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Editorial Team
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Asked: 2026-05-14T21:08:40+00:00

I have a javascript slide show that creates the next slide dynamically and then

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I have a javascript slide show that creates the next slide dynamically and then moves it into view. Since the images are actually sprites, the src is transparent.png and the actual image is mapped via background:url(.. in css.

Every time (well, most of the time) the script creates a new Element, Firefox makes an http request for transparent.png. I have a far-future expires header, and Firefox is respecting all other files’ expiries.

Is there a way to avoid these unnecessary requests. Even though the server is returning 304 unmodified responses, it would be nice if Firefox would respect the expiries on dynamically created images.

I suspect that if I injected a simple string instead of using new Element, this might solve the problem, but I use some methods on Prototypes extended Element object, so I would like to avoid a bunch of html strings in my js file.

This is a nit-picky question, but I’m working on front-end optimization now, so I thought I would address it.

Thanks.

@TJ Crowder Here are two images: http://tinypic.com/r/29kon45/5. The first shows that the requests for trans.png are proliferating. The second shows an example of the headers. Thanks

@all Just to reiterate: what’s real strange is that it only makes these unnecessary requests about half the time, even though all images are created via identical logic.

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1 Answer

  1. Editorial Team

    I know this doesn’t address why Firefox ignores your caching times, but you could always just bypass the issue and not use image tags for the slides. If you make the slides empty div tags and just apply the sprite as a background, Firefox won’t have to make any more requests.

    EDIT:

    According to the explanation at this site, Firefox isn’t ignoring you cache times. If the image has expired, then the browser is supposed to just request the image again. If the time has not expired, which is happening in this case, then the browser is supposed to issue a conditional GET request. I don’t think you can get away from it.

    I think Firefox only issues requests half of the time because it just received the “304 Not Modified” status for the image on a previous request and wants to trust that for subsequent requests if they happen quickly enough.

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