Refactor inbox_block_script library.

Closes #3491252

js/private_message_notification_block.js

@@ -4,8 +4,8 @@
  */
 
 ((Drupal, drupalSettings, window) => {
-  let refreshRate;
   let checkingCountInProgress = false;
+  let updateTimeoutId = null;
 
   /**
    * Private message notification block.
@@ -87,6 +87,24 @@
         pageTitle.textContent = pageTitle.textContent.replace(titlePattern, '');
       }
     },
+
+    /**
+     * Sets a timeout for count updates.
+     */
+    scheduleCountUpdate() {
+      if (updateTimeoutId) {
+        window.clearTimeout(updateTimeoutId);
+      }
+
+      const refreshRate =
+        drupalSettings.privateMessageNotificationBlock.ajaxRefreshRate * 1000;
+      if (refreshRate) {
+        updateTimeoutId = window.setTimeout(
+          Drupal.privateMessageNotificationBlock.triggerCountCallback,
+          refreshRate,
+        );
+      }
+    },
   };
 
   /**
@@ -102,14 +120,8 @@
         context,
       ).shift();
 
-      refreshRate =
-        drupalSettings.privateMessageNotificationBlock.ajaxRefreshRate * 1000;
-
-      if (notificationWrapper && refreshRate) {
-        window.setTimeout(
-          Drupal.privateMessageNotificationBlock.triggerCountCallback,
-          refreshRate,
-        );
+      if (notificationWrapper) {
+        Drupal.privateMessageNotificationBlock.scheduleCountUpdate();
       }
     },
   };
@@ -131,11 +143,6 @@
     );
 
     checkingCountInProgress = false;
-    if (refreshRate) {
-      window.setTimeout(
-        Drupal.privateMessageNotificationBlock.triggerCountCallback,
-        refreshRate,
-      );
-    }
+    Drupal.privateMessageNotificationBlock.scheduleCountUpdate();
   };
 })(Drupal, drupalSettings, window);