Fetch API

So far, we know quite a bit about fetch.

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
});

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:

1. Request to the same origin.
2. Request to another origin.
3. 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 full Referer, for cross-origin send only the origin, unless it’s HTTPS→HTTP request, then send nothing.
• "no-referrer-when-downgrade" – full Referer is always sent, unless we send a request from HTTPS to HTTP (to the less secure protocol).
• "no-referrer" – never send Referer.
• "origin" – only send the origin in Referer, not the full page URL, e.g. only http://site.com instead of http://site.com/path.
• "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.
• "unsafe-url" – always send the full url in Referer, even for HTTPS→HTTP requests.

Here’s a table with all combinations:

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).

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, requires Access-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 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".

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, with response.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 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.