oops missing this file

clock.js

@@ -0,0 +1,16 @@
+let timerID;
+self.addEventListener('message', e => {
+    switch (e.data) {
+        case 'start':
+            if (!timerID) {
+                timerID = setInterval(() => {
+                    self.postMessage('tick');
+                }, 50);
+            }
+            break;
+        case 'stop':
+            clearInterval(timerID);
+            timerID = null;
+            break;
+    };
+}, false);