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:
- parses CSS;
- provides a usable JS API to edit CSS node trees;
- dumps the modified node tree into a CSS string;
- 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.
- postcss-epub to prefix ePub3 properties.
Plugins
- postcss-calc to reduce
calc()
usage (recommanded withpostcss-custom-properties
). - postcss-color-function to transform
color()
function. - postcss-color-gray to transform
gray()
function. - postcss-color-hex-alpha to transform hexadecimal notations with alpha
(
#rrggbbaa
or#rgba
). - postcss-color-hwb to transform
hwb()
function. - postcss-color-rebeccapurple to transform
rebeccapurple
color. - postcss-import to transform
@import
rules by inlining content. - postcss-custom-media to add names for Media Queries.
- postcss-custom-properties to transform Custom Properties for cascading variables.
- 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 ofresult.css
.As a shortcut,
map { inline: true }
is equivalent tomap: '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 withprev: false
.This is a source map option which can be passed to
postcss.parse(css, opts)
. Other options can be passed to thetoResult(opts)
orprocess(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 theannotation
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 iffn
returns true for any child.every(fn)
returns true iffn
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' });