Using // in a <script>'s source

Question

I was wondering if anyone had any resources, proof, or personal experience in using the age-old http/https JavaScript <script> hack:

<script src="//someserver.com/js/script.js"></script>

Has anyone encountered issues in any of these browsers (IE 5.5+, FF2+, Chrome, Opera 9+, Safari 3+)? Has anybody had success stories?

What does this hack do? Uses http/https accordingly with the referrer url? Why don't you just use /, instead of //? — Spidey
– Spidey, Commented Mar 16, 2010 at 23:07
@Spidey: Actually, it is not a hack. It is a perfectly valid relative URI syntax. This is very useful when switching between http and https, because there would be no need to specify the protocol. Using just / wouldn't work in this case, because the OP is requesting the script from a separate domain. — Daniel Vassallo
– Daniel Vassallo, Commented Mar 16, 2010 at 23:12
you're right, Darryl, it is a duplicate. thank you for pointing out where everybody else got the answer, :P (just kidding Daniel / wilmoore). — Dan Beam
– Dan Beam, Commented Mar 17, 2010 at 3:51
By the way, there are some downsides to this - in older browser (IE7-8, I believe) you'll double download some resources due to a bug in IE's network cache (it treats //domain.tld/foo.file differently than domain.tld/foo.file). stevesouders.com/blog/2010/02/10/… — Dan Beam
– Dan Beam, Commented Oct 13, 2011 at 2:46

Daniel Vassallo · Accepted Answer · 2010-03-17 09:48:02Z

24

All modern browsers will understand that format, including IE 6. (Not sure about IE 5.5).

Actually, this is not a hack, but a perfectly valid URI syntax as per RFC 3986: Section 4.2. Therefore, I say you're good to go.

edited Mar 17, 2010 at 9:48

answered Mar 16, 2010 at 23:01

Daniel Vassallo

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Billy ONeal Over a year ago

You're calling IE6 a modern browser? LOL! +1 EDIT: Damn, out of votes :(

Wil Moore III · Accepted Answer · 2011-04-01 21:25:17Z

I can point you to exactly what you are looking for. It is an RFC document so you have to sift through a lot of noise to get to what you want but this is a legit feature (not a hack) of supposed http clients.

       b) If the embedded URL starts with a scheme name, it is
          interpreted as an absolute URL and we are done.

       c) Otherwise, the embedded URL inherits the scheme of
          the base URL.

Read more: http://www.faqs.org/rfcs/rfc1808.html (search for the heading "Resolving Relative URLs" and see steps 1 and 2 below) or here: http://freesoft.org/CIE/RFC/1808/18.htm

As an FYI, I use this in pretty much all of my production projects -- not just for JS resources, but for links to other resources such as images and CSS (UPDATED: I no longer use this for linking stylesheets).

Works pretty much everywhere. I've tried this in IE, FF, Opera, Chrome, Safari/Webkit all going back multiple previous versions (where applicable).

Examples:

< img src="//static.example.com/img/token.png" />
< script type="text/javascript" src="//static.example.com/js/jquery.js">

I find this method to be cleaner than writing code to figure out if we are on http/https.

The only caveat is that you should not use this for stylesheets.

While the following is legal and works:

< link rel="stylesheet" type="text/css" href="//static.example.com/css/screen.css" media="screen" />

In IE, the above will cause two HTTP requests. Currently, this affects IE7, IE8, and early versions of IE9.

In other words, scheme relative URIs should/can be used for all resources except stylesheets.

Community · Accepted Answer · 2017-05-23 12:03:29Z

2

I have been using this schema since I asked this question and I haven't had any problems. I've seen it work in every browser, including IE5.5. (Most of the stuff I work on requires JavaScript and some of the JS is included with this method.)

edited May 23, 2017 at 12:03

CommunityBot

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answered Mar 16, 2010 at 23:40

Darryl Hein

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Comments

Shermozle · Accepted Answer · 2010-03-26 05:13:32Z

1

I think perhaps the reason people get confused about this is that Google Analytics' standard code inclusion does some complicated stuff with the hostname based on the protocol. However I suspect this is due to the fact that their SSL hostname is different to the non-SSL hostname, for some network reason I imagine.

answered Mar 26, 2010 at 5:13

Shermozle

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