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.

Getting started

# via yarn
yarn add resolve-url-loader --dev

# via npm
npm install resolve-url-loader --save-dev

Usage

Plain CSS works fine:

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

or using sass-loader:

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

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

IMPORTANT

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 incoming source-map then it must imply source information at each CSS url() statement.

Don't omit -loader

Your Webpack.config.js should always use the long-form of the loader name (i.e. the -loader suffix).

There is another package called resolve-url which Webpack can confuse with resolve-url-loader.

There are other common examples. Such as jshint and jshint-loader packages being confused.

These conflicts are very hard to debug and will send you crazy. Your Webpack.config.js should always use the long-form of the loader name (i.e. the -loader suffix)

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.

• sourceMap Generate a source-map.

• attempts Limit searching for any files not where they are expected to be. This is unlimited by default so you will want to set it 1 or some small value.

• 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').

• debug Show verbose information on the file paths being searched.

• 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.

There are some additional hacks available without support. Only do this if you know what you are doing.

• absolute Forces the url() to be resolved to an absolute path. This is considered bad practice.

• includeRoot (experimental, non-performant) Include the project root in file search. The root option need not be specified but includeRoot is only really useful if your root directory is shallower than your build working directory.

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 original source is determined by consulting the incoming source-map at the point of the url() statement.

Usually the asset is found relative to the original source file O(1).

However in cases where there is no immediate match, we start searching both deeper and shallower from the starting directory O(n). Note that n may be limited by the attempts option.

This file search "magic" is mainly for historic reasons, to work around broken packages whose assets are not where we would expect.

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.

Use the debug option to see exactly what paths are being searched.

Limitations / Known-issues

File search "magic"

Failure to find an asset will trigger a file search of your project.

This feature was for historic reasons, to work around broken packages, whose assets are not where we would expect. Such problems are rare now and many users may not be aware of the search feature.

We now have the attempts option to limit this feature. However by default it is unlimited (attempts=0) which could make your build non-performant.

You should explicitly set attempts=1 and increase the value only if needed. We will look to make this the default in the next major release.

Mixins

Where url() statements are created in a mixin the source file may then be the mixin file, and not the file calling the mixin. Obviously this is not the desired behaviour.

The incoming source map can vary greatly with different transpilers and their mixins. Use a source map visualiser to see more. If the source-map shows the correct original file and the mixin still doesn't work then raise an issue and point to the visualisation.

Ultimately you will need to work around this. Try to avoid the mixin. Worst case you can try the includeRoot option to force a search of your project sources.

Compatiblity

Webpack

This loader was written for Webpack 1 and has been tweaked to also support with Webpack 2.

If you find any Webpack 2 problems please comment on any similar existing issue or raise a new one.

Node-sass

IMPORTANT

Avoid the combination of Webpack 1 with node-sass@^4.0.0.

Use Webpack 2 if you need latest node-sass

Since node-sass@>=4.0.0 source-maps have sometimes featured negative column values. Since this loader relies on source-maps this can cause a fatal error.

I don't have a lot of data on this. If you are stuck in Webpack 1 and find that this combination actually works ok for you please let me know.

Getting help

Webpack is difficult to configure but extremely rewarding.

I am happy for you to raise an issue to ask a question regarding this package. However ensure you follow the check-list first.

Currently I am not dogfooding this loader in my own work. I may rely on you being able to isolate the problem in a simple example project and to help debug.

I am happy this loader helps so many people. Open-source is provided as-is so please try not project your frustrations. There are some really great people who follow this project who can help.

Issues

Before raising a new issue:

• remove this loader and make sure it is not a problem with a different loader in your config (most often the case)
• check stack overflow for an answer
• review previous issues that may be similar
• be prepared to create a simple open-source project that exhibits your problem, should the solution not be immediately obvious to us
• be prepared to use a source map visualisation to check the transpiler has correct source maps coming out
• (ideally) debug some code and let me know where the problem sits

Pull requests

I am happy to take pull requests, however:

• Ensure your change is backwards compatible - not all users will be using the same version of Webpack or SASS that you do.
• Follow the existing code style.
• Uncomon use-cases/fixes should be opt-in per a new option.
• Do not overwrite existing variables with new values. I would prefer your change variable names elsewhere if necessary.
• Add comments that describe why the code is necessary - i.e. what edge case are we solving. Otherwise we may rewrite later and break your use-case.