Updates relative asset URLs (e.g. url(../images/something.png)) within a string of CSS to reflect a relocation of the CSS file.

Example use case: if you were inlining CSS from styles/main.css into a <style> element in index.html (as part of a build task aiming to reduce HTTP round trips), you could use this function to rewrite all the image/font URLs within the CSS so they still work from the new relative base – in this example, it would prepend all file-relative URLs with style/ (and resolve any .. etc).

npm install reorient-css

var reorientCSS = require('reorient-css');

var result = reorientCSS(
  'body { background: url(something.png); }', // css string
  'some/old/stylesheet/file.css',             // old css location
  'some/new/one.css'                          // new css location (this could even be an .html file,
);                                            // if you're inlining the CSS into a <style> element
                                              // - the basename is ignored anyway)

console.log(result.css);
// > body { background: url('../old/stylesheet/something.png'); }

• It doesn't care whether files exist at the old/new paths – it just uses these path strings to determine the relative path from the new location back to the old location, so it can know how to rewrite the asset URLs.
• Only file-relative URLs are rewritten. Root-relative, absolute and data: URLs will be left untouched, as these are expected to work the same regardless of the CSS file's location.
• behavior properties (a proprietary IE thing) will be left untouched, because any URLs in these properties are relative to the HTML document, not the CSS file, so in theory they should never change.
  • The only exception is if you are moving CSS from one HTML document to another (ie, in <style> elements). If both the new and old CSS locations you provide are .html files, behavior properties will be reoriented.
• Whitespace and formatting are left untouched.
• The output is an object with a property named css, which contains the reoriented CSS string. This object might also contain a map property, if you have specified that option (see below).

The function accepts an optional fourth argument: an object of extra options to pass to PostCSS. For example, {map: true} will tell it to generate a source map (which will be available in the map key of the result object). See the PostCSS docs for more details.

reorient-css can also be used as a PostCSS processor.

var postcss = require('postcss'),
	reorientCSS = require('reorient-css');

var result = postcss()
	.use( reorientCSS.processor( 'some/old/stylesheet/file.css', 'some/new/one.css' ) )
	.process( 'body { background: url(something.png); }' );

console.log(result.css);
// > body { background: url('../old/stylesheet/something.png'); }

Copyright (c) 2014 Callum Locke. Licensed under the MIT license.