Rationale

It turned out that no one CSS preprocessor is able to provide modularity of styles at the development level.

The convenience of preprocessors in the details is completely leveled when the application grows, because the preprocessors cannot provide adequate modularity.

When I have such a powerful tool at hand as JavaScript - it would not be wise not to use it.

Styles written in JavaScript are not so convenient, but when extending even a small project, the power of the JavaScript language fully compensated for this drawback.

Demo

Converter between Postcss-Js-Syntax and CSS

Description

You can use your JS modules for compile CSS files.

CSS styles in JavaScript format represent an simple object:

my-style.js

module.exports = [
    "@at-rule-wiwhout-params", // starts with @
    "@at-rule with params",
    "@at-rule (with params)",
    "// comment", // starts with "//" or "/*"
    {
        ".selector1": {
            color: "#0f0"
        },
        ".selector2": {
            color: "#0f0"
        }
    },
    "// another comment",
    {
        "@at-rule (with params)": {
            "and-content": "value"
        },
        ".selector3": {
            color: "#0f0",
            
            ".sub-selector": { // you can use postcss-nested plugin here
                content: '"quotes is required for this CSS property"'
            }
        }
    }
]

One possible use case:

import postcss from 'postcss'
import jsSyntax from 'postcss-js-syntax'

const css = postcss([...your plugins])
    .process(
        '', // this parameter will be ignored by js parser
        {
            from: require.resolve('./myJsStyle.js'), // required absolute path to real file
            parser: jsSyntax.parser,
            requireFromString: function(code, filename) {
            	// you can provide your own requireFromString function
            	
            	// default:
            	return require(filename)
            }
        }

console.log(css)

I recommend to use require-from-memory npm module in requireFromString option, because it allows you to use ES6 modules with babel like this:

import myModule './my-module.js'
import myNodeModule 'my-node-module'

export default [
    ...myModule.baseStyles({fontSize: "5px"}), // mixin equivalent
    {
    	...myNodeModule?.myCssClasses, // see @babel/plugin-proposal-optional-chaining
    	.override-selector {
    		color: myNodeModule?.colors?.primary, // if color == null it will not be added to CSS
    		content: myNodeModule.myContent?.surroundWithQuotes()
    	}
    }
]

License

CC0-1.0