Advanced Features

All the advanced features of this loader involve customising the join option.

Jump to the “how to” section - * How to: change precedence of source locations * How to: fallback to a theme or other global directory * How to: fallback to some other asset file * How to: perform a file-system search for an asset

What is the “join” function?

The “join” function determines how CSS URIs are combined with one of the possible base paths the algorithm has identified.

⚠️ IMPORTANT - First read how the algorithm works.

The “join” function is a higher-order function created using the options and loader reference. That gives a function that accepts a single item and synchronously returns an absolute asset path to substitute back into the original CSS.

javascript (options:{}, loader:{}) => (item:{ uri:string, query: string, isAbsolute: boolean, bases:{} }) => string | null

Where the bases are absolute directory paths { subString, value, property, selector } per the algorithm. Note that returning null implies no substitution, the original relative uri is retained.

The job of the “join” function is to consider possible locations for the asset based on the bases and determine which is most appropriate. This implies some order of precedence in these locations and some file-system operation to determine if the asset there.

The default implementation is suitable for most users but can be customised per the join option.

A custom join function from scratch is possible but we’ve provided some building blocks to make the task easier.

Building blocks

There are a number of utilities (defined in lib/join-function/index.js) to help construct a custom “join” function . These are conveniently re-exported as properties of the loader.

These utilities are used to create the defaultJoin as follows.

```javascript const { createJoinFunction, createJoinImplementation, defaultJoinGenerator, } = require(‘resolve-url-loader’);

// create a join function equivalent to “defaultJoin” const myJoinFn = createJoinFunction( ‘myJoinFn’, createJoinImplementation(defaultJoinGenerator), }); ```

🤓 If you have some very specific behaviour in mind you can specify your own implementation. This gives full control but still gives you debug logging for free.

javascript createJoinFunction = (name:string, implementation: function): function

For each item, the implementation needs to make multiple attempts at locating the asset. It has mixed concerns of itentifying locations to search and then evaluating those locates one by one.

👉 However its recommended to instead use createJoinImplementation to create the implementation using the generator concept.

javascript createJoinImplementation = (generator: function*): function

The generator has the single concern of identifying locations to search. The work of searching these locations is done by createJoinImplementation. Overall this means less boilerplate code for you to write.

Don’t worry, you don’t need to use function* semantics for the generator unless you want to.

Simple customisation

It is relatively simple to change the precedence of values (from the algorithm) or add further locations to search for an asset. To do this we use createJoinImplementation and write a custom generator.

See the reference or jump directly to the examples.

Reference

The generator identifies [base:string,uri:string] tuples describing locations to search for an asset. It does not return the final asset path.

You may lazily generate tuples as Iterator. Refer to this guide on Iterators and Generators. javascript generator = function* (item: {}, options: {}, loader: {}): Iterator<[string,string]>

Or it can be simpler to write a function that returns Array and convert it to a generator using asGenerator.

javascript generator = asGenerator( function (item: {}, options: {}, loader: {}): Array<string> ) javascript generator = asGenerator( function (item: {}, options: {}, loader: {}): Array<[string,string]> )

When using asGenerator you may return elements as either base:string or [base:string,uri:string] tuples.

How to: change precedence of source locations

Source-map sampling is limited to the locations defined in the algorithm. You can’t change these locations but you can preference them in a different order.

This example shows the default order which you can easily amend. Absolute URIs are rare in most projects but can be handled for completeness.

Using asGenerator

```javascript const { createJoinFunction, createJoinImplementation, asGenerator, defaultJoinGenerator, } = require(‘resolve-url-loader’);

// order source-map sampling location by your preferred precedence (matches defaultJoinGenerator) const myGenerator = asGenerator( ({ isAbsolute, bases: { substring, value, property, selector} }, { root }) => isAbsolute ? [root] : [subString, value, property, selector] );

const myJoinFn = createJoinFunction( ‘myJoinFn’, createJoinImplementation(myGenerator), ); ```

Notes

• The implementation is the default behaviour, so if you want this precedence do not customise the join option.
• Absolute URIs generally use the base path given in the root option as shown.
• The asGenerator utility allows us to return simple Array<string> of potential base paths.

How to: fallback to a theme or other global directory

Additional locations can be added by decorating the default generator. This is popular for adding some sort of “theme” directory containing assets.

This example appends a static theme directory as a fallback location where the asset might reside. Absolute URIs are rare in most projects but can be handled for completeness.

Using asGenerator

```javascript const path = require(‘path’); const { createJoinFunction, createJoinImplementation, asGenerator, defaultJoinGenerator, } = require(‘resolve-url-loader’);

const myThemeDirectory = path.resolve(…);

// call default generator then append any additional paths const myGenerator = asGenerator( (item, …rest) => [ …defaultJoinGenerator(item, …rest), item.isAbsolute ? null : myThemeDirectory, ] );

const myJoinFn = createJoinFunction( ‘myJoinFn’, createJoinImplementation(myGenerator), ); ```

Notes

• By spreading the result of defaultJoinGenerator we are first trying the default behaviour. If that is unsuccessful we then try the theme location.
• It’s assumed that theming doesn’t apply to absolute URIs. Since falsey elements are ignored we can easily null the additional theme element inline as shown.
• The asGenerator utility allows us to return simple Array<string> of potential base paths.

How to: fallback to some other asset file

Lets imagine we don’t have high quality files for all our assets and must sometimes use a lower quality format. For each item we need to try the uri with different file extensions. We can do this by returning tuples of [base:string,uri:string].

In this example we prefer the .svg asset we are happy to use any available .png or .jpg instead.

Using asGenerator

```javascript const { createJoinFunction, createJoinImplementation, asGenerator, defaultJoinGenerator, } = require(‘resolve-url-loader’);

// call default generator then pair different variations of uri with each base const myGenerator = asGenerator( (item, …rest) => { const defaultTuples = […defaultJoinGenerator(item, …rest)]; return /.svg$/.test(item.uri) ? [‘.svg’, ‘.png’, ‘jpg’].flatMap((ext) => defaultTuples.flatMap(([base, uri]) => [base, uri.replace(/.svg$/, ext)] }) ) : defaultTuples; } );

const myJoinFn = createJoinFunction( ‘myJoinFn’, createJoinImplementation(myGenerator), ); ```

Using function*

```javascript const { createJoinFunction, createJoinImplementation, defaultJoinGenerator, } = require(‘resolve-url-loader’);

// call default generator then pair different variations of uri with each base const myGenerator = function* (item, …rest) { if (/.svg$/.test(item.uri)) { for (let ext of [‘.svg’, ‘.png’, ‘jpg’]) { for (let [base, uri] of defaultJoinGenerator(item, …rest)) { yield [base, uri.replace(/.svg$/, ext)]; } } } else { for (let value of defaultJoinGenerator(item, …rest)) { yield value; } } }

const myJoinFn = createJoinFunction( ‘myJoinFn’, createJoinImplementation(myGenerator), ); ```

Notes

• Existing generators such as defaultJoinGenerator will always return [string,string] tuples so we can destruture base and uri values with confidence.
• This implementation attempts all extensions for a given base before moving to the next base. Obviously we may change the nesting and instead do the oposite, attempt all bases for a single extension before moving on to the next extension
• The asGenerator utility allows us to return Array<[string, string]> but is not needed when we use function* semantics.

How to: perform a file-system search for an asset

⚠️ IMPORTANT - This example is indicative only and is not advised.

When this loader was originally released it was very common for packages be broken to the point that a full file search was needed to locate assets referred to in CSS. While this was not performant some users really liked it. By customising the generator we can once again lazily search the file-system.

In this example we search the parent directories of the base paths, continuing upwards until we hit a package boundary. Absolute URIs are rare in most projects but can be handled for completeness.

Using function*

```javascript const path = require(‘path’); const { createJoinFunction, createJoinImplementation, webpackExistsSync } = require(‘resolve-url-loader’);

// search up from the initial base path until you hit a package boundary const myGenerator = function* ( { uri, isAbsolute, bases: { substring, value, property, selector } }, { root, attempts = 1e3 }, { fs }, ) { if (isAbsolute) { yield [root, uri]; } else { for (let base of [subString, value, property, selector]) { for (let isDone = false, i = 0; !isDone && i < attempts; i++) { yield [base, uri]; // unfortunately fs.existsSync() is not present so we must shim it const maybePkg = path.normalize(path.join(base, ‘package.json’)); try { isDone = fs.statSync(maybePkg).isFile(); } catch (error) { isDone = false; } base = base.split(/(\\/)/).slice(0, -2).join(‘’); } } } };

const myJoinFn = createJoinFunction( ‘myJoinFn’, createJoinImplementation(myGenerator), ); ```

Notes

• This implementation is nether tested nor robust, it would need further safeguards to avoid searching the entire file system.

• By using function* the generator is lazy. We only walk the file-system directory tree as necessary.

• The webpack file-system is provided by the enhanced-resolver-plugin and does not contain fs.existsSync(). We must use fs.statsSync() instead and catch any error where the file isn’t present.

• You may set additional options when you configure the loader in webpack and then access them in your generator. In this case we add an attempts option to limit the file search.

Full customisation

The createJoinFunction can give you full control over how the base and uri are joined to create an absolute file path and the definitiion of success for that combination.

It provides additional logging when using debug option so is a better choice then writing a “join” function from scratch.

Limited documentation is given here since it is rare to require a full customisation. Refer to the source code for further information.

Reference

The implementation synchronously returns the final asset path or some fallback value. It makes a number of attempts to search for the given item and returns an element describing each attempt.

javascript implementation = function (item: {}, options: {}, loader: {}): Array<{ base : string, uri : string, joined : string, isSuccess : boolean, isFallback: boolean, }>