How to acces data from variable in begin function

Question

Could someone help me out on this function structure what I try to do is seperate the functions like this:

function getNewResources (currentUrl, nextUrl)
    {
        let resourcesCurrentUrl = getResourceByUrl(currentUrl);
        let resourcesNextUrl = getResourceByUrl(nextUrl);

        // Diff resources
        ACCESS DATA FROM ABOVE VARIABLES HERE!

        // Return new resources            
    }


    function getResourceByUrl (url)
    {
        let xmlhttp = new XMLHttpRequest();
        let xmlResponse;

        xmlhttp.onreadystatechange = function() {
            if (this.readyState == 4) {
                // when succesfull
                var resources = extractResourcesFromXMLResponse(this.response);
                return resources;                   
            }
        };


        xmlhttp.open("GET", url, true);
        xmlhttp.send();
    }


    function extractResourcesFromXMLResponse (xmlResponse)
    {
        let resources = [];

        // Add images
        imagePaths = extractImages(xmlResponse);
        resources.push(imagePaths);
        return resources;
    }

    function extractImages(xmlDoc) 
    {
        let match,
            extractedImages = [],
            newArr = [],
            rex = /<img.*?src="([^">]*\/([^">]*?))".*?>/g;

        while ( match = rex.exec( xmlDoc ) ) {
            extractedImages.push( match[1] );
        }

        return extractedImages;
    }

How do I acces the returned resource variable in the getNewResource() function