URI starting with two slashes ... how do they behave?

Question

Lately I saw working code-blocks like this:

<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>

And according to RFC 2396 (URI Syntax) and RFC 2616 (HTTP 1.1) these URI starting with two slashes are valid, but unfortunately the RFCs don't really explain them.

Can anyone point me to a resource which explains how browsers will/should/do process these URIs?

score 88 · Accepted Answer · answered Nov 01 '10 at 17:03

88

The resource you're looking for is the RFC 3986.

See Section 4.2 and Section 5.4. Quoting from the latter:

Reference Resolution Examples

Within a representation with a well defined base URI of:

    http://a/b/c/d;p?q

a relative reference is transformed to its target URI as follows:

  "g:h"           =  "g:h"
  "g"             =  "http://a/b/c/g"
  "./g"           =  "http://a/b/c/g"
  "g/"            =  "http://a/b/c/g/"
  "/g"            =  "http://a/g"
  "//g"           =  "http://g"
  "?y"            =  "http://a/b/c/d;p?y"
  "g?y"           =  "http://a/b/c/g?y"
  "#s"            =  "http://a/b/c/d;p?q#s"
  "g#s"           =  "http://a/b/c/g#s"
  "g?y#s"         =  "http://a/b/c/g?y#s"
  ";x"            =  "http://a/b/c/;x"
  "g;x"           =  "http://a/b/c/g;x"
  "g;x?y#s"       =  "http://a/b/c/g;x?y#s"
  ""              =  "http://a/b/c/d;p?q"
  "."             =  "http://a/b/c/"
  "./"            =  "http://a/b/c/"
  ".."            =  "http://a/b/"
  "../"           =  "http://a/b/"
  "../g"          =  "http://a/b/g"
  "../.."         =  "http://a/"
  "../../"        =  "http://a/"
  "../../g"       =  "http://a/g"

This means that when the base URI is http://a/b/c/d;p?q and you use //g, the relative reference is transformed to http://g.

answered Nov 01 '10 at 17:03

Daniel Vassallo

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so can this be a solution to using javascript to determine http or https this way whatever it is it will work – Ibu May 14 '11 at 08:54
Since you need to reach the parent level, use `../g` — it will use the current protocol and lead to `http://a/b/c/g`. – Boris S Nov 16 '12 at 10:04
It may be a novice question, but where does this base URI come from? the browser url bar? the server? a tag in the html page? – coderatchet Apr 03 '14 at 05:58
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@thenaglecode: the answer is, it depends. In XML/XHTML/HTML5 you can use `xml:base` to [set it explicitly on any element](http://annevankesteren.nl/2005/08/base-examples). By default in HTML the base URI is the same URI of the page (but inside a CSS file, the base URI is [relative to the CSS uri, not the containing HTML](http://stackoverflow.com/questions/940451), but older IE made it relative to the HTML). In other protocols and languages, it can be different (in XSLT, it is dependent on the current item, for instance). See also [``](http://www.w3.org/TR/html-markup/base.html). – Abel Apr 05 '14 at 12:09
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Important - the result is not always `http`! It depends on the protocol of the page context. If the context was loaded under `http://a/b/c/d;p?q`, then `//g` indeed resolves to `http://g`. But if the page was loaded as `http://a/b/c/d;p?q` then the result will be `https://g`. And also do not forget about other protocols like `file://` (page is loaded from disk) - which will definitely give you a lot of headache. – Andrey Tserkus Oct 14 '16 at 01:33

score 64 · Answer 2 · edited Oct 07 '21 at 05:46

64

These are protocol relative URLs. They point to an address, keeping the current protocol.

This notation is often used to avoid the "mixed content" problem (a IE warning message complaining about http and https resources on the same HTTPS page).

Update: Official documentation in RFC 3986:

A relative reference that begins with two slash characters is termed a network-path reference; such references are rarely used. A relative reference that begins with a single slash character is termed an absolute-path reference. A relative reference that does not begin with a slash character is termed a relative-path reference.

edited Oct 07 '21 at 05:46

Community

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answered Nov 01 '10 at 16:58

Pekka

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Great to know, but how compliant is this with popular browsers. A quick search told me it doesn't work with IE6...is this an HTML5 feature?? – Shane N Mar 03 '11 at 21:49
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@Shane this should work in all browsers. Do you have a link claiming it doesn't work in IE6? – Pekka Mar 03 '11 at 21:51
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Pretty sure that one's an IE1 feature! – Jon Hanna Sep 13 '12 at 02:14
plus for mentioning protocol relative – Tawfik Khalifeh Dec 05 '14 at 09:29

score 30 · Answer 3 · edited May 23 '17 at 10:30

30

They are protocol independent urls. If the web page is served on https then the request uses https, if http then http.

Paul Irish seems to have popularized them by including it in his boilerplate code.

edited May 23 '17 at 10:30

Community

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answered Nov 01 '10 at 16:58

meder omuraliev

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score 2 · Answer 4 · answered Mar 08 '17 at 13:01

Be aware of that it is not only http or https independent, but also file, ftp, etc.

It means if you open .htm file directly in your browser on localhost, browser will resolve // as file protocol and your page won't work. It may cause problems in packed websites as "native" app using tools like Electron, PhoneGap, etc.

Example:

<script src="//mywebsite.com/resource.js"></script>

to

<script src="file://mywebsite.com/resource.js"></script>

URI starting with two slashes ... how do they behave?

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