stylis

What is stylis?

The stylis npm package is a lightweight CSS preprocessor that allows developers to write nested CSS, use mixins, and perform other transformations before the CSS is compiled. It is particularly designed to be used with CSS-in-JS libraries and offers a middleware architecture to extend its capabilities.

What are stylis's main functionalities?

CSS Preprocessing

Stylis allows you to write nested CSS rules, which it will then flatten into valid CSS. The code sample demonstrates how to preprocess a nested CSS string.

const stylis = require('stylis');

const css = `
  .parent {
    color: red;
    .child {
      color: blue;
    }
  }
`;

const output = stylis('.parent', css);
console.log(output);

Prefixing

Stylis automatically adds vendor prefixes to CSS rules when necessary. The code sample shows how to automatically prefix the 'display: flex;' rule.

const stylis = require('stylis');

const css = `display: flex;`;

const output = stylis('', css);
console.log(output);

Middleware

Stylis supports middleware, allowing you to intercept and transform CSS at various stages of processing. The code sample demonstrates a middleware that changes the color property for elements with the class '.button'.

const stylis = require('stylis');

stylis.use((context, content, selectors, parents, line, column, length) => {
  if (context === 2 && selectors[0] === '.button') {
    return content.replace('color: red', 'color: blue');
  }
});

const css = `.button { color: red; }`;

const output = stylis('', css);
console.log(output);

Other packages similar to stylis

postcss

PostCSS is a tool for transforming CSS with JavaScript plugins. It is more established and has a larger ecosystem of plugins compared to stylis. PostCSS can be used for a wide range of tasks including linting, optimization, and applying polyfills.

sass

Sass is a mature and feature-rich CSS extension language that allows for variables, nesting, mixins, and more. Unlike stylis, Sass has its own syntax and requires compilation to CSS, but it offers a more comprehensive set of features for styling.

less

Less is another CSS pre-processor that extends the capabilities of CSS with variables, mixins, functions, and more. It is similar to Sass and offers a different syntax and feature set compared to stylis.

README

STYLIS

A Light–weight CSS Preprocessor.

Installation

• Use a Direct Download: <script src=stylis.js></script>
• Use a CDN: <script src=unpkg.com/stylis></script>
• Use NPM: npm install stylis --save

Features

• nesting a { &:hover {} }
• selector namespacing
• vendor prefixing (flex-box, etc...)
• minification
• esm module compatible
• tree-shaking-able

Abstract Syntax Structure

const declaration = {
	value: 'color:red;',
	type: 'decl',
	props: 'color',
	children: 'red',
	line: 1, column: 1
}

const comment = {
	value: '/*@noflip*/',
	type: 'comm',
	props: '/',
	children: '@noflip',
	line: 1, column: 1
}

const ruleset = {
	value: 'h1,h2',
	type: 'rule',
	props: ['h1', 'h2'],
	children: [/* ... */],
	line: 1, column: 1
}

const atruleset = {
	value: '@media (max-width:100), (min-width:100)',
	type: '@media',
	props: ['(max-width:100)', '(min-width:100)'],
	children: [/* ... */],
	line: 1, column: 1
}

Example:

import {compile, serialize, stringify} from 'stylis'

serialize(compile(`h1{all:unset}`), stringify)

Compile

compile('h1{all:unset}') === [{value: 'h1', type: 'rule', props: ['h1'], children: [/* ... */]}]
compile('--foo:unset;') === [{value: '--foo:unset;', type: 'decl', props: '--foo', children: 'unset'}]

Tokenize

tokenize('h1 h2 h3 [h4 h5] fn(args) "a b c"') === ['h1', 'h2', 'h3', '[h4 h5]', 'fn', '(args)', '"a b c"']

Serialize

serialize(compile('h1{all:unset}'), stringify)

Vendor Prefixing

import {compile, serialize, stringify, middleware, prefixer } from 'stylis';

serialize(compile('div{display:flex;}'), middleware([prefixer, stringify]))

Middleware

The middleware helper is a convenient helper utility, that for all intents and purposes you can do without if you intend to implement your own traversal logic. The stringify middleware is one such middleware that can be used in conjunction with it.

Elements passed to middlewares have a root property that is the immediate root/parent of the current element in the compiled output, so it references the parent in the already expanded CSS-like structure. Elements have also parent property that is the immediate parent of the current element from the input structure (structure representing the input string).

Traversal

serialize(compile('h1{all:unset}'), middleware([(element, index, children) => {
	assert(children === element.root.children && children[index] === element.children)
}, stringify])) === 'h1{all:unset;}'

The abstract syntax tree also includes an additional return property for more niche uses.

Prefixing

serialize(compile('h1{all:unset}'), middleware([(element, index, children, callback) => {
	if (element.type === 'decl' && element.props === 'all' && element.children === 'unset')
		element.return = 'color:red;' + element.value
}, stringify])) === 'h1{color:red;all:unset;}'

serialize(compile('h1{all:unset}'), middleware([(element, index, children, callback) => {
	if (element.type === 'rule' && element.props.indexOf('h1') > -1)
		return serialize([{...element, props: ['h2', 'h3']}], callback)
}, stringify])) === 'h2,h3{all:unset;}h1{all:unset;}'

Reading

serialize(compile('h1{all:unset}'), middleware([stringify, (element, index, children) => {
	assert(element.return === 'h1{all:unset;}')
}])) === 'h1{all:unset;color:red;}'

The middlewares in src/Middleware.js dive into tangible examples of how you might implement a middleware, alternatively you could also create your own middleware system as compile returns all the nessessary structure to fork from.

Variables

CSS variables are supported but a note should be made about the exotic use of css variables. The css spec mentions the following

The allowed syntax for custom properties is extremely permissive. The production matches any sequence of one or more tokens, so long as the sequence does not contain , , unmatched <)-token>, <]-token>, or <}-token>, or top-level tokens or tokens with a value of "!".

That is to say css variables according to the spec allows: --foo: if(x > 5) this.width = 10; and while this value is obviously useless as a variable, and would be invalid in any normal property, it still might be read and acted on by JavaScript and this is supported by Stylis, however things become slightly undefined when we start to include the { and } productions in our use of exotic css variables.

For example consider the following: --foo: {};

While this is valid CSS and supported. It is unclear what should happen when the rule collides with the implicit block termination rule that allows i.e h1{color:red}(notice the omitted semicolon) to also be a valid CSS production. This results in the following contradiction in: h1{--example: {} is it to be treated as h1{--foo:{;} or h1{--foo:{} the later of which is an unterminated block or in the following: h1{--foo:{} h1{color:red;} should it be h1 {--foo:{}h1{color:red;}; where {}h1{color:red; is part of the css variable --foo and not a new rule or should it be something else?

Never the less Stylis still supports the exotic forms highlighted in the spec, however you should consider it as a general rule to delimit such exotic uses of variables in strings or parentheses i.e: h1{--foo:'{'} or h1{--foo:({)}.

Benchmark

Stylis is at-least 2X faster than its predecesor.

License

Stylis is MIT licensed.