resolve-url-loader

What is resolve-url-loader?

The resolve-url-loader package is a webpack loader that resolves relative paths in url() statements based on the original source file. This is particularly useful when dealing with source maps and pre-processors like Sass, as it allows assets referenced in CSS to be correctly found and bundled by webpack.

What are resolve-url-loader's main functionalities?

Resolving relative URLs

This feature allows resolve-url-loader to adjust relative paths in url() statements so that they point to the correct location in a webpack build. The code sample shows how to include resolve-url-loader in a webpack configuration.

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          'style-loader',
          'css-loader',
          'resolve-url-loader'
        ]
      }
    ]
  }
};

Source map support

resolve-url-loader can handle source maps, which is essential for debugging processed stylesheets like those written in Sass. The code sample demonstrates how to enable source map support in webpack loaders, including resolve-url-loader.

module.exports = {
  module: {
    rules: [
      {
        test: /\.scss$/,
        use: [
          'style-loader',
          {
            loader: 'css-loader',
            options: { sourceMap: true }
          },
          {
            loader: 'resolve-url-loader',
            options: { sourceMap: true }
          },
          {
            loader: 'sass-loader',
            options: { sourceMap: true }
          }
        ]
      }
    ]
  }
};

Other packages similar to resolve-url-loader

file-loader

file-loader resolves import/require() on a file into a url and emits the file into the output directory. It's similar to resolve-url-loader in that it helps with asset management, but it doesn't resolve relative URLs based on the original source file.

url-loader

url-loader works like file-loader but can return a DataURL if the file is smaller than a byte limit. It's similar to resolve-url-loader as it deals with URLs in CSS and other files, but it doesn't specifically address the resolution of relative paths.

README

Resolve URL Loader

Webpack loader that resolves relative paths in url() statements based on the original source file.

Use in conjunction with the sass-loader and specify your asset url() relative to the .scss file in question.

This loader will use the source-map from the SASS compiler to locate the original .scss source file and write a more Webpack-friendly path for your asset. The CSS loader can then locate your asset for individual processing.

Usage

Plain CSS works fine:

var css = require('!css!resolve-url!./file.css');

or using sass-loader:

var css = require('!css!resolve-url!sass?sourceMap!./file.scss');

Use in tandem with the style-loader to compile sass and to add the css rules to your document:

require('!style!css!resolve-url!./file.css');

and

require('!style!css!resolve-url!sass?sourceMap!./file.scss');

Source maps required

Note that source maps must be enabled on any preceding loader. In the above example we use sass?sourceMap.

In some use cases (no preceding transpiler) there will be no incoming source map. Therefore we do not warn if the source-map is missing.

However if there is an incomming source-map then it must imply source information at each CSS url() statement.

Apply via webpack config

It is preferable to adjust your webpack.config so to avoid having to prefix every require() statement:

module.exports = {
  module: {
    loaders: [
      {
        test   : /\.css$/,
        loaders: ['style', 'css', 'resolve-url']
      }, {
        test   : /\.scss$/,
        loaders: ['style', 'css', 'resolve-url', 'sass?sourceMap']
      }
    ]
  }
};

Options

Options may be set using query parameters or by using programmatic parameters. Programmatic means the following in your webpack.config.

module.exports = {
   resolveUrlLoader: {
      ...
   }
}

Where ... is a hash of any of the following options.

• absolute Forces the url() to be resolved to an absolute path. This is considered bad practice so only do it if you know what you are doing.

• sourceMap Generate a source-map.

• silent Do not display warnings on CSS syntax or source-map error.

• fail Syntax or source-map errors will result in an error.

• keepQuery Keep query string and hash within url. I.e. url('./MyFont.eot?#iefix'), url('./MyFont.svg#oldiosfix').

• root An optional directory within which search may be performed. Relative paths are permitted. Where omitted process.cwd() is used and should be sufficient for most use cases.

Note that query parameters take precedence over programmatic parameters.

How it works

A rework process is run on incoming CSS.

Each url() statement that implies an asset triggers a file search using node fs operations. The asset should be relative to the original source file that was transpiled. This file is determined by consulting the incomming source-map at the point of the url() statement.

Usually the asset is found relative to the original source file. However in some cases there is no immediate match (cough bootstrap cough) and we so we start searching both deeper and shallower from the starting directory.

Shallower paths must be limited to avoid the whole file system from being considered. Progressively shallower paths within the root will be considered. Paths featuring a package.json or bower.json file will not be considered.

If the asset is not found then the url() statement will not be updated with a Webpack module-relative path. However if the url() statement has no source-map source information the loader will fail.

The loader will also fail when input source-map sources cannot all be resolved relative to some consistent path within root.