Does every web request send the browser's cookies?
I'm not talking page views, but a request for an image, .js file, etc.
Update If a web page has 50 elements, that is 50 requests. Why would it send the SAME cookie(s) for each request, doesn't it cache or know it already has it?
As others have said, if the cookie's host, path, etc. restrictions are met, it'll be sent, 50 times.
But you also asked why: because cookies are an HTTP feature, and HTTP is stateless. HTTP is designed to work without the server storing any state between requests.
In fact, the server doesn't have a solid way of recognizing which user is sending a given request; there could be a thousand users behind a single web proxy (and thus IP address). If the cookies were not sent every request, the server would have no way to know which user is requesting whatever resource.
Finally, the browser has no clue if the server needs the cookies or not, it just knows the server instructed it to send the cookie for any request to foo.com, so it does so. Sometimes images need them (e.g., dynamically-generated per-user), sometimes not, but the browser can't tell.
Yes. Every request sends the cookies that belong to the same domain. They're not cached as HTTP is stateless, what means every request must be enough for the server to figure out what to do with it. Say you have images that are only accessible by certain users; you must send your auth cookie with every one of those 50 requests, so the server knows it's you and not someone else, or a guest, among the pool of requests it's getting.
Having said that, cookies might not be sent given other restrictions mentioned in the other responses, such as HTTPS setting, path or domain. Especially there, an important thing to notice: cookies are not shared between domains. That helps with reducing the size of HTTP calls for static files, such as the images and scripts you mentioned.
Example: you have 4 cookies at www.stackoverflow.com; if you make a request to www.stackoverflow.com/images/logo.png, all those 4 cookies will be sent.
However, if you request stackoverflow.com/images/logo.png (notice the subdomain change) or images.stackoverflow.com/logo.png, those 4 cookies won't be present - but maybe those related to these domains will.
You can read more about cookies and images requesting, for example, at this StackOverflow Blog Post.
No. Not every request sends the cookies. It depends on the cookie configuration and client-server connection.
For example, if your cookie's secure option is set to true then it must be transmitted over a secure HTTPS connection. Means when you see that website with HTTP protocol then these cookies won't be sent by browsers as the secure flag is true.
3 years have passed
There's another reason why a browser wouldn't send cookies. You can add a crossOrigin attribute to your <script> tag, and the value to "anonymous". This will prevent cookies to be sent to the destination server. 99.9% of the time, your javascripts are static files, and you don't generate that js code based on the request's cookies. If you have 1KB of cookies, and you have 200 resources on your page, then your user is uploading 200KB, and that might take some time on 3G and have zero effect on the result page. Visit HTML attribute: crossorigin for reference.
I know this is an old thread. But I've just noticed that most browsers won't sent cookies for a domain if you add a trailing dot. For example http://example.com. won't receive cookies set for .example.com. Apache on the other hand treats them as the same host. I find this useful to make cross domain tracking more difficult for external resources I include, but you could also use it for performance reasons. Note this brakes validation of https certificates. I've run a few tests using browsershots and my own devices. The hack works on almost all browsers except for safari (mobile and desktop), which will include cookies in the request.
Short answer is Yes. The below lines are from the JS documentation
Cookies were once used for general client-side storage. While this was legitimate when they were the only way to store data on the client, it is now recommended to use modern storage APIs. Cookies are sent with every request, so they can worsen performance (especially for mobile data connections).
