javascript - How can I add a callback function to chrome extension api? -

actually, want use webrequest modify cookie before request sending. own cookie data stored in extension's storage, getting data storage asynchronous. how can modify request's cookie?

i want this:

var beforesendheadershandler = {     func: function(details, addcookie){         addcookie(details)         return {requestheaders: details.requestheaders}     },     filter: {         urls: ["<all_urls>"]     },     extra: ["blocking", "requestheaders"] }; chrome.webrequest.onbeforesendheaders.addlistener(beforesendheadershandler.func,     beforesendheadershandler.filter,     beforesendheadershandler.extra); 

how can add callback argument chrome.webrequest.onbeforesendheaders.addlistener callback function, or other solution?

if want that, use promise

var beforesendheadershandler = {     func: function(details, addcookie){         var promise = new promise(function(resolve) {             chrome.storage.local.get("your key", function(results) {                 resolve(results["your key"]);             });         };         promise.then(function(value) {            return {requestheaders: value};         });     },     filter: {         urls: ["<all_urls>"]     },     extra: ["blocking", "requestheaders"] }; chrome.webrequest.onbeforesendheaders.addlistener(beforesendheadershandler.func,     beforesendheadershandler.filter,     beforesendheadershandler.extra); 

recommended

see synchronous call in google chrome extension , asynchronous operation in blocking chrome.webrequest.onbeforesendheaders listener, it's recommended if make copy outside of listener, or if possible, save storage using localstorage, synchronous.