Question

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How to detect when XHR returns a cached resource?

I built an auto loader that only loads a javascript file if it has not already been loaded on this page using jquery with:

$.getScript();

I first check if the functions exist and if they do not I load them. However, this is unable to account for the browsers cache, I was hoping I might be able to figure out how.

One thing I would like to add to it is to see if the user already has the file saved in their browsers cache. Is it possible to check if a file from your server has already been cached on the users browser, so that I can skip making the call to the server at all?

Answer 1

The cache works transparently. If you request a file and the file exists in the user’s cache, then the cached file will be used and there will not be an additional request to the server. The browser takes care of that, so you don’t have to worry about it for the case that you’re describing.

.getScript by default disables this using timestamped query arguments, so you simply need to disable the cache bypass which getScript uses:

$.ajaxSetup({
  cache: true
});