postcss-custom-properties

What is postcss-custom-properties?

The postcss-custom-properties npm package is a PostCSS plugin that allows you to use CSS Custom Properties (also known as CSS variables) in environments that do not support them natively. It transforms CSS variables into static values based on your configurations, making it easier to maintain themes and styles dynamically across your project.

What are postcss-custom-properties's main functionalities?

Transform CSS Custom Properties

This feature allows the transformation of CSS custom properties into their corresponding static values. It is useful for supporting older browsers that do not understand CSS variables.

/* Input CSS */
:root {
  --main-color: red;
}

a {
  color: var(--main-color);
}

/* Output CSS */
a {
  color: red;
}

Preserve option

With the preserve option set to true, the plugin outputs both the transformed static value and the original variable. This is useful for progressive enhancement.

/* Input CSS */
:root {
  --main-color: red;
}

a {
  color: var(--main-color);
}

/* Output CSS with preserve: true */
a {
  color: red;
  color: var(--main-color);
}

Other packages similar to postcss-custom-properties

cssnext

cssnext is a PostCSS plugin that allows you to use future CSS features today. It includes support for CSS custom properties among other features. Compared to postcss-custom-properties, cssnext offers a broader range of CSS features but might be heavier due to its comprehensive nature.

postcss-preset-env

postcss-preset-env lets you convert modern CSS into something most browsers can understand, determining the polyfills you need based on your targeted browsers or runtime environments. It includes handling of CSS custom properties as part of its feature set, similar to postcss-custom-properties, but is more configurable and includes various stages of CSS specifications.

README

postcss-custom-properties

PostCSS plugin to transform W3C CSS Custom Properties for cascading variables syntax to more compatible CSS.

Per w3c specifications, the usage of var() is limited to property values. Do not expect the plugin to transform var() in media queries or in selectors.

N.B. The transformation is not complete and cannot be (dynamic cascading variables based on custom properties relies on the DOM tree). It currently just aims to provide a future-proof way of using a limited subset (to :root selector) of the features provided by native CSS custom properties. Since we do not know the DOM in the context of this plugin, we cannot produce safe output. Read #1 & #9 to know why this limitation exists.

If you are looking for a full support of CSS custom properties, please follow the opened issue for runtime support.

N.B.² If you are wondering why there is a different plugin (postcss-css-variables) that claims to do more than this plugin, be sure to understand the explanation above about limitation. This plugins have a behavior that is not reflecting the specifications.

This plugin works great with postcss-calc.

Installation

$ npm install postcss-custom-properties

Usage

// dependencies
var fs = require("fs")
var postcss = require("postcss")
var customProperties = require("postcss-custom-properties")

// css to be processed
var css = fs.readFileSync("input.css", "utf8")

// process css using postcss-custom-properties
var output = postcss()
  .use(customProperties())
  .process(css)
  .css

Using this input.css:

:root {
  --color: red;
}

div {
  color: var(--color);
}

you will get:

div {
  color: red;
}

Note that plugin returns itself in order to expose a setVariables function that allow you to programmatically change the variables.

var variables = {
  "--a": "b",
}
var plugin = customProperties()
plugin.setVariables(variables)
var result = postcss()
  .use(plugin)
  .process(input)

This might help for dynamic live/hot reloading.

Checkout tests for more.

Options

strict

Default: true

Per specifications, all fallbacks should be added since we can't verify if a computed value is valid or not. This option allows you to avoid adding too many fallback values in your CSS.

preserve

Default: false

Allows you to preserve custom properties & var() usage in output.

var out = postcss()
  .use(customProperties({preserve: true}))
  .process(css)
  .css

You can also set preserve: "computed" to get computed resolved custom properties in the final output. Handy to make them available to your JavaScript.

variables

Default: {}

Allows you to pass an object of variables for :root. These definitions will override any that exist in the CSS. The keys are automatically prefixed with the CSS -- to make it easier to share variables in your codebase.

appendVariables

Default: false

If preserve is set to true (or "computed"), allows you to append your variables at the end of your CSS.

warnings

Default: true Type: Boolean|Object

Allows you to enable/disable warnings. If true, will enable all warnings. For now, it only allow to disable messages about custom properties definition not scoped in a :root selector.

---

Contributing

Fork, work on a branch, install dependencies & run tests before submitting a PR.

$ git clone https://github.com/YOU/postcss-custom-properties.git
$ git checkout -b patch-1
$ npm install
$ npm test

Changelog

License