As I understand it, a "tracking pixel" is a script that contains code that includes a header that make the response appear to be an image, which is then called using an image tag, like:

<img src="mytrackingpixel.php">

...so that the code runs automatically/silently, when the page loads the images, for tracking or other purposes.

If I'm correct so far, then my questions are:

  1. What's the advantage? How is this any different from just includeing another .js file, with the same code?

  2. Images are often cached (but also refreshed sometimes) - so won't the code run at somewhat unpredictably/irregularly?

  • oh and # - do they sometimes have actual images attached?
    – ashleedawg
    Jan 15 '19 at 9:50

Here are a few reasons one might choose tracking pixels over a script:

  • Images are less likely to be blocked on the client-side than scripts.

  • You can easily prevent the pixel from being cached (and therefore prevent any cache jank) by using a Cache-Control HTTP header that tells the browser to never cache the pixel.

  • For basic server-side tracking, JavaScript is not needed anyways, as the server can get all the info it wants from the HTTP headers.

  • If you are doing server-side tracking, a tracking pixel is simpler to implement than JavaScript code.

Tracking pixels and tracking JavaScript both have their pros and cons. The tracking pixel is a simple, basic server-side approach that is often enough if the webmaster's needs are not too advanced.

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