:00Z').getUTCDay(); return d === 0 ? 7 : d; })();
+  const firstISO = toISO(currentYear, currentMonth, 1);
+  const firstWD = getWeekday(firstISO);
   for (let i = 1; i < firstWD; i++) {
     const empty = document.createElement('div');
     empty.className = 'cal-day empty-cell';

js/excel.js

@@ -1,3 +1,5 @@
+import { formatDateDE, buildNameMap } from './utils.js';
+
 // Reads employee names from first column of xlsx (skips header row 0)
 export function readEmployeeNames(arrayBuffer) {
   const wb = XLSX.read(arrayBuffer, { type: 'array' });
@@ -19,7 +21,7 @@ export function sanitizeCell(value) {
 // Pure helper: returns { names: [...], dates: [...] } columns for a schedule.
 // Multiple employees per date are joined alphabetically with " & ".
 export function buildScheduleColumns(assignments, employees) {
-  const nameById = Object.fromEntries(employees.map(e => [e.id, e.name]));
+  const nameById = buildNameMap(employees);
   const byDate = new Map();
   for (const a of assignments) {
     if (!byDate.has(a.date)) byDate.set(a.date, []);
@@ -29,10 +31,7 @@ export function buildScheduleColumns(assignments, employees) {
   const names = sortedDates.map(d =>
     sanitizeCell(byDate.get(d).slice().sort((x, y) => x.localeCompare(y, 'de')).join(' & '))
   );
-  const dates = sortedDates.map(d => {
-    const [y, m, day] = d.split('-');
-    return day + '.' + m + '.' + y;
-  });
+  const dates = sortedDates.map(formatDateDE);
   return { names, dates };
 }
 

js/history.js

@@ -1,6 +1,7 @@
 import { state, saveState } from './app.js';
 import { loadFromJSON, saveToJSON, removeHistoryEntry } from './data.js';
 import { downloadBlob } from './excel.js';
+import { formatDateDE, buildNameMap } from './utils.js';
 
 export function renderDataTab() {
   const container = document.getElementById('tab-data');
@@ -102,7 +103,7 @@ function renderHistoryTable() {
     return;
   }
 
-  const nameById = Object.fromEntries(state.employees.map(e => [e.id, e.name]));
+  const nameById = buildNameMap(state.employees);
   const sorted = [...state.history].sort((a, b) => {
     const dateCmp = b.date.localeCompare(a.date);
     if (dateCmp !== 0) return dateCmp;
@@ -124,11 +125,10 @@ function renderHistoryTable() {
 
   const tbody = document.createElement('tbody');
   for (const h of sorted) {
-    const [y, m, d] = h.date.split('-');
     const tr = document.createElement('tr');
 
     const dateTd = document.createElement('td');
-    dateTd.textContent = d + '.' + m + '.' + y;
+    dateTd.textContent = formatDateDE(h.date);
 
     const nameTd = document.createElement('td');
     nameTd.textContent = nameById[h.employeeId] ?? '(unbekannt)'; // textContent: safe

js/utils.js

@@ -0,0 +1,28 @@
+// Shared pure helpers used across modules.
+
+// "YYYY-MM-DD" from numeric parts.
+export function toISO(year, month, day) {
+  return year + '-' + String(month).padStart(2, '0') + '-' + String(day).padStart(2, '0');
+}
+
+// "YYYY-MM" month prefix (used to filter history by month).
+export function monthPrefix(year, month) {
+  return year + '-' + String(month).padStart(2, '0');
+}
+
+// Weekday for an ISO date string: 1=Mo, 2=Di, 3=Mi, 4=Do, 5=Fr, 6=Sa, 7=So.
+export function getWeekday(isoDate) {
+  const d = new Date(isoDate + 'T12:00:00Z').getUTCDay(); // 0=Sun
+  return d === 0 ? 7 : d;
+}
+
+// "DD.MM.YYYY" from an ISO date string.
+export function formatDateDE(isoDate) {
+  const [y, m, d] = isoDate.split('-');
+  return d + '.' + m + '.' + y;
+}
+
+// { [id]: name } lookup map from an employees array.
+export function buildNameMap(employees) {
+  return Object.fromEntries(employees.map(e => [e.id, e.name]));
+}