Transform for Lasso.js to replace URLs in CSS files with URLs of bundled resources

CSS files (which might be derived from LESS or SASS resources) often contain references to other assets.

For example:

.app {
    background-image: url(myfile.png);
}

When CSS URL resolving is enabled, image assets referenced in CSS files will automatically be copied to the output directory and the URL reference will be replaced with the resultant URL. In the example above, the file myfile.png will be moved to the output directory and the URL in the CSS file will be adjusted accordingly.

Resource URLs that begin with data:, //, https://, and https:// are ignored during URL resolving.

In the typical use case, relative URLs are resolved relative to the source file. However, it is also possible to resolve URLs that are paths using rules of require.resolve().

var config = {
    resolveCssUrls: true
    ...
};
var myLasso = lasso.create(config);
myLasso.lassoPage(...);

var config = {
    resolveCssUrls: {
      urlResolver: function(url, lassoContext, callback) {
        url = url.replace('SOME_TOKEN', 'something else');
        callback(null, url);
      }
    }
    ...
};
var myLasso = lasso.create(config);
myLasso.lassoPage(...);

Consider this CSS snippet:

.app {
    background-image: url(require:assets-module/images/myfile.png);
}

In this example, the actual path to assets-module/images/myfile.png will be resolved using the rules of require.resolve(). The path will resolved relative to the source file. Therefore, if the target is relative (e.g. ./myfile.png), then the target will be resolved relative to the source file.

Consider this CSS snippet:

.app {
    background-image: url(myfile.png?base64);
}

The special "?base64" suffix will trigger the resolver to automatically encode the image content using Base64 which will inline the data.