So far, we know quite a bit about fetch
.
Let’s see the rest of API, to cover all its abilities.
Please note: most of these options are used rarely. You may skip this chapter and still use fetch
well.
Still, it’s good to know what fetch
can do, so if the need arises, you can return and read the details.
Here’s the full list of all possible fetch
options with their default values (alternatives in comments):
let promise = fetch(url, {
method: "GET", // POST, PUT, DELETE, etc.
headers: {
// the content type header value is usually auto-set
// depending on the request body
"Content-Type": "text/plain;charset=UTF-8"
},
body: undefined, // string, FormData, Blob, BufferSource, or URLSearchParams
referrer: "about:client", // or "" to send no Referer header,
// or an url from the current origin
referrerPolicy: "strict-origin-when-cross-origin", // no-referrer-when-downgrade, no-referrer, origin, same-origin...
mode: "cors", // same-origin, no-cors
credentials: "same-origin", // omit, include
cache: "default", // no-store, reload, no-cache, force-cache, or only-if-cached
redirect: "follow", // manual, error
integrity: "", // a hash, like "sha256-abcdef1234567890"
keepalive: false, // true
signal: undefined, // AbortController to abort request
window: window // null
});
An impressive list, right?
We fully covered method
, headers
and body
in the chapter Fetch.
The signal
option is covered in Fetch: Abort.
Now let’s explore the remaining capabilities.
referrer, referrerPolicy
These options govern how fetch
sets the HTTP Referer
header.
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.
The referrer
option allows to set any Referer
(within the current origin) or remove it.
To send no referrer, set an empty string:
fetch('/page', {
referrer: "" // no Referer header
});
To set another url within the current origin:
fetch('/page', {
// assuming we're on https://javascript.info
// we can set any Referer header, but only within the current origin
referrer: "https://javascript.info/anotherpage"
});
The referrerPolicy
option sets general rules for Referer
.
Requests are split into 3 types:
- Request to the same origin.
- Request to another origin.
- Request from HTTPS to HTTP (from safe to unsafe protocol).
Unlike the referrer
option that allows to set the exact Referer
value, referrerPolicy
tells the browser general rules for each request type.
Possible values are described in the Referrer Policy specification:
"strict-origin-when-cross-origin"
– the default value: for same-origin send the fullReferer
, for cross-origin send only the origin, unless it’s HTTPS→HTTP request, then send nothing."no-referrer-when-downgrade"
– fullReferer
is always sent, unless we send a request from HTTPS to HTTP (to the less secure protocol)."no-referrer"
– never sendReferer
."origin"
– only send the origin inReferer
, not the full page URL, e.g. onlyhttp://site.com
instead ofhttp://site.com/path
."origin-when-cross-origin"
– send the fullReferer
to the same origin, but only the origin part for cross-origin requests (as above)."same-origin"
– send the fullReferer
to the same origin, but noReferer
for cross-origin requests."strict-origin"
– send only the origin, not theReferer
for HTTPS→HTTP requests."unsafe-url"
– always send the full url inReferer
, even for HTTPS→HTTP requests.
Here’s a table with all combinations:
Value | To same origin | To another origin | HTTPS→HTTP |
---|---|---|---|
"no-referrer" |
- | - | - |
"no-referrer-when-downgrade" |
full | full | - |
"origin" |
origin | origin | origin |
"origin-when-cross-origin" |
full | origin | origin |
"same-origin" |
full | - | - |
"strict-origin" |
origin | origin | - |
"strict-origin-when-cross-origin" or "" (default) |
full | origin | - |
"unsafe-url" |
full | full | full |
Let’s say we have an admin zone with a URL structure that shouldn’t be known from outside of the site.
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
).
E.g. Referer: https://javascript.info/admin/secret/paths
.
If we’d like other websites know only the origin part, not the URL-path, we can set the option:
fetch('https://another.com/page', {
// ...
referrerPolicy: "origin-when-cross-origin" // Referer: https://javascript.info
});
We can put it to all fetch
calls, maybe integrate into JavaScript library of our project that does all requests and uses fetch
inside.
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).
fetch
Referrer policy, described in the specification, is not just for fetch
, but more global.
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">
.
mode
The mode
option is a safe-guard that prevents occasional cross-origin requests:
"cors"
– the default, cross-origin requests are allowed, as described in Fetch: Cross-Origin Requests,"same-origin"
– cross-origin requests are forbidden,"no-cors"
– only safe cross-origin requests are allowed.
This option may be useful when the URL for fetch
comes from a 3rd-party, and we want a “power off switch” to limit cross-origin capabilities.
credentials
The credentials
option specifies whether fetch
should send cookies and HTTP-Authorization headers with the request.
"same-origin"
– the default, don’t send for cross-origin requests,"include"
– always send, requiresAccess-Control-Allow-Credentials
from cross-origin server in order for JavaScript to access the response, that was covered in the chapter Fetch: Cross-Origin Requests,"omit"
– never send, even for same-origin requests.
cache
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.
The cache
options allows to ignore HTTP-cache or fine-tune its usage:
"default"
–fetch
uses standard HTTP-cache rules and headers,"no-store"
– totally ignore HTTP-cache, this mode becomes the default if we set a headerIf-Modified-Since
,If-None-Match
,If-Unmodified-Since
,If-Match
, orIf-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 whenmode
is"same-origin"
.
redirect
Normally, fetch
transparently follows HTTP-redirects, like 301, 302 etc.
The redirect
option allows to change that:
"follow"
– the default, follow HTTP-redirects,"error"
– error in case of HTTP-redirect,"manual"
– allows to process HTTP-redirects manually. In case of redirect, we’ll get a special response object, withresponse.type="opaqueredirect"
and zeroed/empty status and most other properies.
integrity
The integrity
option allows to check if the response matches the known-ahead checksum.
As described in the specification, supported hash-functions are SHA-256, SHA-384, and SHA-512, there might be others depending on the browser.
For example, we’re downloading a file, and we know that its SHA-256 checksum is “abcdef” (a real checksum is longer, of course).
We can put it in the integrity
option, like this:
fetch('http://site.com/file', {
integrity: 'sha256-abcdef'
});
Then fetch
will calculate SHA-256 on its own and compare it with our string. In case of a mismatch, an error is triggered.
keepalive
The keepalive
option indicates that the request may “outlive” the webpage that initiated it.
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.
When the visitor leaves our page – we’d like to save the data to our server.
We can use the window.onunload
event for that:
window.onunload = function() {
fetch('/analytics', {
method: 'POST',
body: "statistics",
keepalive: true
});
};
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.
It has a few limitations:
- We can’t send megabytes: the body limit for
keepalive
requests is 64KB.- 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 multiplekeepalive
requests in parallel, but the sum of their body lengths should not exceed 64KB.
- 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
- We can’t handle the server response if the document is unloaded. So in our example
fetch
will succeed due tokeepalive
, 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.
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