PostCSS

PostCSS is a framework for CSS postprocessors, to modify CSS with JavaScript with full source map support.

It takes care of the most common CSS tool tasks:

1. parses CSS;
2. provides a usable JS API to edit CSS node trees;
3. dumps the modified node tree into a CSS string;
4. generates a source map (or modifies an pre-existing source map) containing your changes;

You can use this framework to write your own:

• CSS minifier or beautifier.
• CSS polyfills.
• Grunt plugin to generate sprites, include data-uri images or any other work.
• Text editor plugin to automate CSS routines.
• Command-line CSS tool.

Sponsored by Evil Martians.

Built with PostCSS

Tools

• Autoprefixer adds vendor prefixes by Can I Use data.
• BEM linter lints CSS for SUIT CSS methodology.
• CSS MQPacker joins same media queries.
• css2modernizr analyzes your CSS and output only used Modernizr’s settings.
• cssnext is a transpiler (CSS4+ to CSS3) that allow you to use tomorrow’s CSS syntax today.
• CSSWring is a CSS minifier with full source map support.
• data-separator splits data-uri into a separate CSS file.
• pixrem is a rem unit polyfill.
• webpcss to duplicate images in CSS to WebP for supported browsers.
• Pleeease is a pack of various postprocessors.
• Pleeease Filters converts WebKit filters to SVG filter for other browsers.
• RTLCSS mirrors styles for right-to-left locales.
• CSS Byebye explicitly removes the CSS rules that you don't want.

Plugins

• postcss-calc to reduce calc() usage (recommanded with postcss-custom-properties).
• postcss-color to transform latest W3C CSS color module syntax to more compatible CSS.
• postcss-color-gray to transform gray() function to more compatible CSS.
• postcss-import to transform @import rules by inlining content.
• postcss-custom-media to transform W3C CSS Custom Media Queries to more compatible CSS.
• postcss-custom-properties to transform W3C CSS Custom Properties for cascading variables to more compatible CSS.
• postcss-url to rebase or inline on url().

Quick Example

Let’s fix a forgotten content property in ::before and ::after:

var postcss = require('postcss');

var contenter = postcss(function (css) {
    css.eachRule(function (rule) {
        if ( rule.selector.match(/::(before|after)/) ) {
            // In each ::before/::after rule

            // Did we forget the content property?
            var good = rule.some(function (i) { return i.prop == 'content'; });

            if ( !good ) {
                // Add content: "" if we forget it
                rule.prepend({ prop: 'content', value: '""' });
            }

        }
    });
});

And the CSS with a forgotten content property:

a::before {
    width: 10px;
    height: 10px
}

will be fixed by our new contenter:

var fixed = contenter.process(css).css;

to:

a::before {
    content: "";
    width: 10px;
    height: 10px
}

Features

Source Map

PostCSS generates a source map of its changes:

result = processor.process(css, { map: true, from: 'from.css', to: 'to.css' });
result.css // String with processed CSS
result.map // Source map

And modifies a source map from previous steps (for example, a Sass preprocessor):

var sass = compiler.compile(sass);

processor.process(sass.css, {
    map:  { prev: sass.map },
    from: 'from.sass.css',
    to:   'to.css'
});

Preserves code formatting and indentations

PostCSS will not change any byte of a rule, if you do not modify its node:

postcss(function (css) { }).process(css).css == css;

And when you modify CSS nodes, PostCSS will try to copy the coding style:

contenter.process("a::before{color:black}")
// a::before{content:'';color:black}

contenter.process("a::before {\n  color: black;\n  }")
// a::before {
//   content: '';
//   color: black;
//   }

Which allows you to use PostCSS in text editor plugins while preserving the user’s code style.

Why PostCSS Better Than …

Preprocessors

Preprocessors (like Sass or Stylus) give us special languages with variables, mixins, and statements, which are compiled to CSS. Compass, nib and other mixins libraries use these languages to work with prefixes, sprites and inline images.

But the Sass and Stylus languages were created to be syntax-sugar for CSS. Writing complicated programs using preprocessor languages can be very difficult. For example, it would be impossible to implement Autoprefixer on top of Sass.

PostCSS gives you the comfort and power of JS or CoffeeScript while you are working with CSS. Applying the depth and variety of npm’s libraries allows you to perform quite magical things using PostCSS.

An important point is that postprocessors are not the enemies of preprocessors. Preprocessors and postprocessors can be easily combined, so that you can take advantage of the readability and syntactical sugar offered by Sass and Stylus; and PostCSS will preserve their source maps.

Regular Expressions

Some Grunt plugins modify CSS with regular expressions, however a parser and its node tree provide a much safer interface to edit CSS. Furthermore, regular expressions typically break the source maps generated by preprocessors.

CSS Parsers

There are a lot of good CSS parsers, such as Gonzales, but they only help you to read in the CSS. PostCSS provides you with full source map support and a high level API. Safe iterators, and other features, are unique to PostCSS.

Rework

Rework and PostCSS are very similar, but they have different targets.

Rework was created to build a new CSS sublanguage that replaced Stylus (like Myth). PostCSS was created for CSS tools which work with legacy CSS code (one such tool is Autoprefixer).

Because of this fundamental difference, PostCSS:

• Handles source map better, because it updates the map from the previous step (for example, Sass compilation).
• Preserves all your spaces and code style, so that it can function in text editor plugins.
• Has a safer parser, so that it can be used for legacy code. Only PostCSS can parse all of the hacks from Browserhacks.com.
• Has a high level API to provide an simple interface for your processor to perform typical tasks.

Usage

Grunt

Grunt plugin grunt-postcss allows you to pipe your CSS files through an array of PostCSS processors.

grunt.initConfig({
    postcss: {
        options: {
            map: true,
            processors: [
                require('autoprefixer-core').postcss,
                require('csswring').postcss
            ]
        },
        dist: {
            src: 'css/*.css'
        }
    }
});

grunt.loadNpmTasks('grunt-postcss');

Gulp

There is a Gulp plugin for PostCSS called gulp-postcss that allows you to pipe your CSS files through an array of PostCSS processors.

Support for external source maps is provided by gulp-sourcemaps.

var postcss    = require('gulp-postcss');
var sourcemaps = require('gulp-sourcemaps');

gulp.task('css', function () {
    var processors = [
        require('autoprefixer-core'),
        require('csswring')
     ];
     return gulp.src('./src/style.css')
        .pipe(sourcemaps.init())
        .pipe(postcss(processors))
        .pipe(sourcemaps.write('.'))
        .pipe(gulp.dest('./dest'));
});

Webpack

In webpack you can use postcss-loader to process CSS files through an array of PostCSS processors.

module.exports = {
    module: {
        loaders: [
            {
                test:   /\.css$/,
                loader: "style-loader!css-loader!postcss-loader"
            }
        ]
    },
    postcss: [require('autoprefixer-core'), require('csswring')]
}

Write Own Processor

You can parse CSS with the postcss.parse() method, which returns a CSS AST:

var postcss = require('postcss');

var css = postcss.parse('a { color: black }');

You can easily make changes to this AST. Use css.list to get children. Properties rule.selector, decl.prop, decl.value, atrule.name and atrule.params contain data.

Try to avoid using underscore-prefixed properties (such as _selector, _params and _value), as they are used for comment-preserving magic (See Raw Properties below). Use the getters and setters instead (for example, selector, selectors, params and value).

css.list[0].value = 'white';

After changes have been made you can get the new CSS and a source map reflecting the modifications:

var result = css.toResult(options);

result.css //=> 'a { color: white }'
result.map //=> '{"version":3, … }'

The methods postcss.parse() and CSS#toResult() are part of a low level API, and - in most cases - it will be better to create processors with a simpler API and chaining.

Processor

The function postcss(fn) creates a processor from your function:

var postcss = require('postcss');

var processor = postcss(function (css, opts) {
    // Code to modify CSS
});

If you want to combine multiple processors (and parse the CSS only once), you can add several functions using the use(fn) method:

var all = postcss().
          use(prefixer).
          use(minifing);

You can also add processor objects with the postcss function:

postcss().use( autoprefixer.postcss ); // via function
postcss().use( autoprefixer );         // via object

A processor function can change the current CSS node tree:

postcss(function (css) {
    css.append( /* new rule */ )
});

or create a completely new CSS root node and return it instead:

postcss(function (css) {
    var newCSS = postcss.root()
    // Add rules and declarations
    return newCSS;
});

This generated processor transforms some CSS using the process(css, opts) method:

var doubler = postcss(function (css) {
    // Clone each declaration
    css.eachDecl(function (decl) {
        decl.parent.prepend( decl.clone() );
    });
});

var css    = "a { color: black; }";
var result = doubler.process(css);

result.css //=> "a { color: black; color: black; }"

You can change the original CSS filename via the from option, which can make syntax error more helpful:

var wrong = "a {";
processor.process(wrong, { from: 'main.css' });
//=> Can't parse CSS: Unclosed block at line 1:1 in main.css

Options from process(css, opts) will be sent to processors as the second argument.

You can also use the result from a previous postprocessor, or an already-parsed Root, as an argument to the next one:

result = processor1.process(css)
processor2.process(result)

Multiple Inputs

The function postcss() generates a processor for only one input. If you need to process several inputs (for example, when concatenating files) you can use postcss.parse().

Let’s join two CSS strings with full source map support in only 5 lines of code:

var file1 = postcss.parse(css1, { from: 'a.css' });
var file2 = postcss.parse(css2, { from: 'b.css' });

file1.append( file2 );

var result = file1.toResult({ to: 'app.css', map: true });

Source Map

By using source maps, a browser’s development tools can indicate the original position of your styles before the css file was transformed. For example, an inspector will show the position in a Sass file, even if the file has been compiled to CSS, concatenated, and minified.

To ensure a correct source map is generated, every CSS processing step should update the map generated by the previous step. For example, a Sass compiler will generate the first map, a concatenation tool should update the Sass step’s map, and a minifier should update the map generated by the concatenation tool.

There are two ways to store a source map:

• You can place it in a separate file which contains a special annotation comment pointing to another file:

a { } /*# sourceMappingURL=main.out.css.map */

* Or you can inline a base64-encoded source map within a CSS comment:

```css
a { }
/*# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoibWFpbi5taW4uY3NzIiwic291cmNlcyI6WyJtYWluLmNzcyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiQUFBQSxJQUFLIn0= */

PostCSS has great source map support. To ensure that you generate the correct source map, you must indicate the input and output CSS files paths (using the options from and to respectively).

To generate a new source map with the default options, provide map: true in the options passed to processor.process(css, opts).

var result = processor.process(css, {
    from: 'main.css',
    to:   'main.out.css'
    map:  true,
});

result.map //=> '{"version":3,"file":"main.out.css","sources":["main.css"],"names":[],"mappings":"AAAA,KAAI"}'

fs.writeFileSync('main.out.css',     result.css);
fs.writeFileSync('main.out.css.map', result.map);

Or set from in postcss.parse(css, opts) and to in root.toResult(opts):

var root = postcss.parse(css, { from: 'main.css' });
root.last.removeSelf(); // Example transformation

var result = root.toResult({ to: 'main.out.css' });
fs.writeFileSync('main.out.css',     result.css);
fs.writeFileSync('main.out.css.map', result.map);

If PostCSS is handling CSS and finds source maps from previous transformations, it will automatically update the CSS with the same options.

// main.sass.css has an annotation comment with a link to main.sass.css.map
var result = minifier.process(css, { from: 'main.sass.css', to: 'main.min.css' });
result.map //=> Source map from main.sass to main.min.css

If you want more control over source map generation, you can define the map option as an object with the following parameters:

• inline (boolean): indicates the source map should be inserted into the CSS string as a comment. By default, PostCSS will inline new source maps only if the source map from a previous step inserted an inline source map.

  If you inline a source map, result.map will be empty, as the source map will be contained within the text of result.css.

  As a shortcut, map { inline: true } is equivalent to map: 'inline'.

• prev (string, object, or boolean): map content from a previous processing step (for example, Sass compilation). PostCSS will try to read the previous source map automatically from the comment within origin CSS, but you can also set manually. If desired, you can omit the previous map with prev: false.

  This is a source map option which can be passed to postcss.parse(css, opts). Other options can be passed to the toResult(opts) or process(css, opts) methods.

• sourcesContent (boolean): indicates that we should set the origin content (for example, Sass source) of the source map. By default, PostCSS will add content only if previous map contains it.

• annotation (boolean or string): indicates if we should add annotation comments to the CSS. By default, PostCSS will always add a comment with a path to the source map. But if the previous CSS does not have an annotation comment, PostCSS will omit it too.

  By default, PostCSS presumes that you want to save the source map as opts.to + '.map' and will use this path in the annotation comment. But you can set another path by providing a string value as the annotation option.

  If you set inline: true, annotation cannot be disabled.

Safe Mode

If you provide a safe: true option to the process or parse methods, PostCSS will try to correct any syntax error that it finds in the CSS. For example, it will parse a { as a {}.

postcss.parse('a {');                 // will throw "Unclosed block"
postcss.parse('a {', { safe: true }); // will return CSS root for a {}

This is useful for legacy code filled with plenty of hacks. Another use case is interactive tools with live input, for example, the Autoprefixer demo.

Helpers

Vendor

PostCSS contains height optimized code to split vendor prefix:

var vendor = require('postcss/lib/vendor');

vendor.prefix('-moz-tab-size')     //=> '-moz-'
vendor.unprefixed('-moz-tab-size') //=> 'tab-size'

List

To safely split comma- or space-separated values (such as those in background-image or transform) with brackets and quotes support, you can use the list helper:

var list = require('postcss/lib/list');

list.space(image.value)     //=> ['linear-gradient(white, black)', 'blue']
list.comma(transform.value) //=> ['color 200ms', 'background 200ms']

Nodes

Processor functions receive a Root node which contains the CSS node tree.

var processor = postcss(function (cssRoot) {
});

There are 4 types of child nodes: Comment, AtRule, Rule and Declaration. All nodes possess toString() and clone() methods.

You can parse CSS and get a Root node by calling the postcss.parse(css, opts) method:

var cssRoot = postcss.parse('a { }');

Many of the methods on a node will return the current node, which enables you to build method chains:

root.append( rule1 ).append( rule2 ).toString();

Node Source

Every node stores its origin file (if you provide the from option to the process or parse methods) and position:

var root = postcss.parse(css, { from: 'main.css' });
var rule = root.rules[0];

rule.source.file  //=> 'main.css'
rule.source.start //=> { line: 5,  position: 1 }
rule.source.end   //=> { line: 10, position: 5 }

Whitespace

All nodes (excluding the Root) have a before property which contains the indentation and any previous whitespace.

Nodes with children (Root, AtRule and Rule) also contain an after property which indicates the spaces after the last child and before a } character or the end of the file.

Every Declaration has a between property with colon, spaces and comments between the property name and value. Rule stores the spaces and comments between the selector and { in the between property. AtRule uses between to indicate the spaces and comments before either a { or ;, if the at-rule is bodiless.

var root = postcss.parse("a {\n  color: black;\n}\n");

root.rules[0].between          //=> " " between selector and {
root.rules[0].decls[0].before  //=> "\n  " before color: black
root.rules[0].decls[0].between //=> ": " between property name and value
root.rules[0].after            //=> "\n" before }
root.after                     //=> "\n" from end of file

The simplest way to minify CSS is to set before, between and after properties to an empty string:

var minifier = postcss(function (css) {
    css.eachDecl(function (decl) {
        decl.before  = '';
        decl.between = ':';
    });
    css.eachRule(function (rule) {
        rule.before  = '';
        rule.between = '';
        rule.after   = '';
    });
    css.eachAtRule(function (atRule) {
        atRule.before  = '';
        atRule.between = '';
        atRule.after   = '';
    });
    css.eachComment(function (comment) {
        comment.removeSelf();
    });
});

var css = "a {\n  color:black\n}\n";
minifier.process(css).css //=> "a{color:black}"

Note that nodes may have not before or between properties:

• If node was created by hand via postcss.rule().
• node.clone() will clean all style properties to use the style for a new CSS root.

Raw Properties

Some CSS values (selectors, comment text, at-rule params and declaration values) can contain comments. PostCSS will clean them to remove trailing spaces:

var root = postcss.parse("a /**/ b {}");
var rule  = root.rules[0];

rule.selector      //=> 'a  b' trimmed and cleaned from comments
rule._selector.raw //=> 'a /**/ b' original raw value

But PostCSS preservers the raw content in order to stringify it back to CSS, in case you don’t change the original value. In general, PostCSS tries to preserve the original CSS byte-to-byte whenever possible:

rule.toString() //=> 'a /**/ b {}' with comment

rule.selector = '.link b';
rule.toString() //=> '.link b {}' you change value and origin comment was gone

Containers

Root, AtRule and Rule nodes can contain children in rules or decls properties.

There are some common methods to perform work on children:

• append(newChild) adds a child at the end of the children list.
• prepend(newChild) adds a child at the beginning of the children list.
• insertBefore(existsChild, newChild) inserts a new child before a pre-existing child.
• insertAfter(existsChild, newChild) inserts a new child after some pre-existing child.
• remove(existsChild) removes a child.
• index(existsChild) returns a child’s index.
• some(fn) returns true if fn returns true for any child.
• every(fn) returns true if fn returns true for all children.

Methods append, prepend, insertBefore and insertAfter will also accept arrays and Root nodes as an argument.

Methods insertBefore, insertAfter and remove will accept child nodes or indexes as the existsChild argument. Note that providing a child index will result in the method completing much faster.

There are two shortcuts to provide the first and last child of a node:

rule.first //=> First declaration in rule
rule.last  //=> Last declaration in rule

Children

Comment, AtRule, Rule and Declaration nodes should be wrapped in other nodes.

All children contain a parent property which indicates the parent node:

rule.decls[0].parent == rule;

All children have a removeSelf() method:

rule.decls[0].removeSelf();

But invoking the remove(index) method on the parent is much faster:

rule.each(function (decl, i) {
    rule.remove(i);
});

Iterators

All parent nodes have an each method which allows you to iterate over its child nodes:

root = postcss.parse('a { color: black; display: none }');

root.each(function (rule, i) {
    if ( rule.type == 'rule' ) {
        console.log(rule.selector, i); // Will log "a 0"
    }
});

root.rules[0].each(function (decl, i) {
    if ( rule.type != 'comment' ) {
        console.log(decl.prop, i); // Will log "color 0" and "display 1"
    }
});

Unlike for {}-cycle construct or Array#forEach() this iterator is safe. So you can mutate the children during iteration and PostCSS will fix the current index:

rule.rules.forEach(function (decl, i) {
    rule.prepend( decl.clone() );
    // Will infinitely cycle as prepending the current declaration will
    // cause the second and successive indexes to interact with the
    // current declaration endlessly
});

rule.each(function (decl, i) {
    rule.prepend( decl.clone() );
    // Will work correctly (each declaration will be cloned only once),
    // because the iterator index will be recalculated only after the prepend
});

Because CSS has a nested structure, PostCSS also features a recursive iterator eachInside:

root.eachInside(function (node, i) {
    console.log(node.type + ' inside ' + node.parent.type);
});

There are also shortcuts so that you can recursively iterate over nodes of a specific type:

root.eachDecl(function (decl, i) {
    // Each declaration inside root
});

root.eachRule(function (rule, i) {
    // Each rule inside root and any nested at-rules
});

root.eachAtRule(function (atRule, i) {
    // Each at-rule inside root and any nested at-rules
});

root.eachComment(function (comment, i) {
    // Each comment inside root
})

You can break out from the iteration by returning false.

Root Node

Root node contains the entire CSS tree. Its children can only be Comment, AtRule, or Rule nodes in the rules property.

You can create a new root using the shortcut:

var root = postcss.root();

Method toString() stringifies the entire node tree and returns a CSS string:

root = postcss.parse(css);
root.toString() == css;

If PostCSS found previous source map, it will save all the relevant information within Root#prevMap:

root = postcss.parse(css);
if (root.prevMap && root.prevMap.inline) {
    console.log('Inlined map: ' + root.prevMap.annotation)
}

Comment Node

/* Block comment */

PostCSS creates Comment nodes only for comments found between rules or declarations. Comments found within selectors, at-rules params, or declaration values will be stored in the Raw property.

Comment has only one property: text which contains the trimmed text inside the comment.

comment.text //=> "Block comment"

You can create a new comment using a shortcut:

var comment = postcss.comment({ text: 'New comment' });

AtRule Node

@charset 'utf-8';

@font-face {
    font-family: 'Cool'
}

@media print {
    img { display: none }
}

AtRule has two own properties: name and params.

As illustrated above, some at-rules do not contain any children (for example, @charset or @import), some at-rules can only contain declarations (for example, @font-face or @page), but most of them can contain rules and nested at-rules (for example, @media, @keyframes and others).

The parser selects AtRule content type by its name. If you create an AtRule node manually, it will infer its content type by the first child added via the append or other methods:

var atRule = postcss.atRule({ name: '-x-animations' });
atRule.rules        //=> undefined
atRule.decls        //=> undefined

atRule.append( postcss.rule({ selector: 'from' }) );
atRule.rules.length //=> 1
atRule.decls        //=> undefined

You can create a new at-rule using a shortcut:

var atRule = postcss.atRule({ name: 'charset', params: 'utf-8' });

Rule Node

a {
    color: black;
}

Rule nodes have a selector property and contain their Declaration and Comment children within the decls property.

They also possess a selectors shortcut, which returns an array:

rule.selector  //=> "a, b"
rule.selectors //=> ['a', 'b']

You can avoid using the Declaration constructor for append and other insert methods, by:

rule.append({ prop: 'color', value: 'black' });

The property semicolon indicates if the last declaration within the rule has a semicolon or not:

var root = postcss.parse('a { color: black }');
root.rules[0].semicolon //=> false

var root = postcss.parse('a { color: black; }');
root.rules[0].semicolon //=> true

You can create a new rule using a shortcut:

var rule = postcss.rule({ selector: 'a' });

Declaration Node

color: black

Declaration nodes have prop, value and important properties.

You can create a new declaration using a shortcut:

var decl = postcss.decl({ prop: 'color', value: 'black' });

Or you can use the short form available via a rule’s append() and other add methods:

rule.append({ prop: 'color', value: 'black' });