@RiversideRocks is not expecting data to be returned.
He is simply pointing out that when a link (as he posted) has a .js extension, that the 404 pages is not displayed as expected, hence his topic:
Seems legit that a properly formatted 404 page should be returned when the extension is wrong (in this case .js), because when other extensions are used (see .xml) example, the 404 page is formatted correctly:
@RiversideRocks is simply pointing out that when a .js extension is processed as a 404 (file not found), it the 404 page is not formatted nicely, as expected.