javascript - How to wait for an asynchronous method's callback return value?

Maybe you can solve the problem by keeping track of the tab URLs?:

1. When the app starts, get all open tab URLs by using chrome.tabs.query


2. Subscribe to chrome.tabs.onUpdated and chrome.tabs.onRemoved and add/remove/update the URLs when they change.

Some kind of code example based on the documentation:

var tabUrlHandler = (function() {
   // All opened urls
   var urls = {},
   queryTabsCallback = function(allTabs) {
      allTabs && allTabs.forEach(function(tab) {
          urls[tab.id] = tab.url;
      });
   },
   updateTabCallback = function(tabId, changeinfo, tab) {
       urls[tabId] = tab.url;
   },
   removeTabCallback = function(tabId, removeinfo) {
       delete urls[tabId];
   };
   // init
   chrome.tabs.query({ active: true }, queryTabsCallback);
   chrome.tabs.onUpdated.addListener(updateTabCallback);
   chrome.tabs.onRemoved.addListener(removeTabCallback);
   return {
     contains: function(url) {
        for (var urlId in urls) {
           if (urls[urlId] == url) {
              return true;
           }
        }
        return false;
     }
   };
}());

Now you should be able to ask the tabUrlHandler directly in your onBeforeRequestMethod:

function onBeforeRequest(details) {
  var websiteAlreadyOpenInOtherTab = tabUrlHandler.contains(details.url);
  if (websiteAlreadyOpenInOtherTab) {
    return { cancel: true };
  }
}