javascript and text/javascript content types
What is the difference between these headers?
Content-Type: application/javascript
Content-Type: application/x-javascript
Content-Type: text/javascript
Which one is best and why?
Please do not say they are identical - if they were identical there would not have been two of them. I know both work - but I would like to know the difference.
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 .