diff --git a/js/data.js b/js/data.js
new file mode 100644
index 0000000..4deced3
--- /dev/null
+++ b/js/data.js
@@ -0,0 +1,59 @@
+export function loadFromJSON(json) {
+  const parsed = JSON.parse(json);
+  return {
+    employees: parsed.employees ?? [],
+    calendar: parsed.calendar ?? { holidays: [], companyClosures: [] },
+    history: parsed.history ?? []
+  };
+}
+
+export function saveToJSON(state) {
+  return JSON.stringify({
+    employees: state.employees,
+    calendar: state.calendar,
+    history: state.history
+  }, null, 2);
+}
+
+export function createEmployee(name) {
+  return {
+    id: crypto.randomUUID(),
+    name: name.trim(),
+    constraints: {
+      neverDays: [],
+      lowPriorityDays: [],
+      onlyDays: [],
+      maxPerMonth: null,
+      minGapDays: 0,
+      vacations: []
+    }
+  };
+}
+
+export function removeEmployee(state, id) {
+  state.employees = state.employees.filter(e => e.id !== id);
+}
+
+export function toggleHoliday(state, isoDate) {
+  const arr = state.calendar.holidays;
+  const idx = arr.indexOf(isoDate);
+  if (idx >= 0) arr.splice(idx, 1); else arr.push(isoDate);
+}
+
+export function toggleClosure(state, isoDate) {
+  const arr = state.calendar.companyClosures;
+  const idx = arr.indexOf(isoDate);
+  if (idx >= 0) arr.splice(idx, 1); else arr.push(isoDate);
+}
+
+// Adds entries to history, replacing any existing entries for the same dates
+export function addHistoryEntries(state, entries) {
+  const dates = new Set(entries.map(e => e.date));
+  state.history = state.history.filter(h => !dates.has(h.date));
+  state.history.push(...entries);
+  state.history.sort((a, b) => a.date.localeCompare(b.date));
+}
+
+export function removeHistoryEntry(state, isoDate) {
+  state.history = state.history.filter(h => h.date !== isoDate);
+}