Svelte preprocess CSS Modules

Generate CSS Modules classnames on Svelte components

``bash
npm install --save-dev svelte-preprocess-cssmodules
`

for svelte 4 and below, use version 2 of the preprocessor.

Table of Content

- Usage
- Approach
- Class objects and arrays
- Class directive
- Local selector
- CSS binding
- Scoped class on child components
- Import styles from an external stylesheet
- Destructuring import
- kebab-case situation
- Unnamed import
- Directive and dynamic class
- Preprocessor Modes
- Native
- Mixed
- Scoped
- Comparative
- Why CSS Modules over Svelte scoping?
- Configuration
- Rollup
- Webpack
- SvelteKit
- Svelte Preprocess
- Vite
- Options
- Code example

Usage

Add the module attribute to

My red text

`

The component will be compiled to

`html


My red text


`

$3

The default svelte scoping appends every css selectors with a unique class to only affect the elements of the component.

CSS Modules scopes each class name with a unique id/name in order to affect the elements of the component. As the other selectors are not scoped, it is recommended to write each selector with a class.

`html



lorem ipsum tut moue

lorem ipsum tut moue


`
`html



lorem ipsum tut moue

lorem ipsum tut moue


`

_transformed to_

`html



lorem ipsum tut moue

lorem ipsum tut moue


`

`html



lorem ipsum tut moue

lorem ipsum tut moue


`

$3

#### Object with thruthy values

`html



`

generating

`html


...


`

#### Array with thruthy values

`html



`

generating

`html


...


`

$3

Toggle a class on an element.

`html


Home

Home
`

_generating_

`html


Home
`

#### Use of shorthand

`html


Home
`

_generating_

`html


Home
`

$3

Force a selector to be scoped within its component to prevent style inheritance on child components.

:local() is doing the opposite of :global() and can only be used with the native and mixed modes (see preprocessor modes). The svelte scoping is applied to the selector inside :local().

`html



`
`html

My secondary lorem ipsum tuye

`

generating

`html



`
`html

My secondary lorem ipsum tuye

`

When used with a class, :local() cssModules is replaced by the svelte scoping system. This could be useful when targetting global classnames.

`html



`

generating

`html



`

$3

Link the value of a CSS property to a dynamic variable by using bind().

`html


My lorem ipsum text


`

A scoped css variable, binding the declared statement, will be created on the component root elements which the css property will inherit from.

`html



My lorem ipsum text



`

An object property can also be targetted and must be wrapped with quotes.

`html



`

_generating_

`html



`

$3

CSS Modules allows you to pass a scoped classname to a child component giving the possibility to style it from its parent. (Only with the native and mixed modes – See preprocessor modes).

`html




{@render props.children?.()}



`

`html



`

_generating_

`html



`




Import styles from an external stylesheet

Alternatively, styles can be created into an external file and imported onto a svelte component. The name referring to the import can then be used on the markup to target any existing classname of the stylesheet.

- The option parseExternalStylesheet need to be enabled.
- The css file must follow the convention [FILENAME].module.css in order to be processed.

Note: That import is only meant for stylesheets relative to the component. You will have to set your own bundler in order to import node_modules css files.

`css
/ style.module.css /
.red { color: red; }
.blue { color: blue; }
`
`html



My red text

My blue text

`

generating

`html


My red text

My blue text

`

$3

`css
/ style.module.css /
section { padding: 10px; }
.red { color: red; }
.blue { color: blue; }
.bold { font-weight: bold; }
`
`html



My red text

My blue text


`

generating

`html


My red text

My blue text


`

$3

The kebab-case class names are being transformed to a camelCase version to facilitate their use on Markup and Javascript.

`css
/ style.module.css /
.success { color: green; }
.error-message {
color: red;
text-decoration: line-through;
}
`
`html


My success text

My error message

My success message

My error message

`

generating

`html


My success messge

My error message

`

$3

If a css file is being imported without a name, CSS Modules will still apply to the classes of the stylesheet.

`css
/ style.module.css /
p { font-size: 18px; }
.success { color: green; }
`
`html


My success message

My another message

`

generating

`html


My success messge

My error message

`

$3

Use the Svelte's builtin class: directive or javascript template to display a class dynamically.
Note: the shorthand directive is NOT working with imported CSS Module identifiers.

`html


isSuccess = !isSuccess}>Toggle




Success




class:success={isSuccess}
class:error={!isSuccess}>Notice

Notice

Notice

`

Preprocessor Modes

The mode can be set globally from the config or locally to override the global setting.

$3

Scopes classes with CSS Modules, anything else is unscoped.

Pros:

- uses default CSS Modules approach
- creates unique ID to avoid classname conflicts and unexpected inheritances
- passes scoped class name to child components

Cons:

- does not scope non class selectors.
- forces to write selectors with classes.
- needs to consider third party plugins with useAsDefaultScoping on – Read more.

$3

Scopes non-class selectors with svelte scoping in addition to native (same as preprocessor v1)

`html


My red text


`

_generating_

`html


My red text


`

Pros:

- creates class names with unique ID to avoid conflicts and unexpected inheritances
- uses svelte scoping on non class selectors
- passes scoped class name to child components

Cons:

- adds more weight to tag selectors than class selectors (because of the svelte scoping)

`html


• Home


• About

• Home


• About


`

$3

Scopes classes with svelte scoping in addition to mixed.

`html


My red text


`

_generating_

`html


My red text


`

Pros:

- creates class names with unique ID to avoid conflicts and unexpected inheritances
- scopes every selectors at equal weight

Cons:

- does not pass scoped classname to child components

$3

| | Svelte scoping | Preprocessor Native | Preprocessor Mixed | Preprocessor Scoped |
| -------------| ------------- | ------------- | ------------- | ------------- |
| Scopes classes | O | O | O | O |
| Scopes non class selectors | O | X | O | O |
| Creates unique class ID | X | O | O | O |
| Has equal selector weight | O | O | X | O |
| Passes scoped classname to a child component | X | O | O | X |

Why CSS Modules over Svelte scoping?

- On a full svelte application: it is just a question of taste as the default svelte scoping is largely enough. Component styles will never inherit from other styling.

- On a hybrid project (like using svelte to enhance a web page): the default scoping may actually inherits from a class of the same name belonging to the style of the page. In that case using CSS Modules to create a unique ID and to avoid class inheritance might be advantageous.

Configuration

$3

To be used with the plugin rollup-plugin-svelte.

`js
import svelte from 'rollup-plugin-svelte';
import { cssModules } from 'svelte-preprocess-cssmodules';

export default {
...
plugins: [
svelte({
preprocess: [
cssModules(),
]
}),
]
...
}
`

$3

To be used with the loader svelte-loader.

`js
const { cssModules } = require('svelte-preprocess-cssmodules');

module.exports = {
...
module: {
rules: [
{
test: /\.svelte$/,
exclude: /node_modules/,
use: [
{
loader: 'svelte-loader',
options: {
preprocess: [
cssModules(),
]
}
}
]
}
]
}
...
}
`

$3

As the module distribution is targetting esnext, Node.js 14 or above is required
in order to work.

`js
// svelte.config.js

import { cssModules } from 'svelte-preprocess-cssmodules';

const config = {
...
preprocess: [
cssModules(),
]
};

export default config;
`

$3

The CSS Modules preprocessor requires the compoment to be a standard svelte component (using vanilla js and vanilla css). if any other code, such as Typescript or Sass, is encountered, an error will be thrown. Therefore CSS Modules needs to be run at the very end.

`js
import { typescript, scss } from 'svelte-preprocess';
import { cssModules } from 'svelte-preprocess-cssmodules';

...
// svelte config:
preprocess: [
typescript(),
scss(),
cssModules(), // run last
],
...
`

$3

Set the svelte.config.js accordingly.

`js
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
import { cssModules } from 'svelte-preprocess-cssmodules';

export default {
preprocess: [
vitePreprocess(),
cssModules()
]
};
`




$3

Pass an object of the following properties

| Name | Type | Default | Description |
| ------------- | ------------- | ------------- | ------------- |
| cssVariableHash | {String} | [hash:base64:6] | The hash type (see locatonIdentName) |
| getLocalIdent | Function | undefined | Generate the classname by specifying a function instead of using the built-in interpolation |
| hashSeeder | {Array} | ['style', 'filepath', 'classname'] | An array of keys to base the hash on |
| includeAttributes | {Array} | [] | An array of attributes to parse along with class |
| includePaths | {Array} | [] (Any) | An array of paths to be processed |
| localIdentName | {String} | "[local]-[hash:base64:6]" | A rule using any available token |
| mode | native\|mixed\|scoped | native | The preprocess mode to use
| parseExternalStylesheet | {Boolean} | false | Enable parsing on imported external stylesheet |
| parseStyleTag | {Boolean} | true | Enable parsing on style tag |
| useAsDefaultScoping | {Boolean} | false | Replace svelte scoping globally |

#### getLocalIdent

Customize the creation of the classname instead of relying on the built-in function.

`ts
function getLocalIdent(
context: {
context: string, // the context path
resourcePath: string, // path + filename
},
localIdentName: {
template: string, // the template rule
interpolatedName: string, // the built-in generated classname
},
className: string, // the classname string
content: {
markup: string, // the markup content
style: string, // the style content
}
): string {
return your_generated_classname;
}
`


Example of use

`bash


Directory


SvelteApp
└─ src
├─ App.svelte
└─ components
└─ Button.svelte
`
`html

Ok


`

`js
// Preprocess config
...
preprocess: [
cssModules({
localIdentName: '[path][name]__[local]',
getLocalIdent: (context, { interpolatedName }) => {
return interpolatedName.toLowerCase().replace('src_', '');
// svelteapp_components_button__red;
}
})
],
...
`

#### hashSeeder

Set the source of the hash (when using [hash] / [contenthash]).

The list of available keys are:

- style the content of the style tag (or the imported stylesheet)
- filepath the path of the component
- classname the local classname

Example of use: creating a common hash per component
`js
// Preprocess config
...
preprocess: [
cssModules({
hashSeeder: ['filepath', 'style'],
})
],
...
`
`html
Ok
Cancel

`

_generating_

`html
Ok
Cancel

`

#### includeAttributes

Add other attributes than class to be parsed by the preprocesser

`js
// Preprocess config
...
preprocess: [
cssModules({
includeAttributes: ['data-color', 'classname'],
})
],
...
`
`html
Red
Red or Blue

`

_generating_

`html
Red
Red or Blue

`

#### localIdentName

Inspired by webpack interpolateName, here is the list of tokens:

- [local] the targeted classname
- [ext] the extension of the resource
- [name] the basename of the resource
- [path] the path of the resource
- [folder] the folder the resource is in
- [contenthash] or [hash] (they are the same) the hash of the resource content (by default it's the hex digest of the md5 hash)
- [:contenthash::] optionally one can configure
- other hashTypes, i. e. sha1, md5, sha256, sha512
- other digestTypes, i. e. hex, base26, base32, base36, base49, base52, base58, base62, base64
- and length the length in chars

#### useAsDefaultScoping

Globally replace the default svelte scoping by the CSS Modules scoping. As a result, the module attribute to
`

_generating_

`html


Welcome



`

Potential issue with third party plugins

The preprocessor requires you to add the module attribute to

My Modal Title

Lorem ipsum dolor sit, amet consectetur.

`

Final html code generated by svelte

`html


My Modal Title

Lorem ipsum dolor sit, amet consectetur.


``

License

MIT