I do not find it cozy to leave footprints on Facebook, Google and alike upon every single step I do on the Internet. For this purpose I use a script blocker. Now I found that Stack Overflow does not allow login without allowing it to use ajax.googleapis.com.
Why does Stack Overflow require JavaScript from Google in order to work? Which information is forwarded to Google upon acting on Stack Overflow?
We use jQuery - a very popular JavaScript library - at Stack Exchange.
For technical reasons, we use the Google CDN for the jQuery files (a CDN is a Content Delivery Network - a bit of technology that makes distributing things like images and JavaScript libraries across the world faster) - that's what is hosted on ajax.googleapis.com. This is something many many sites do and as result many people will already have this library on their computer.
You will note that the full URL of the blocked request is:
http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js
That is the jQuery library we use.
Much of our JavaScript code leverages jQuery, so if it is not loaded, things do not work.
max-age of 365 days with a 30 day stale-while-revalidate cache on jQuery. Stack Exchange could easily do the same. The performance difference between hosting this on Google's CDN vs. internally on SE is minimal and would only be realized by users who either clear their cache daily or never visit the site. For users who clear their cache often, they would welcome this change as it is one less signal to google. I personally use Decentraleyes to block google CDN. It would also help Chinese users, and be one less DNS query & server that can have outages.
Aug 16, 2017 at 17:51
There is a very popular Javascript library known as jQuery. Google hosts a free CDN for a number of Javascript libraries including jQuery.
As a lot of websites use Google's CDN for jQuery, a large number of visitors to Stack Exchange will already have it cached and not need to reload it, thus resulting in a faster user experience. In contrast, if SE were to personally serve jQuery on their CDN, every first-time visitor to Stack Exchange would have to retrieve it.
The only information Google gets from you when you fetch jQuery is basic information that your browser sends to every website you visit. Because Google's CDN is not hosted on google.com, Google's standard cookies are not sent, and as far as I can tell, Google does not set cookies on googleapis.com, partly because that would only decrease the efficiency of a CDN.
For example, my browser sent this
GET /ajax/libs/jquery/1.10.2/jquery.min.js HTTP/1.1
Host: ajax.googleapis.com
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.63 Safari/537.36
DNT: 1
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
If-Modified-Since: Tue, 09 Jul 2013 11:31:25 GMT
If-Modified-Since (based on the Last-Modified date) or If-None-Match (based on the ETag) request when the user simply hits refresh. (Even for a regular refresh; don't ask me why. It seems Chrome has recently stopped doing so.) But indeed, Google's CDN does not return any specific information that a browser can accidentally emit when asking if a cached resource is still valid.