Issue 6717 - Part 1: run eslint through npm

ext/background.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 77 matching lines @@
       // but sometimes we only have the tab id when we create a Page object.
       // In that case we get the url from top frame of the tab, recorded by
       // the onBeforeRequest handler.
       let frames = framesOfTabs.get(this.id);
       if (frames)
       {
         let frame = frames.get(0);
         if (frame)
           return frame.url;
       }
+
+      return undefined;
     },
     sendMessage(message, responseCallback)
     {
       browser.tabs.sendMessage(this.id, message, responseCallback);
     }
   };
 
   ext.getPage = id => new Page({id: parseInt(id, 10)});
 
   ext.pages = {
@@ skipping 491 matching lines @@
           return frames.get(0) || null;
         }
       };
     }
 
     return ext.onMessage._dispatch(
       message, sender, sendResponse
     ).includes(true);
   });
 }