Fix bug: Clock can't be stopped when 'Start' clicked while running

2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.view/index.html

@@ -43,12 +43,17 @@
     }
 
     function clockStart() {
-      timerId = setInterval(update, 1000);
+      // set a new interval only if the clock is stopped
+      // otherwise we would rewrite the timerID reference to the running interval and wouldn't be able to stop the clock ever again
+      if (!timerId) { 
+        timerId = setInterval(update, 1000);
+      }
       update(); // <--  start right now, don't wait 1 second till the first setInterval works
     }
 
     function clockStop() {
       clearInterval(timerId);
+      timerId = null; // <-- clear timerID to indicate that the clock has been stopped, so that it is possible to start it again in clockStart()
     }
 
     clockStart();