Usually that header is set automatically and contains the url of the page that made the request. In most scenarios, it's not important at all, sometimes, for security purposes, it makes sense to remove or shorten it.
- **`"no-referrer-when-downgrade"`** -- the default value: full `Referer` is always sent, unless we send a request from HTTPS to HTTP (to the less secure protocol).
- **`"origin-when-cross-origin"`** -- send the full `Referer` to the same origin, but only the origin part for cross-origin requests (as above).
- **`"same-origin"`** -- send the full `Referer` to the same origin, but no `Referer` for cross-origin requests.
- **`"strict-origin"`** -- send only the origin, not the `Referer` for HTTPS→HTTP requests.
- **`"strict-origin-when-cross-origin"`** -- for same-origin send the full `Referer`, for cross-origin send only the origin, unless it's HTTPS→HTTP request, then send nothing.
- **`"unsafe-url"`** -- always send the full url in `Referer`, even for HTTPS→HTTP requests.
If we send a `fetch`, then by default it always sends the `Referer` header with the full url of our page (except when we request from HTTPS to HTTP, then no `Referer`).
Its only difference compared to the default behavior is that for requests to another origin `fetch` sends only the origin part of the URL (e.g. `https://javascript.info`, without path). For requests to our origin we still get the full `Referer` (maybe useful for debugging purposes).
In particular, it's possible to set the default policy for the whole page using the `Referrer-Policy` HTTP header, or per-link, with `<a rel="noreferrer">`.
- **`"include"`** -- always send, requires `Access-Control-Allow-Credentials` from cross-origin server in order for JavaScript to access the response, that was covered in the chapter <info:fetch-crossorigin>,
By default, `fetch` requests make use of standard HTTP-caching. That is, it respects the `Expires` and `Cache-Control` headers, sends `If-Modified-Since` and so on. Just like regular HTTP-requests do.
- **`"default"`** -- `fetch` uses standard HTTP-cache rules and headers,
- **`"no-store"`** -- totally ignore HTTP-cache, this mode becomes the default if we set a header `If-Modified-Since`, `If-None-Match`, `If-Unmodified-Since`, `If-Match`, or `If-Range`,
- **`"reload"`** -- don't take the result from HTTP-cache (if any), but populate the cache with the response (if the response headers permit this action),
- **`"no-cache"`** -- create a conditional request if there is a cached response, and a normal request otherwise. Populate HTTP-cache with the response,
- **`"force-cache"`** -- use a response from HTTP-cache, even if it's stale. If there's no response in HTTP-cache, make a regular HTTP-request, behave normally,
- **`"only-if-cached"`** -- use a response from HTTP-cache, even if it's stale. If there's no response in HTTP-cache, then error. Only works when `mode` is `"same-origin"`.
- **`"manual"`** -- allows to process HTTP-redirects manually. In case of redirect, we'll get a special response object, with `response.type="opaqueredirect"` and zeroed/empty status and most other properies.
As described in the [specification](https://w3c.github.io/webappsec-subresource-integrity/), supported hash-functions are SHA-256, SHA-384, and SHA-512, there might be others depending on the browser.
For example, we gather statistics on how the current visitor uses our page (mouse clicks, page fragments he views), to analyze and improve the user experience.
Normally, when a document is unloaded, all associated network requests are aborted. But the `keepalive` option tells the browser to perform the request in the background, even after it leaves the page. So this option is essential for our request to succeed.
- If we need to gather a lot of statistics about the visit, we should send it out regularly in packets, so that there won't be a lot left for the last `onunload` request.
- This limit applies to all `keepalive` requests together. In other words, we can perform multiple `keepalive` requests in parallel, but the sum of their body lengths should not exceed 64KB.
- We can't handle the server response if the document is unloaded. So in our example `fetch` will succeed due to `keepalive`, but subsequent functions won't work.
- In most cases, such as sending out statistics, it's not a problem, as the server just accepts the data and usually sends an empty response to such requests.
