Refresh image with Javascript, but only if changed on server

I want to reload an image on a page if it has been updated on the server. In other questions it has been suggested to do something like

newImage.src = "http://localhost/image.jpg?" + new Date().getTime();

to force the image to be re-loaded, but that means that it will get downloaded again even if it really hasn't changed.

Is there any Javascript code that will cause a new request for the same image to be generated with a proper If-Modified-Since header so the image will only be downloaded if it has actually changed?

UPDATE: I'm still confused: if I just request the typical URL, I'll get the locally cached copy. (unless I make the server mark it as not cacheable, but I don't want to do that because the whole idea is to not re-download it unless it really changes.) if I change the URL, I'll always re-download, because the point of the new URL is to break the cache. So how do I get the in-between behavior I want, i.e. download the file only if it doesn't match the locally cached copy?

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