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Overview.src.html
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Overview.src.html
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<!doctype html>
<html lang=en-US>
<meta charset=utf-8>
<title>Fetch Standard</title>
<link rel=stylesheet href=https://resources.whatwg.org/standard.css>
<link rel=icon href=https://resources.whatwg.org/logo-fetch.svg>
<div class=head>
<p><a class=logo href=https://whatwg.org/><img alt=WHATWG src=https://resources.whatwg.org/logo-fetch.svg width=100 height=100></a>
<h1 id=cors>Fetch</h1>
<h2 class="no-num no-toc">Living Standard — Last Updated [DATE: 3 August 2002]</h2>
<dl>
<dt>Participate:
<dd><a href=https://github.com/whatwg/fetch>GitHub whatwg/fetch</a>
(<a href=https://github.com/whatwg/fetch/issues/new>file an issue</a>,
<a href=https://github.com/whatwg/fetch/issues>open issues</a>,
<a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=WHATWG&component=Fetch&resolution=---">legacy open bugs</a>)
<dd><a href=https://wiki.whatwg.org/wiki/IRC>IRC: #whatwg on Freenode</a>
<dt>Commits:
<dd><a href=https://github.com/whatwg/fetch/commits>GitHub whatwg/fetch/commits</a>
<dd><a href=https://twitter.com/fetchstandard>@fetchstandard</a>
</dl>
<script src=https://resources.whatwg.org/file-issue.js async></script>
<script id=head src=https://resources.whatwg.org/dfn.js defer></script>
</div>
<h2 class="no-num no-toc short">Abstract</h2>
<p>The Fetch standard defines requests, responses, and the process that binds them:
fetching.
<h2 class="no-num no-toc">Table of Contents</h2>
<!-- toc -->
<h2 class="no-num short">Goals</h2>
<p>To unify fetching across the web platform this specification supplants a number of algorithms and specifications:
<ul class="brief no-backref">
<li>HTML Standard's fetch and potentially CORS-enabled fetch algorithms
<span data-anolis-ref class=informative>HTML</span>
<li>CORS <span data-anolis-ref class=informative>CORS</span>
<li>HTTP `<code title=http-origin>Origin</code>` header semantics
<span data-anolis-ref class=informative>ORIGIN</span>
</ul>
<p>Unifying fetching provides consistent handling of:
<ul class=brief>
<li>URL schemes
<li>Redirects
<li>Cross-origin semantics
<li>CSP <span data-anolis-ref>CSP</span>
<li>Service workers <span data-anolis-ref>SW</span>
<li>Mixed Content <span data-anolis-ref>MIX</span>
<li>`<code title>Referer</code>` <span data-anolis-ref>REFERRER</span>
</ul>
<h2 class=short>Preface</h2>
<p>At a high level, fetching a resource is a fairly simple operation. A request goes in, a
response comes out. <!--You can't explain that! -->The details of that operation are
however quite involved and used to not be written down carefully and differ from one API
to the next.
<p>Numerous APIs provide the ability to fetch a resource, e.g. HTML's <code>img</code> and
<code>script</code> element, CSS' <code>cursor</code> and <code>list-style-image</code>,
the <code>navigator.sendBeacon()</code> and <code>self.importScripts()</code> JavaScript
APIs. The Fetch Standard provides a unified architecture for these features so they are
all consistent when it comes to various aspects of fetching, such as redirects and the
CORS protocol.
<p>The Fetch Standard also defines the <code title=dom-global-fetch>fetch()</code>
JavaScript API, which exposes most of the networking functionality at a fairly low level
of abstraction.
<h2 class="short">Conformance</h2>
<p>All diagrams, examples, and notes in this specification are
non-normative, as are all sections explicitly marked non-normative.
Everything else in this specification is normative.
<p>The key words "MUST", "MUST NOT", "REQUIRED", <!--"SHALL", "SHALL
NOT",--> "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in the normative parts of this specification are to be
interpreted as described in RFC2119. For readability, these words do
not appear in all uppercase letters in this specification.
<span data-anolis-ref>RFC2119</span>
<h2>Infrastructure</h2>
<p>This specification uses terminology from the ABNF, Encoding, HTML, IDL, Streams, and URL Standards.
<span data-anolis-ref>ABNF</span>
<span data-anolis-ref>ENCODING</span>
<span data-anolis-ref>HTML</span>
<span data-anolis-ref>WEBIDL</span>
<span data-anolis-ref>STREAMS</span>
<span data-anolis-ref>URL</span>
<p>A <span data-anolis-spec=encoding>byte</span> sequence with bytes in the range 0x00 to
0x7F, inclusive, is represented as a <span data-anolis-spec=encoding>utf-8</span>-encoded
<span data-anolis-spec=encoding>string</span> with code points in the range U+0000 to
U+007F, inclusive. To avoid confusion with an actual string, backticks are used.
<p class=example>"<code title>true</code>" is a
<span data-anolis-spec=encoding>string</span>, while `<code title>true</code>` is a
byte sequence.
<p>Comparing two byte sequences in a <dfn>byte-case-insensitive</dfn> manner means
comparing them exactly, byte for byte, except that the bytes in the range 0x41 to 0x5A,
inclusive, are considered to also match their corresponding byte in the range 0x61 to
0x7A, inclusive.
<p>A <dfn>case-insensitive byte sequence</dfn> is a byte sequence that when compared to
another byte sequence does so in a <span>byte-case-insensitive</span> manner.
<p class=example>The
<span title="case-insensitive byte sequence">case-insensitive byte sequences</span>
`<code title>Content-Type</code>` and `<code title>content-TYPE</code>` are equal.
<p>To <dfn>byte-lowercase</dfn> a byte sequence, increase each byte it contains, in the
range 0x41 to 0x5A, inclusive, by 0x20.
<p>To <dfn>byte-uppercase</dfn> a byte sequence, subtract each byte it contains, in the
range 0x61 to 0x7A, inclusive, by 0x20.
<hr>
<p>A <dfn>fetch URL</dfn> is a <span data-anolis-spec=url title=concept-url>URL</span> for
which implementations need not store the
<span data-anolis-spec=url title=concept-url-fragment>fragment</span> as it is never
exposed. When <span data-anolis-spec=url title=concept-url-serializer>serialized</span>,
the <i title>exclude fragment flag</i> is set, meaning implementations can store the
<span data-anolis-spec=url title=concept-url-fragment>fragment</span> nonetheless.
<hr>
<p><span data-anolis-spec=html title=concept-task>Tasks</span> that are
<span data-anolis-spec=html title="queue a task">queued</span> by this standard are annotated as one
of:
<ul class=brief>
<li><dfn>process request body</dfn>
<li><dfn>process request end-of-file</dfn>
<li><dfn>process response</dfn>
<li><dfn>process response end-of-file</dfn>
</ul>
<p>To <dfn>queue a fetch task</dfn> on <span title=concept-request>request</span>
<var>request</var> to <var>run an operation</var>, run these steps:
<ol>
<li><p>If <var>request</var>'s <span title=concept-request-client>client</span> is
null, terminate these steps.
<li><p><span data-anolis-spec=html>Queue a task</span> to
<var>run an operation</var> on <var>request</var>'s
<span title=concept-request-client>client</span>'s
<span data-anolis-spec=html>responsible event loop</span> using the
<span data-anolis-spec=html>networking task source</span>.
</ol>
<p>To <dfn>queue a fetch-done task</dfn>, given a <var>request</var> and
<var>response</var>, run these steps:
<ol>
<li><p>Set <var>request</var>'s <span>done flag</span>.
<li><p><span>Queue a fetch task</span> on <var>request</var> to
<span>process response end-of-file</span> for <var>response</var>.
</ol>
<hr>
<p>To <dfn>wait for a <var>response</var></dfn>, if <var>response</var>'s
<span title=concept-response-body>body</span> is non-null, wait for <var>response</var>'s
<span title=concept-response-body>body</span>'s <span title=concept-body-stream>stream</span> to be
<span title=concept-ReadableStream-closed>closed</span> or
<span title=concept-ReadableStream-errored>errored</span>.
<p>To <dfn>read a <var>request</var></dfn>, if <var>request</var>'s
<span title=concept-request-body>body</span> is non-null, whenever
<var>request</var>'s <span title=concept-request-body>body</span> is read from (i.e.
is transmitted or read by script), increase <var>request</var>'s
<span title=concept-request-body>body</span>'s
<span title=concept-body-transmitted>transmitted bytes</span> with the amount of payload body
bytes transmitted and then <span>queue a fetch task</span> on <var>request</var> to
<span>process request body</span> for <var>request</var>.
<!-- XXX xref "read", "payload body" -->
<h3>HTTP</h3>
<p>While <span title=concept-fetch>fetching</span> encompasses more than just HTTP, it
borrows a number of concepts from HTTP and applies these to resources obtained via other
means (e.g., <code title>data</code> URLs).
<p>The <dfn>HTTP whitespace bytes</dfn> are 0x09, 0x0A, 0x0D, and 0x20.
<h4>Methods</h4>
<p>A <dfn title=concept-method>method</dfn> is a byte sequence that matches the
<span data-anolis-spec=http>method</span> token production.
<p>A <dfn>simple method</dfn> is a <span title=concept-method>method</span> that is
`<code title>GET</code>`, `<code title>HEAD</code>`, or `<code title>POST</code>`.
<p>A <dfn>forbidden method</dfn> is a <span title=concept-method>method</span> that is a
<span>byte-case-insensitive</span> match for `<code title>CONNECT</code>`,
`<code title>TRACE</code>`, or `<code title>TRACK</code>`.
<span data-anolis-ref class=informative>HTTPVERBSEC</span>
<p>To <dfn title=concept-method-normalize>normalize</dfn> a
<span title=concept-method>method</span>, if it is a <span>byte-case-insensitive</span>
match for `<code title>DELETE</code>`, `<code title>GET</code>`,
`<code title>HEAD</code>`, `<code title>OPTIONS</code>`, `<code title>POST</code>`, or
`<code title>PUT</code>`, <span>byte-uppercase</span> it.
<p class="note no-backref"><span title=concept-method-normalize>Normalization</span> is
done for backwards compatibility and consistency across APIs as
<span title=concept-method>methods</span> are actually "case-sensitive".
<h4 id=terminology-headers>Headers</h4>
<p>A <dfn title=concept-header-list>header list</dfn> consists of zero or more
<span title=concept-header>headers</span>.
<p class="note no-backref">A <span title=concept-header-list>header list</span> is essentially a
specialized multimap. An ordered list of key-value pairs with potentially duplicate keys.
<p>To <dfn title=concept-header-list-append>append</dfn> a
<span title=concept-header-name>name</span>/<span title=concept-header-value>value</span>
(<var>name</var>/<var>value</var>) pair to a
<span title=concept-header-list>header list</span> (<var>list</var>), append a new
<span title=concept-header>header</span> whose <span title=concept-header-name>name</span>
is <var>name</var>, <span title="byte-lowercase">byte-lowercased</span>, and
<span title=concept-header-value>value</span> is <var>value</var>, to
<var>list</var>.
<p>To <dfn title=concept-header-list-delete>delete</dfn> a
<span title=concept-header-name>name</span> (<var>name</var>) from a
<span title=concept-header-list>header list</span> (<var>list</var>), remove all
<span title=concept-header>headers</span> whose
<span title=concept-header-name>name</span> is <var>name</var>,
<span title="byte-lowercase">byte-lowercased</span>, from <var>list</var>.
<p>To <dfn title=concept-header-list-set>set</dfn> a
<span title=concept-header-name>name</span>/<span title=concept-header-value>value</span>
(<var>name</var>/<var>value</var>) pair in a
<span title=concept-header-list>header list</span> (<var>list</var>), run these
steps:
<ol>
<li><p><span>Byte-lowercase</span> <var>name</var>.
<li><p>If there are any <span title=concept-header>headers</span> in
<var>list</var> whose <span title=concept-header-name>name</span> is
<var>name</var>, set the <span title=concept-header-value>value</span> of the first
such <span title=concept-header>header</span> to <var>value</var> and remove the
others.
<li><p>Otherwise, append a new <span title=concept-header>header</span> whose
<span title=concept-header-name>name</span> is <var>name</var> and
<span title=concept-header-value>value</span> is <var>value</var>, to
<var>list</var>.
</ol>
<p>To <dfn title=concept-header-list-combine>combine</dfn> a
<span title=concept-header-name>name</span>/<span title=concept-header-value>value</span>
(<var>name</var>/<var>value</var>) pair in a
<span title=concept-header-list>header list</span> (<var>list</var>), run these
steps:
<ol>
<li><p><span>Byte-lowercase</span> <var>name</var>.
<li><p>If there are any <span title=concept-header>headers</span> in <var>list</var> whose
<span title=concept-header-name>name</span> is <var>name</var>, set the
<span title=concept-header-value>value</span> of the first such
<span title=concept-header>header</span> to its <span title=concept-header-value>value</span>,
followed by 0x2C 0x20, followed by <var>value</var>.
<li><p>Otherwise, append a new <span title=concept-header>header</span> whose
<span title=concept-header-name>name</span> is <var>name</var> and
<span title=concept-header-value>value</span> is <var>value</var>, to
<var>list</var>.
</ol>
<p class="note no-backref"><span title=concept-header-list-combine>Combine</span> is used by
<code data-anolis-spec=xhr>XMLHttpRequest</code> and the
<span title=concept-websocket-establish>WebSocket protocol handshake</span>.
<hr>
<p>A <dfn title=concept-header>header</dfn> consists of a
<dfn title=concept-header-name>name</dfn> and <dfn title=concept-header-value>value</dfn>.
A <span title=concept-header-name>name</span> is a
<span>case-insensitive byte sequence</span> that matches the
<span data-anolis-spec=http>field-name</span> token production. A
<span title=concept-header-value>value</span> is a byte sequence that matches the
<span data-anolis-spec=http>field-content</span> token production.
<p class="note no-backref"><span data-anolis-spec=http>field-value</span> allows 0x0A and
0x0D bytes which can lead to reparsing issues.
<p>To <dfn title=concept-header-value-normalize>normalize</dfn> a
<span title=concept-header-value>value</span>, remove any leading and trailing
<span>HTTP whitespace bytes</span> from it.
<p>A <dfn title=concept-header-value-combined>combined value</dfn>, given a
<span title=concept-header-name>name</span> (<var>name</var>) and
<span title=concept-header-list>header list</span> (<var>list</var>), is the
<span title=concept-header-value>values</span> of all <span title=concept-header>headers</span> in
<var>list</var> whose <span title=concept-header-name>name</span> is <var>name</var>, separated from
each other by `<code title>,</code>`, in order.
<hr>
<p>A <dfn>simple header</dfn> is a <span title=concept-header>header</span> whose
<span title=concept-header-name>name</span> is `<code title>Accept</code>`,
`<code title>Accept-Language</code>`, or `<code title>Content-Language</code>`, or whose
<span title=concept-header-name>name</span> is `<code title>Content-Type</code>` and
<span title=concept-header-value>value</span>, <span title=concept-header-parse>once parsed</span>,
has a MIME type (ignoring parameters) that is
`<code title>application/x-www-form-urlencoded</code>`, `<code title>multipart/form-data</code>`, or
`<code title>text/plain</code>`.
<!-- XXX * needs better xref
* ignoring parameters has been the standard for a long time now
* interesting test: "Content-Type: text/plain;" -->
<p>A <dfn>forbidden header name</dfn> is a <span title=concept-header>header</span>
<span title=concept-header-name>name</span> that is one of
<ul class=brief>
<li>`<code title>Accept-Charset</code>`
<li>`<code title>Accept-Encoding</code>`
<li>`<code title>Access-Control-Request-Headers</code>`
<li>`<code title>Access-Control-Request-Method</code>`
<li>`<code title>Connection</code>`
<li>`<code title>Content-Length</code>`
<li>`<code title>Cookie</code>`
<li>`<code title>Cookie2</code>`
<li>`<code title>Date</code>`
<li>`<code title>DNT</code>`
<li>`<code title>Expect</code>`
<li>`<code title>Host</code>`
<li>`<code title>Keep-Alive</code>`
<li>`<code title=http-origin>Origin</code>`
<li>`<code title>Referer</code>`
<li>`<code title>TE</code>`
<li>`<code title>Trailer</code>`
<li>`<code title>Transfer-Encoding</code>`
<li>`<code title>Upgrade</code>`
<li>`<code title>Via</code>`
</ul>
<p>or a <span title=concept-header>header</span> <span title=concept-header-name>name</span> that
starts with `<code title>Proxy-</code>` or `<code title>Sec-</code>` (including being just
`<code title>Proxy-</code>` or `<code title>Sec-</code>`).
<p class=note>These are forbidden so the user agent remains in full control over them.
<span title=concept-header-name>Names</span> starting with `<code title>Sec-</code>` are
reserved to allow new <span title=concept-header>headers</span> to be minted that are safe
from APIs using <span title=concept-fetch>fetch</span> that allow control over
<span title=concept-header>headers</span> by developers, such as
<code data-anolis-spec=xhr>XMLHttpRequest</code>.
<span data-anolis-ref class=informative>XHR</span>
<p>A <dfn>forbidden response-header name</dfn> is a <span title=concept-header>header</span>
<span title=concept-header-name>name</span> that is one of:
<ul class=brief>
<li>`<code title>Set-Cookie</code>`
<li>`<code title>Set-Cookie2</code>`
</ul>
<p>A <dfn>CORS-safelisted response-header name</dfn>, given a
<span title=concept-response-header-list>header list</span> <var>list</var>, is a
<span title=concept-header>header</span> <span title=concept-header-name>name</span> that is one of:
<ul>
<li>`<code title>Cache-Control</code>`
<li>`<code title>Content-Language</code>`
<li>`<code title>Content-Type</code>`
<li>`<code title>Expires</code>`
<li>`<code title>Last-Modified</code>`
<li>`<code title>Pragma</code>`.
<li>Any <span title=concept-header-value>value</span> resulting from
<span title=concept-header-parse>parsing</span>
`<code title=http-access-control-expose-headers>Access-Control-Expose-Headers</code>` in
<var>list</var> that is not a <span>forbidden response-header name</span>.
</ul>
<hr>
<p>To <dfn title=concept-header-parse>parse a header value</dfn> given a
<span title=concept-header-name>name</span> (<var>name</var>) and a
<span title=concept-header>header</span> or a <span title=concept-header-list>header list</span>
(<var>headers</var>), run these steps:
<ol>
<li><p>If <var>name</var> is not in <var>headers</var>, return null.
<li>
<p>If the ABNF for <var>name</var> allows a single
<span title=concept-header>header</span> and <var>headers</var> contains more than
one, return failure.
<p class="note no-backref">If different error handling is required, extract the desired
<span title=concept-header>header</span> first.
<li><p>If parsing all the <span title=concept-header>headers</span>
<span title=concept-header-name>named</span> <var>name</var> in
<var>headers</var>, per the ABNF for <var>name</var>, failed, return failure.
<li><p>Return one or more <span title=concept-header-value>values</span> resulting from
parsing all the <span title=concept-header>headers</span>
<span title=concept-header-name>named</span> <var>name</var> in
<var>headers</var>, per the ABNF for <var>name</var>.
</ol>
<p>To <dfn title=concept-header-extract-MIME-type>extract a MIME type</dfn> from a
<span title=concept-header-list>header list</span> (<var>headers</var>), run these
steps:
<ol>
<li><p>Let <var>MIMEType</var> be the result of
<span title=concept-header-parse>parsing</span> `<code title>Content-Type</code>` in
<var>headers</var>.
<li><p>If <var>MIMEType</var> is null or failure, return the empty byte sequence.
<li><p>Return <var>MIMEType</var>,
<span title="byte-lowercase">byte-lowercased</span>.
</ol>
<hr>
<p>A <dfn id=default-user-agent-value>default `<code>User-Agent</code>` value</dfn> is a
user-agent-defined <span title=concept-header-value>value</span> for the
`<code title=http-user-agent>User-Agent</code>` <span title=concept-header>header</span>.
<h4>Statuses</h4>
<p>A <dfn id=concept-status>status</dfn> is a code.
<p>A <dfn>null body status</dfn> is a <span>status</span> that is <code>101</code>,
<code>204</code>, <code>205</code>, or <code>304</code>.
<p>An <dfn>ok status</dfn> is any <span>status</span> in the range <code>200</code> to
<code>299</code>, inclusive.
<p>A <dfn>redirect status</dfn> is a <span>status</span> that is <code>301</code>, <code>302</code>,
<code>303</code>, <code>307</code>, or <code>308</code>.
<h4>Bodies</h4>
<p>A <dfn title=concept-body>body</dfn> consists of:
<ul>
<li>
<p>A <dfn title=concept-body-stream>stream</dfn> (a
<code title=concept-ReadableStream>ReadableStream</code> object).
<p class="XXX"><a href="https://github.com/whatwg/streams/issues/379">This might become a
<code>ReadableByteStream</code> object</a>. While the type might change, the behavior specified
will be equivalent since the hypothetical <code title>ReadableByteStream</code> object will have
the same members as the <code title=concept-ReadableStream>ReadableStream</code> object has today.
<li><p>A <dfn title=concept-body-transmitted>transmitted bytes</dfn> (an integer), initially 0.
<li><p>A <dfn title=concept-body-total-bytes>total bytes</dfn> (an integer), initially 0.
</ul>
<p>To <dfn title=concept-body-clone>clone</dfn> a <span title=concept-body>body</span>
<var>body</var>, run these steps:
<ol>
<li><p>Let «<var>out1</var>, <var>out2</var>» be the result of
<span title=concept-tee-ReadableStream>teeing</span> <var>body</var>'s
<span title=concept-body-stream>stream</span>. Rethrow any exceptions.
<li><p>Set <var>body</var>'s <span title=concept-body-stream>stream</span> to <var>out1</var>.
<li><p>Return a <span title=concept-body>body</span> whose
<span title=concept-body-stream>stream</span> is <var>out2</var> and other members are copied from
<var>body</var>.
</ol>
<p>To <dfn>handle content codings</dfn> given <var>codings</var> and
<var>bytes</var>, run these substeps:
<ol>
<li><p>If <var>codings</var> are not supported, return <var>bytes</var>.
<li><p>Return the result of decoding bytes with the given
<var>codings</var> as explained in HTTP. <span data-anolis-ref>HTTP</span>
</ol>
<!-- XXX no hook in HTTP -->
<h4>Requests</h4>
<p>The input to <span title=concept-fetch>fetch</span> is a
<dfn title=concept-request>request</dfn>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-method>method</dfn> (a
<span title=concept-method>method</span>). Unless stated otherwise it is
`<code title>GET</code>`.
<p class="note no-backref">This can be updated during redirects to `<code title>GET</code>` as
described in <span title=concept-http-fetch>HTTP fetch</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-url>url</dfn> (a <span>fetch URL</span>).
<p>A <span title=concept-request>request</span> has an associated
<dfn>local-URLs-only flag</dfn>. Unless stated otherwise it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn>sandboxed-storage-area-URLs flag</dfn>. Unless stated otherwise it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-header-list>header list</dfn> (a
<span title=concept-header-list>header list</span>). Unless stated otherwise it is empty.
<p>A <span title=concept-request>request</span> has an associated
<dfn>unsafe-request flag</dfn>. Unless stated otherwise it is unset.
<p class="note no-backref">The <span>unsafe-request flag</span> is set by APIs such as
<code title=dom-global-fetch>fetch()</code> and
<code data-anolis-spec=xhr>XMLHttpRequest</code> to ensure a
<span>CORS-preflight fetch</span> is done based on the supplied
<span title=concept-request-method>method</span> and
<span title=concept-request-header-list>header list</span>. It does not free an API from
outlawing <span title="forbidden method">forbidden methods</span> and
<span title="forbidden header name">forbidden header names</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-body>body</dfn> (null or a
<span title=concept-body>body</span>). Unless stated otherwise it is null.
<p class="note no-backref">This can be updated during redirects to null as described in
<span title=concept-http-fetch>HTTP fetch</span>.
<hr>
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-client>client</dfn> (null or an
<span data-anolis-spec=html>environment settings object</span>).
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-window>window</dfn> ("<code>no-window</code>",
"<code>client</code>", or an
<span data-anolis-spec=html>environment settings object</span> whose
<span data-anolis-spec=html>global object</span> is a
<code data-anolis-spec=html>Window</code> object). Unless stated otherwise it is
"<code>client</code>".
<p class="note no-backref">The "<code>client</code>" value is changed to "<code>no-window</code>" or
<span title=concept-request>request</span>'s <span title=concept-request-client>client</span> during
<span title=concept-fetch>fetching</span>. It provides a convenient way for standards to not have to
explicitly set <span title=concept-request>request</span>'s
<span title=concept-request-window>window</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-target-browsing-context>target browsing context</dfn> (null or a
<span data-anolis-spec=html>browsing context</span>). Unless stated otherwise it is null.
<p>A <span title=concept-request>request</span> has an associated
<dfn>keep-alive flag</dfn>. Unless stated otherwise it is unset.
<p class="note no-backref">This is used by <code>navigator.sendBeacon</code> and the HTML
<code>img</code> element to outlive the
<span data-anolis-spec=html>environment settings object</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn>skip-service-worker flag</dfn>. Unless stated otherwise it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-initiator>initiator</dfn>, which is
the empty string,
"<code title>download</code>",
"<code title>imageset</code>",
"<code title>manifest</code>", or
"<code title>xslt</code>". Unless stated otherwise it is the empty string.
<p class="note no-backref">A <span title=concept-request>request</span>'s
<span title=concept-request-initiator>initiator</span> is not particularly granular for
the time being as other specifications do not require it to. It is primarily a
specification device to assist defining CSP and Mixed Content. It is not exposed to
JavaScript. <span data-anolis-ref>CSP</span> <span data-anolis-ref>MIX</span>
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-type>type</dfn>, which is
the empty string,
"<code title>audio</code>",
"<code title>font</code>",
"<code title>image</code>",
"<code title>script</code>",
"<code title>style</code>",
"<code title>track</code>", or
"<code title>video</code>". Unless stated otherwise it is the empty string.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-destination>destination</dfn>, which is
the empty string,
"<code title>document</code>",
"<code title>embed</code>",
"<code title>font</code>",
"<code title>image</code>",
"<code title>manifest</code>",
"<code title>media</code>",
"<code title>object</code>",
"<code title>report</code>",
"<code title>script</code>",
"<code title>serviceworker</code>",
"<code title>sharedworker</code>",
"<code title>style</code>",
"<code title>worker</code>", or
"<code title>xslt</code>". Unless stated otherwise it is the empty string.
<div class=note>
<p>The following table illustrates the relationship between a
<span title=concept-request>request</span>'s
<span title=concept-request-initiator>initiator</span>,
<span title=concept-request-type>type</span>,
<span title=concept-request-destination>destination</span>, CSP directives, and features.
<table>
<tr>
<th><span title=concept-request-initiator>Initiator</span>
<th><span title=concept-request-type>Type</span>
<th><span title=concept-request-destination>Destination</span>
<th>CSP directive
<th>Features
<tr>
<td rowspan=16>""
<td rowspan=6>""
<td>"<code title>report</code>"
<td rowspan=2>?
<td>CSP, NEL reports.
<tr>
<td>"<code title>document</code>"
<td>HTML's navigate algorithm.
<tr>
<td>"<code title>document</code>"
<td><code title>child-src</code>
<td>HTML's <code><iframe></code> and <code><frame></code>
<tr>
<td>""
<td><code title>connect-src</code>
<td><code>navigator.sendBeacon()</code>, <code>EventSource</code>,
HTML's <code>ping=""</code>, <code title=dom-global-fetch>fetch()</code>,
<code>XMLHttpRequest</code>, <code>WebSocket</code>, Cache API?
<tr>
<td>"<code title>object</code>"
<td><code title>object-src</code>
<td>HTML's <code><object></code>
<tr>
<td>"<code title>embed</code>"
<td><code title>object-src</code>
<td>HTML's <code><embed></code>
<tr>
<td>"<code title>audio</code>"
<td>"<code title>media</code>"
<td><code title>media-src</code>
<td>HTML's <code><audio></code>
<tr>
<td>"<code title>font</code>"
<td>"<code title>font</code>"
<td><code title>font-src</code>
<td>CSS' <code>@font-face</code>
<tr>
<td>"<code title>image</code>"
<td>"<code title>image</code>"
<td><code title>img-src</code>
<td>HTML's <code><img src></code>, <code title>/favicon.ico</code> resource,
SVG's <code><image></code>, CSS' <code>background-image</code>, CSS'
<code>cursor</code>, CSS' <code>list-style-image</code>, …
<tr>
<td rowspan=4>"<code title>script</code>"
<td>"<code title>script</code>"
<td><code title>script-src</code>
<td>HTML's <code><script></code>, <code>importScripts()</code>
<tr>
<td>"<code title>serviceworker</code>"
<td>?
<td><code title>navigator.serviceWorker.register()</code>
<tr>
<td>"<code title>sharedworker</code>"
<td><code title>child-src</code>
<td><code>SharedWorker</code>
<tr>
<td>"<code title>worker</code>"
<td><code title>child-src</code>
<td><code>Worker</code>
<tr>
<td>"<code title>style</code>"
<td>"<code title>style</code>"
<td><code title>style-src</code>
<td>HTML's <code><link rel=stylesheet></code>, CSS' <code>@import</code>
<tr>
<td>"<code title>track</code>"
<td>"<code title>media</code>"
<td><code title>media-src</code>
<td>HTML's <code><track></code>
<tr>
<td>"<code title>video</code>"
<td>"<code title>media</code>"
<td><code title>media-src</code>
<td>HTML's <code><video></code> element
<tr>
<td>"<code title>download</code>"
<td>""
<td>""
<td>?
<td>HTML's <code>download=""</code>, "Save Link As…" UI
<tr>
<td>"<code title>imageset</code>"
<td>"<code title>image</code>"
<td>"<code title>image</code>"
<td><code title>img-src</code>
<td>HTML's <code><img srcset></code> and <code><picture></code>
<tr>
<td>"<code title>manifest</code>"
<td rowspan=2>""
<td>"<code title>manifest</code>"
<td><code title>manifest-src</code>
<td>HTML's <code><link rel=manifest></code>
<tr>
<td>"<code title>xslt</code>"
<td>"<code title>xslt</code>"
<td><code title>script-src</code>
<td><code><?xml-stylesheet></code>
</table>
<p>CSP's <code>form-action</code> needs to be a hook directly in HTML's navigate or form
submission algorithm.
<p>CSP will also need to check
<span title=concept-request>request</span>'s
<span title=concept-request-client>client</span>'s
<span data-anolis-spec=html>browsing context</span>'s
<span data-anolis-spec=html title="ancestor browsing context">ancestor browsing contexts</span>
for various CSP directives.
</div>
<hr>
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-priority>priority</dfn> (null or a user-agent-defined object).
Unless otherwise stated it is null.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-origin>origin</dfn>, which is "<code>client</code>" or an
<span data-anolis-spec=html>origin</span>. Unless stated otherwise it is "<code>client</code>".
<p class="note no-backref">"<code>client</code>" is changed to an
<span data-anolis-spec=html>origin</span> during <span title=concept-fetch>fetching</span>. It
provides a convenient way for standards to not have to set
<span title=concept-request>request</span>'s <span title=concept-request-origin>origin</span>.
<span title=concept-request>Request</span>'s <span title=concept-request-origin>origin</span> can be
changed during redirects too.
<p>A <span title=concept-request>request</span> has an associated
<dfn>omit-<code>Origin</code>-header flag</dfn>. Unless stated otherwise it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn>same-origin data-URL flag</dfn>. Unless stated otherwise it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-referrer>referrer</dfn>, which is "<code>no-referrer</code>",
"<code>client</code>", or a <span data-anolis-spec=url title=concept-url>URL</span>.
Unless stated otherwise it is "<code>client</code>".
<p class="note no-backref">"<code>client</code>" is changed to "<code>no-referrer</code>" or a
<span data-anolis-spec=url title=concept-url>URL</span> during
<span title=concept-fetch>fetching</span>. It provides a convenient way for standards to not have to
set <span title=concept-request>request</span>'s
<span title=concept-request-referrer>referrer</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-referrer-policy>referrer policy</dfn>, which is a
<span title=concept-referrer-policy>referrer policy</span>. Unless stated otherwise it is
the empty string.
<p class="note no-backref">This can be used to override a referrer policy associated with
an <span data-anolis-spec=html>environment settings object</span>.
<span data-anolis-ref>REFERRER</span>
<p>A <span title=concept-request>request</span> has an associated
<dfn>synchronous flag</dfn>. Unless stated otherwise it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-mode>mode</dfn>, which is "<code title>same-origin</code>",
"<code title>cors</code>", "<code title>no-cors</code>", "<code title>navigate</code>", or
"<code title>websocket</code>". Unless stated otherwise, it is "<code title>no-cors</code>".
<p class="note no-backref">Even though the default <span title=concept-request>request</span>
<span title=concept-request-mode>mode</span> is "<code title>no-cors</code>", standards are highly
discouraged from using it for new features. It is rather unsafe. "<code title>navigate</code>" and
"<code title>websocket</code>" are special values for the HTML Standard.
<span data-anolis-ref>HTML</span>
<p>A <span title=concept-request>request</span> has an associated
<dfn>use-CORS-preflight flag</dfn>. Unless stated otherwise, it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-credentials-mode>credentials mode</dfn>, which is
"<code title>omit</code>", "<code title>same-origin</code>", or
"<code title>include</code>". Unless stated otherwise, it is "<code title>omit</code>".
<p class="note no-backref">When <span title=concept-request>request</span>'s
<span title=concept-request-mode>mode</span> is "<code title>navigate</code>", its
<span title=concept-request-credentials-mode>credentials mode</span> is assumed to be
"<code title>include</code>" and <span title=concept-fetch>fetch</span> does not currently account
for other values. If <cite>HTML</cite> changes here, this standard will need corresponding changes.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-use-url-credentials-flag>use-URL-credentials flag</dfn>.
Unless stated otherwise, it is unset.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-cache-mode>cache mode</dfn>, which is
"<code title>default</code>", "<code title>no-store</code>", "<code title>reload</code>",
"<code title>no-cache</code>", or "<code title>force-cache</code>". Unless stated otherwise, it is
"<code title>default</code>".
<div class="note no-backref">
<dl>
<dt>"<code title>default</code>"
<dd><span title=concept-fetch>Fetch</span> will inspect the HTTP cache on the way to the network.
If there is a fresh response it will be used. If there is a stale response a conditional request
will be created, and a normal request otherwise. It then updates the HTTP cache with the response.
<span data-anolis-ref>HTTP</span>
<dt>"<code title>no-store</code>"
<dd>Fetch behaves as if there is no HTTP cache at all.
<dt>"<code title>reload</code>"
<dd>Fetch behaves as if there is no HTTP cache on the way to the network. Ergo, it creates a
normal request and updates the HTTP cache with the response.
<dt>"<code title>no-cache</code>"
<dd>Fetch creates a conditional request if there is a response in the HTTP cache and a normal
request otherwise. It then updates the HTTP cache with the response.
<dt>"<code title>force-cache</code>"
<dd>Fetch uses any response in the HTTP cache matching the request, not paying attention to
staleness. If there was no response, it creates a normal request updates the HTTP cache with the
response.
</dl>
<p>If <span title=concept-request-header-list>header list</span> contains a
<span title=concept-header>header</span> whose <span title=concept-header-name>name</span> is one
of
`<code title>If-Modified-Since</code>`,
`<code title>If-None-Match</code>`,
`<code title>If-Unmodified-Since</code>`,
`<code title>If-Match</code>`, and
`<code title>If-Range</code>`,
<span title=concept-fetch>fetch</span> will set
<span title=concept-request-cache-mode>cache mode</span> to "<code title>no-store</code>" if it is
"<code title>default</code>".
</div>
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-redirect-mode>redirect mode</dfn>, which is
"<code title>follow</code>", "<code title>error</code>", or "<code title>manual</code>".
Unless stated otherwise, it is "<code title>follow</code>".
<p>A <span title=concept-request>request</span> has associated
<dfn title=concept-request-integrity-metadata>integrity metadata</dfn> (a string). Unless
stated otherwise, it is the empty string.
<hr>
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-url-list>url list</dfn> (a list of one or more
<span title="fetch URL">fetch URLs</span>). Unless stated otherwise, it is a list
containing a copy of <span title=concept-request>request</span>'s
<span title=concept-request-url>url</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-current-url>current url</dfn>. It is a pointer to the last
<span>fetch URL</span> in <span title=concept-request>request</span>'s
<span title=concept-request-url-list>url list</span>.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-redirect-count>redirect count</dfn>. Unless stated otherwise,
it is zero.
<p>A <span title=concept-request>request</span> has an associated
<dfn title=concept-request-response-tainting>response tainting</dfn>, which is
"<code title>basic</code>", "<code title>cors</code>", or "<code title>opaque</code>".
Unless stated otherwise, it is "<code title>basic</code>".
<p>A <span title=concept-request>request</span> has an associated <dfn>done flag</dfn>.
Unless stated otherwise, it is unset.
<p class="note no-backref">A <span title=concept-request>request</span>'s
<span title=concept-request-url-list>url list</span>,
<span title=concept-request-current-url>current url</span>,
<span title=concept-request-redirect-count>redirect count</span>,
<span title=concept-request-response-tainting>response tainting</span>, and
<span>done flag</span> are used as bookkeeping details by the
<span title=concept-fetch>fetch</span> algorithm.
<hr>
<p>A <dfn>subresource request</dfn> is a <span title=concept-request>request</span>
whose <span title=concept-request-destination>destination</span> is "<code title>font</code>",
"<code title>image</code>", "<code title>manifest</code>", "<code title>media</code>",
"<code title>script</code>", "<code title>style</code>", "<code title>xslt</code>", or the empty
string.
<p>A <dfn>potential-navigation-or-subresource request</dfn> is a
<span title=concept-request>request</span> whose
<span title=concept-request-destination>destination</span> is
"<code title>object</code>" or "<code title>embed</code>".
<p>A <dfn>non-subresource request</dfn> is a <span title=concept-request>request</span>
whose <span title=concept-request-destination>destination</span> is "<code title>document</code>",
"<code title>report</code>", "<code title>serviceworker</code>", "<code title>sharedworker</code>",
or "<code title>worker</code>".
<p>A <dfn>navigation request</dfn> is a <span title=concept-request>request</span> whose
<span title=concept-request-destination>destination</span> is
"<code title>document</code>".
<p class=note>See <span data-anolis-spec=sw>handle fetch</span> for usage of these terms.
<span data-anolis-ref>SW</span>
<p>To <dfn title=concept-request-clone>clone</dfn> a <span title=concept-request>request</span>
<var>request</var>, run these steps:
<ol>
<li><p>Let <var>newRequest</var> be a copy of <var>request</var>, except for its
<span title=concept-request-body>body</span>.
<li><p>If <var>request</var>'s <span title=concept-request-body>body</span> is non-null, set
<var>newRequest</var>'s <span title=concept-request-body>body</span> to the result of
<span title=concept-body-clone>cloning</span> <var>request</var>'s
<span title=concept-request-body>body</span>. Rethrow any exceptions.
<li><p>Return <var>newRequest</var>.
</ol>
<h4>Responses</h4>
<p>The result of <span title=concept-fetch>fetch</span> is a
<dfn title=concept-response>response</dfn>. A <span title=concept-response>response</span>
evolves over time. That is, not all its fields are available straight away.
<p>A <span title=concept-response>response</span> has an associated
<dfn title=concept-response-type>type</dfn> which is
"<code title>basic</code>",
"<code title>cors</code>",
"<code title>default</code>",
"<code title>error</code>",
"<code title>opaque</code>", or
"<code title>opaqueredirect</code>".
Unless stated otherwise, it is "<code title>default</code>".
<p>A <span title=concept-response>response</span> can have an associated
<dfn title=concept-response-termination-reason>termination reason</dfn> which is
<i title>end-user abort</i>, <i title>fatal</i>, or <i title>timeout</i>.
<p>A <span title=concept-response>response</span> has an associated
<dfn title=concept-response-url>url</dfn>. It is a pointer to the last
<span>fetch URL</span> in <span title=concept-response>response</span>'s
<span title=concept-response-url-list>url list</span> and null if
<span title=concept-response>response</span>'s
<span title=concept-response-url-list>url list</span> is the empty list.
<p>A <span title=concept-response>response</span> has an associated
<dfn title=concept-response-url-list>url list</dfn> (a list of zero or more
<span title="fetch URL">fetch URLs</span>). Unless stated otherwise, it is the empty list.