Optional sequence number for iCalendar events

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@hebcal/icalendar",
-  "version": "4.20.1",
+  "version": "4.21.0",
   "author": "Michael J. Radwin (https://github.com/mjradwin)",
   "keywords": [
     "ical",
@@ -54,10 +54,10 @@
     "@rollup/plugin-commonjs": "^25.0.2",
     "@rollup/plugin-json": "^6.0.0",
     "ava": "^5.3.1",
-    "eslint": "^8.43.0",
+    "eslint": "^8.44.0",
     "eslint-config-google": "^0.14.0",
     "jsdoc": "^4.0.2",
     "jsdoc-to-markdown": "^8.0.0",
-    "rollup": "^3.25.3"
+    "rollup": "^3.26.0"
   }
 }

src/icalendar.js

@@ -71,6 +71,11 @@ export class IcalEvent {
     this.ev = ev;
     this.options = options;
     this.dtstamp = options.dtstamp || IcalEvent.makeDtstamp(new Date());
+    if (typeof ev.sequence === 'number') {
+      this.sequence = ev.sequence;
+    } else if (typeof options.sequence === 'number') {
+      this.sequence = options.sequence;
+    }
     const timed = this.timed = Boolean(ev.eventTime);
     const locale = options.locale;
     let subj = shouldRenderBrief(ev) ? ev.renderBrief(locale) : ev.render(locale);
@@ -186,8 +191,11 @@ export class IcalEvent {
    */
   getLongLines() {
     if (this.lines) return this.lines;
-    const categoryLine = this.category ? `CATEGORIES:${this.category}` : [];
+    const categoryLine = this.category ? [`CATEGORIES:${this.category}`] : [];
     const uid = this.ev.uid || this.getUid();
+    if (this.sequence) {
+      categoryLine.unshift(`SEQUENCE:${this.sequence}`);
+    }
     const arr = this.lines = [
       'BEGIN:VEVENT',
       `DTSTAMP:${this.dtstamp}`,

src/icalendar.spec.js

@@ -678,3 +678,18 @@ test('yerushalmi-yomi', (t) => {
   ];
   t.deepEqual(lines, expected);
 });
+
+test('sequence', (t) => {
+  const ev = new TestEvent(new HDate(22, 'Iyyar', 5781));
+  const icalEvent = new IcalEvent(ev, {sequence: 73});
+  const lines = icalEvent.toString().split('\r\n').slice(0, 5);
+  lines[1] = 'DTSTAMP:X';
+  const expected = [
+    'BEGIN:VEVENT',
+    'DTSTAMP:X',
+    'SEQUENCE:73',
+    'CATEGORIES:Holiday',
+    'SUMMARY:Test Event',
+  ];
+  t.deepEqual(lines, expected);
+});