Function doesn't change a global variable

Question

Been struggling with a little problem for a while. Here, I have a function which searches for album covers via album-cover and Last.FM API. Here's the function itself:

global.url = ' '
function CoverReturn(artist_name, release_name) {
    search_covers(last_fm_api_token).search({
        artist: artist_name,
        album: release_name,
        size: 'extralarge'
    }, function(err, res) {
        res = res.replace("300x300", "1024x1024")
        url = res
    });
}
CoverReturn(...)
console.log(url)

The problem is that the function doesn't change the url variable at all (and yeah, I really need to change it). I've tried different ways to change this variable, but it just doesn't. What's the problem?