Difference between application/x-javascript and text/javascript content types

Question

text/javascript is obsolete, and application/x-javascript was experimental (hence the x- prefix) for a transitional period until application/javascript could be standardised.

You should use application/javascript. This is documented in the RFC.

As far a browsers are concerned, there is no difference (at least in HTTP headers). This was just a change so that the text/* and application/* MIME type groups had a consistent meaning where possible. (text/* MIME types are intended for human readable content, JavaScript is not designed to directly convey meaning to humans).

Note that using application/javascript in the type attribute of a script element will cause the script to be ignored (as being in an unknown language) in some older browsers. Either continue to use text/javascript there or omit the attribute entirely (which is permitted in HTML 5).

This isn't a problem in HTTP headers as browsers universally (as far as I'm aware) either ignore the HTTP content-type of scripts entirely, or are modern enough to recognise application/javascript.

mime-types starting with x- are not standardized. In case of javascript it's kind of outdated. Additional the second code snippet

<?Header('Content-Type: text/javascript');?>

requires short_open_tags to be enabled. you should avoid it.

<?php Header('Content-Type: text/javascript');?>

However, the completely correct mime-type for javascript is

application/javascript

http://www.iana.org/assignments/media-types/application/index.html

According to RFC 4329 the correct MIME type for JavaScript should be application/javascript. Howerver, older IE versions choke on this since they expect text/javascript.

Use type="application/javascript"

In case of HTML5, the type attribute is obsolete, you may remove it. Note: that it defaults to "text/javascript" according to w3.org, so I would suggest to add the "application/javascript" instead of removing it.

http://www.w3.org/TR/html5/scripting-1.html#attr-script-type
The type attribute gives the language of the script or format of the data. If the attribute is present, its value must be a valid MIME type. The charset parameter must not be specified. The default, which is used if the attribute is absent, is "text/javascript".

Use "application/javascript", because "text/javascript" is obsolete:

RFC 4329: http://www.rfc-editor.org/rfc/rfc4329.txt

3. Deployed Scripting Media Types and Compatibility

   Various unregistered media types have been used in an ad-hoc fashion to label and exchange programs written in ECMAScript and JavaScript. These include:

   +-----------------------------------------------------+ | text/javascript | text/ecmascript | | text/javascript1.0 | text/javascript1.1 | | text/javascript1.2 | text/javascript1.3 | | text/javascript1.4 | text/javascript1.5 | | text/jscript | text/livescript | | text/x-javascript | text/x-ecmascript | | application/x-javascript | application/x-ecmascript | | application/javascript | application/ecmascript | +-----------------------------------------------------+

Use of the "text" top-level type for this kind of content is known to be problematic. This document thus defines text/javascript and text/
ecmascript but marks them as "obsolete". Use of experimental and
unregistered media types, as listed in part above, is discouraged.
The media types,

  * application/javascript
  * application/ecmascript

which are also defined in this document, are intended for common use and should be used instead.

This document defines equivalent processing requirements for the
types text/javascript, text/ecmascript, and application/javascript.
Use of and support for the media type application/ecmascript is
considerably less widespread than for other media types defined in
this document. Using that to its advantage, this document defines
stricter processing rules for this type to foster more interoperable
processing.

x-javascript is experimental, don't use it.