chrome extension change download file name form

1chrome.downloads.onDeterminingFilename.addListener(function(item, suggest) {
2    if (item.referrer.search("gutenberg.org") == -1) {
3    // If the file does not come from gutenberg.org, suggest nothing new.
4        suggest({filename: item.filename});
5    } else {
6    // Otherwise, fetch the book's title in storage...
7        chrome.storage.local.get([item.referrer], function(result) {
8            if (result[item.referrer] == null) {
9                // ...and if we find don't find it, suggest nothing new.
10                suggest({filename: item.filename});
11                console.log('Nothing done.');
12            }
13            else {
14                // ...if we find it, suggest it.
15                fileExt = item.filename.split('.').pop();
16                var newFilename = "gutenberg/" + result[item.referrer] + "." + fileExt;
17                suggest({filename: newFilename});
18                console.log('New filename: ' + newFilename);
19            }
20          });
21        // Storage API is asynchronous so we need to return true
22        return true;
23    }
24  });
25