PostCSS CSS Variables

PostCSS plugin to transform CSS Custom Properties(CSS variables) syntax into a static representation. This plugin provides a future-proof way of using most of CSS variables featuers.

CSS variables or CSS Custom Properties limited subset polyfill/shim.

We strive for the most complete transformation but we/no plugin can achieve true complete parity according to the speficification because of the DOM cascade unknowns.

Latest Version: 0.2.2

Changelog

Usage

var postcss = require('postcss');
var cssvariables = require('postcss-css-variables');

var fs = require('fs');

var mycss = fs.readFileSync('input.css', 'utf8');

// Process your CSS with postcss-css-variables
var output = postcss([
        cssvariables(/*options*/)
    ])
    .process(mycss)
    .css;

console.log(output);

For more general PostCSS usage, see this section

At-rule support @media, @support, etc

You can add rules that declare CSS variables nested inside at-rules. You can even nest the at-rules however deep you want.

The following CSS:

:root {
    --width: 100px;
}

@media (max-width: 1000px) {
    :root {
        --width: 200px;
    }
}

.box {
    width: var(--width);
}

will be processed to:

.box {
    width: 100px;
}

@media (max-width: 1000px) {
    .box {
        width: 200px;
    }
}

Nested rules

When using this feature, postcss-css-variables will output invalid CSS by itself(but you did input invalid CSS anyway). This feature is best paired with postcss-nested in order to properly expand the rules.

var postcss = require('postcss');
var cssvariables = require('postcss-css-variables');
var nestedcss = require('postcss-nested');

var fs = require('fs');

var mycss = fs.readFileSync('input.css', 'utf8');

// Process your CSS with postcss-css-variables
var output = postcss([
        // Process any CSS variables
        cssvariables(/*options*/),
        // After we processed all of the variables, flatten/unnest rules
        nestedcss
    ])
    .process(mycss)
    .css;

console.log(output);

Simple example

The following CSS:

.box-foo {
    --some-width: 150px;
    width: var(--some-width);

    .box-bar {
        width: var(--some-width);
    }
}

will be processed to:

.box-foo {
    width: 150px;

    .box-bar {
        width: 150px;
    }
}

Complex example

The following CSS:

:root {
    --some-width: 150px;
}

.box-foo {
    width: var(--some-width);

    .box-bar {
        width: var(--some-width);
    }
}

@media (max-width: 800px) {
    .box-foo {
        --some-width: 300px;
    }
}

will be processed to:

.box-foo {
    width: 150px;

    .box-bar {
        width: 150px;
    }

    @media (max-width: 800px) {
        .box-bar {
            width: 300px;
        }
    }
}

@media (max-width: 800px) {
    .box-foo {
        width: 300px;
    }
}

Interoperability

post-css-variables plays really nice with postcss-nested in order to get a larger subset of CSS variables features. See Nested rules, Usage section

If you are using postcss-custom-properties previosly, we have a compatible feature set and more so you can switch over without having to refactor any of your code. You can just start writing the new awesome stuff.

Why?

This plugin was spawned out of a discussion on the cssnext repo and a personal need.

There is another similar plugin available, postcss-custom-properties, which at the moment, covers less of the CSS-variable/custom-property draft/spec compared to this plugin. It also happened to not cover the feature I was after and after dealing with it, making an incomplete prototype piggybacking off of it; I decided to write my own starting from scratch. I do use some of the same/similar unit tests because that just makes sense to get proper code coverage.

Differences from postcss-custom-properties

The main features that wepost-css-variables add/provide are:

• No limitation on what scope CSS variables can be declared or used (:root or wherever)
  • Proper value substition based on explicit DOM/structure traversal
• At-rule support @media, @support, etc
• Nested rules which can be fully deduced with postcss-nested.

Syntax

Defining Custom Properties: the --* family of properties

A custom property is any property whose name starts with two dashes (U+002D HYPHEN-MINUS). A property must be in a rule.

Note: :root is nothing more than the selector for the root DOM node. You could use any other selector (class, id, etc)

:root {
    --foo-width: 100px;
    --foo-bg-color: rgba(255, 0, 0, 0.85);
}

A custom property can be declared multiple times.

See: W3C Draft: CSS Custom Properties for Cascading Variables, section 2 for more information.

Using Cascading Variables: the var() notation

.foo {
    width: var(--foo-width);
    /* You can even provide a fallback */
    background: var(--foo-bg-color, #ff0000);
}

See: W3C Draft: CSS Custom Properties for Cascading Variables, section 3 for more information.

Options:

preserve (default: false)

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

• false: Removes --var declarations and replaces var() with their resolved/computed values.
• true: Keeps var() declarations in the output and has the computed value as a fallback declaration. Also keeps computed --var declarations
• 'computed': Keeps computed --var declarations in the output. Handy to make them available to your JavaScript.

variables (default: {})

Define an object map of variables in JavaScript that will be declared at the :root scope.

Can be a simple key-value pair or an object with a value property and an optional isImportant bool property

The object keys are automatically prefixed with -- (according to CSS custom property syntax) if you do not provide it.

var postcss = require('postcss');
var cssvariables = require('postcss-css-variables');

postcss([
    cssvariables({
        variables: {
            '--some-var': '100px',
            '--other-var': {
                value: '#00ff00'
            },
            '--important-var': {
                value: '#ff0000',
                isImportant: true
            }
        }
    })
])
.process(css, opts);

Quick Reference/Notes

• This plugin was spawned out of a discussion on the cssnext repo
• We provide a larger CSS variable feature subset than postcss-custom-properties.
• Related links and issues:
  • var declared in media query should pull in properties that use/reference that var on cssnext/cssnext
  • Investigate support for media-query scoped properties on postcss/postcss-custom-properties
  • remove :root limitation by injecting rules with new declarations that just contains modified properties. on postcss/postcss-custom-properties

Testing

We have a suite of Mocha tests. If you see something that doesn't have coverage, make an issue or pull request.

Run once:

npm install

Run whenever you want to test:

npm run test