Is it possible to test whether XmlHttpRequest will hit the cache instead of the server?

Question

In IE using Javascript, is there way to know if doing a regular GET on a URL using XmlHttpRequest will pull its data from the cache or hit the server?

If there is no way to know in advance, is there a way to tell XmlHttpRequest to NOT hit the server, and instead pull from the cache if possible?

To clarify: I understand how browser caching works, I'm trying to understand if I can use javascript to determine in advance if an ajax call will use the browsers cache. In my case, I'm writing a JS library, so I don't control the server's behavior, so I'm highly constrained in terms of knowing apriori what cache settings/headers are.

Answer 1

It's actually what decent HTTP user agents do by default: they fetch from the server only if there is no valid cache entry for the location that you request. So your best bet to control (or at least understand) caching behaviour is to look at the HTTP cache headers the web server returns for the location you request in your script.

Answer 2

It seems to me you can never know for sure if something is cached, because even if it is at one point, it may be invalidated immediately afterwards. What you can do is forcing the browser to hit the server by adding a random querystring parameter to your request.