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Resolve URL Loader

A webpack loader that rewrites relative paths in url() statements based on the original source file.

Why?

TL;DR Making Sass work with a feature based project structure

With webpack you can import a .scss file (or some other compile-to-css file) and have a loader take care of the transpilation. With Sass (at least) this file can include a whole tree of source files into a single output.

We can imagine a virtual .css file at the location the original .scss import. Webpack expects any assets found in this CSS to be relative to the original imported file.

For projects with a feature based structure this will be a problem, since you will want to co-locate your assets with your .scss partials.

Example - webpack imports index.scss which includes feature foo.

files	content
src /
index.scss	`@import features/foo`
features /
_foo.scss	`url(bar.png)`
bar.png

Intuatively we want the assets in partial _foo.scss relative to the partial, meaning url(bar.png).

However webpack's css-loader will encounter url(bar.png) and expect to find src/bar.png. This is not the correct location and the build will fail.

Thankfully resolve-url-loader provides the "url rewriting" that Sass is missing. Use it after the transpiler (such as sass-loader). It makes use of the source-map to find the original source file and rewrite url() statements.

In our example it rewrites url(bar.png) to url(features/bar.png) as required.

Version 3

Features

Use postcss parser by default. This is long overdue as the old rework parser doesn't cope with modern css.
Lots of automated tests running actual webpack builds. If you have an interesting use-case let me know.

Breaking Changes

Multiple options changed or deprecated.
Removed file search "magic" in favour of join option.
Errors always fail and are no longer swallowed.
Processing absolute asset paths requires root option to be set.

Migrating

Initially set option engine: 'rework' for parity with your existing build. Once working you can remove this option or set engine: 'postcss' explicitly.

Retain keepQuery option if you are already using it.

The root option now has a different meaning. Previously it limited file search. Now it is the base path for absolute or root-relative URIs, consistent with css-loader. If you are already using it you can probably remove it.

If you build on Windows platform and your content contains absolute asset paths, then css-loader could fail. The root option here may fix the URIs before they get to css-loader. Try to leave it unspecified, otherwise (windows only) set to empty string root: ''.

Getting started

Install

via npm

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

via yarn

yarn add resolve-url-loader --dev

Configure Webpack

The typical use case is resolve-url-loader between sass-loader and css-loader.

⚠️ IMPORTANT

source-maps required for loaders preceding resolve-url-loader (regardless of devtool).
Always use full loader package name (don't omit -loader) otherwise you can get errors that are hard to debug.

rules: [
  {
    test: /\.scss$/,
    use: [
      ...
      {
        loader: 'css-loader',
        options: {...}
      }, {
        loader: 'resolve-url-loader',
        options: {...}
      }, {
        loader: 'sass-loader',
        options: {
          sourceMap: true,
          sourceMapContents: false
        }
      }
    ]
  },
  ...
]

Refer to test directory for full webpack configurations (as used in automated tests).

Options

option	type	default		description
`engine`	`'rework'` `'postcss'`	`'postcss'`		The css parser engine.
`sourceMap`	boolean	`false`		Generate a source-map.
`keepQuery`	boolean	`false`		Keep query-string and/or hash suffixes. e.g. `url('./MyFont.eot?#iefix')` Be aware downstream loaders may remove query-string or hash.
`removeCR`	boolean	`false`		Convert orphan CR to whitespace (postcss only). See known issues below.
`debug`	boolean	`false`		Display debug information.
`silent`	boolean	`false`		Do not display warnings.
`root`	string	unset		Similar to the (now defunct) option in `css-loader`. This string, possibly empty, is prepended to absolute URIs. Absolute URIs are only processed if this option is set.
`join`	function	inbuilt	advanced	Custom join function. Use custom javascript to fix asset paths on a per-case basis. Refer to the default implementation for more information.
`absolute`	boolean	`false`	useless	Forces URIs to be output as absolute file paths. This is retained for historical compatibility but is likely to be removed in the future, so let me know if you use it.

How it works

A rework or postcss process is run on incoming CSS.

Each url() statement may imply an asset or may not. Generally only relative URIs are considered. However if root is specified then absolute or root-relative URIs are considered.

For each URI considered, the incomming source-map is consulted to determine the original file where the url() was specified. This becomes the base argument to the join function, whose default implementation is something like the following pseudocode.

join(uri, base?) =>
  compose(path.normalize, path.join)(base || options.join, uri);

Note that for absolute uri then the base is absent. In the default implementation the root option is used instead.

Full file search has been discontinued in version 3, however it is possible to specify a custom join function.

There is the added complexity that base may be an iterator. However resolve-url-loader exports some useful functions that makes a custom join easier.

Following join the URI has become an absolute path. Back-slashes are then converted to forward-slashes and the path is made relative to the initial resource being considered.

Use the debug option to see verbose information from the join function.

Limitations / Known-issues

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.

Ensure this is indeed the problem as there are many ways to misconfigure webpack. Try inlining the mixin and check that everything works correctly. However ultimately you will need to work around this.

Compatiblity

Tested macOS and Windows for node 6.x.

All webpack1-webpack4 with contemporaneous loaders/plugins.

Refer to test directory for full webpack configurations (as used in automated tests).

Some edge cases with libsass on Windows (see below).

Engines

The engine:postcss is by far the more reliable option.

The engine:rework option is retained for historical compatibility but is likely to be removed in the future, so let me know if you use it.

If you need production css source-map it is best to avoid the combination webpack4 with engine:rework. Tests show a systematic flaw in the outgoing source-map mappings.

Absolute URIs

By "absolute URIs" we more correctly mean assets with root-relative URLs or absolute file paths.

Absolute paths are not processed by default

These are not processed unless a root is specified.

However recall that any paths that are processed will have windows back-slash converted to posix forward-slash. This can be useful since some webpack loaders can choke on windows paths. By using root: '' then resolve-url-loader effectively does nothing to absolute paths except change the back-slash.

It can also be useful to process absolute URIs if you have a custom join function and want to process all paths. However this is perhaps better done with some separate postcss plugin.

Windows line breaks

Normal windows linebreaks are CRLF. But sometimes libsass will output single CR characters.

This problem is specific to multiline declarations. Refer to the libsass bug #2693.

If you have any such multiline declarations preceding url() statements it will fail your build.

Libsass doesn't consider these orphan CR to be newlines but postcss engine does. The result being an offset in source-map line-numbers which crashes resolve-url-loader.

Module build failed: Error: resolve-url-loader: CSS error
  source-map information is not available at url() declaration

Some users find the node-sass linefeed option solves the problem.

Solutions

Try the node-sass linefeed option by way of sass-loader.

Work arounds

Enable removeCR option here.
Remove linebreaks in declarations.

Diagnosis

Run a stand-alone sass build npx node-sass index.scss output.css
Use a hex editor to check line endings Format-Hex output.css
Expect 0DOA (or desired) line endings. Single 0D confirms this problem.

Getting help

Webpack is difficult to configure but extremely rewarding.

Remove this loader and make sure it is not a problem with a different loader in your config (most often the case)

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.
(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. I know it is old but it maintains backwards compatibility.
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.

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