return value of chrome.webRequest.onBeforeRequest based on data in chrome.storage

return value of chrome.webRequest.onBeforeRequest based on data in
chrome.storage

I am trying to to block certain webrequests in my google chrome extension
based on data stored in chrome.storage.local. However I can't find a way
to return "{cancel: true };" inside the callback function of
onBeforeRequest.addListener. Or to access data from storage.local outside
of it's respective callback function due to the asynchronous way of
chrome.Storage.local.get().
Here is my relevant code.
chrome.webRequest.onBeforeRequest.addListener( function(info) {
chrome.storage.local.get({requests: []}, function (result) {
// depending on the value of result.requests.[0].item I want to
return "{cancel: true };" in order to block the webrequest
if(result.requests.[0].item == 0) return {cancel: true}; //
however this is obviously in the wrong place
});
// if I put return {cancel: true} here, where it should be, I can't
access the data of storage.local.get anymore
// if(result.requests.[0].item == 0) return {cancel: true};
});
Has anyone a solution for this problem? Thanks for your help.