PostCSS

PostCSS is a framework for CSS postprocessors, to modify CSS by your JS function.

It takes care of most common CSS tool tasks:

1. parses CSS;
2. gives you usable JS API to edit CSS node tree;
3. saves modified node tree to new CSS;
4. generates (or modifies existent) source map for your changes;

You can use this framework to write you own:

• CSS minifier or beautifizer.
• Grunt plugin to generate sprites, include data-uri images or any other works.
• Text editor plugin to automate CSS routine.
• Command-line CSS tool.

Sponsored by Evil Martians.

Build with PostCSS

• Autoprefixer
• grunt-pixrem

Quick Example

Let’s fix 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 every ::before/::after rule

            // Did we forget 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 then CSS with forgotten content:

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 source map for its transformations:

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

And modifies source map from previous step (like Sass preprocessor):

var sassMap = fs.readFileSync('from.sass.map');
processor.process(css, { map: sassMap, from: 'from.sass.css', to: 'to.css' });

Preserves code formatting and indentations

PostCSS will not change any byte of rule if you don't modify node:

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

And when you modify CSS nodes, PostCSS will try to copy 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;
//   }

Why PostCSS Better Than …

Preprocessors

Preprocessors (like Sass or Stylus) give us special language with variables, mixins, statements and compile it to CSS. Compass, nib and other mixins libraries use this languages to work with prefixes, sprites and inline images.

But Sass and Stylus languages were created to be syntax-sugar for CSS. Writing really complicated programs using preporcessor languages is very difficult. Autoprefixer is absolutely impossible on Sass.

PostCSS gives you comfort and power of JS or CoffeeScript to working with CSS. You can do really magic things with wide range of npm libraries.

But postprocessors are not enemies for preprocessors. Sass and Stylus is still the best way to improve readability and add some syntax sugar to CSS. You can easily combine preprocessors and postprocessors.

RegExp

Some Grunt plugins modify CSS with regular expressions. But CSS parser and node tree are much safer way to edit CSS. Also regexps will break source maps generated by preprocessors.

CSS Parsers

There are a lot of good CSS parsers, like Gonzales. But they help you only with first step.

Unlike them PostCSS gives you useful high level API (for example, safe iterators) and changes source map generator (or modifier for existing source map from preprocessors).

Rework

Rework was a first CSS postprocessors framework. PostCSS is very similar to it.

But Rework has no high level API and rewrite your CSS code style and indentations. So it can’t be used in text editor plugins.

Unlike it PostCSS preserves all spaces and code formatting. If you don't change rule, output will be byte‑to‑byte equal.

Usage

Processor

Function postcss(fn) creates processor by your function:

var postcss = require('postcss');

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

If you want to combine multiple processors (and parse CSS only once), you can create empty processor and add several functions by use(fn) method:

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

Processor function can just change current CSS node tree:

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

or create totally new CSS root and return it:

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

Processor will transform some CSS by 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 = processor.process(css);

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

You can set original CSS filename by from options and make syntax error messages much more helpful:

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

Source Map

PostCSS will generate source map, if you set map option to true in process(css, opts) method.

You must set input and output CSS files paths (by from and to options) to generate correct map.

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

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

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

PostCSS can also modify previous source map (for example, from Sass compilation). So, if you compile: Sass to CSS and then minify CSS by postprocessor, final source map will contain mapping from Sass code to minified CSS.

Just set original source map content (as string or JS object) to map option:

var result = minifier.process(css, {
    map:   fs.readFileSync('main.sass.map'),
    from: 'main.sass.css',
    to:   'main.min.css'
});

result.map //=> Source map from main.sass to main.min.css

Nodes

Processor function will receive Root node with CSS node tree inside.

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

There are 3 types of child nodes: AtRule, Rule and Declaration. All nodes contain toString() and clone() methods.

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

var postcss = require('postcss');

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

Node Source

Every node stores it origin file (if you set from option to process or parse method) and position at source property:

var root = postcss.parse(css, { from: 'main.css' });
var rule = root.rules[1];

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

Whitespaces

All nodes (exclude Root) have before property with all earlier spaces and comments.

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

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

root.after                    //=> "\n" from end of file
root.rules[0].after           //=> "\n" before }
root.rules[0].decls[0].before //=> "\n  " before color: black

So, the simplest way to minify CSS is to clean before and after properties:

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

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

Raw Properties

Some CSS values (like selectors, at-rule params and declaration values) can contain comments. PostCSS will clean them for you:

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

ab.selector //=> 'a  b' trimmed and cleaned from comments

But PostCSS saves raw content to stringify it to CSS, if you don’t set new value. As you can remember, PostCSS tries to save origin CSS byte-to-byte, when it’s possible:

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

ab.selector = '.link b';
ab.toString() //=> '.link b' you change value and magic was gone

Containers

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

There are common method to work with children:

• append(newChild) to add child at the end of children list.
• prepend(newChild) to add child at the beginning of children list.
• insertBefore(existsChild, newChild) to insert new child before some existent children.
• insertAfter(existsChild, newChild) to insert new child after some existent children.
• remove(child) to remove child.
• index(child) to return child index.
• some(fn) to return true if fn return true on any child.
• every(fn) to return true if fn return true on all children.

Methods insertBefore, insertAfter and remove can receive child node or child index number as existent child argument. Have in mind that index works much faster.

Children

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

All children contain parent property with parent node:

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

All children has removeSelf() method:

rule.decls[0].removeSelf();

But remove(index) in parent with child index is much faster:

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

Iterators

All parent nodes have each method to iterate through children nodes:

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

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

root.rules[0].each(function (decl, i) {
    console.log(decl.prop, i); // Will log "color 0" and "display 1"
});

Instead of simple for or Array#forEach() this iterator is safe. You can change children inside iteration and it will fix current index:

rule.rules.forEach(function (decl, i) {
    rule.prepend( decl.clone() );
    // Will be infinity cycle, because on prepend current declaration become
    // second and next index will go to current declaration again
});

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

Because CSS is nested structure, PostCSS contains recursive iterator by node 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 Node

Root node contains all CSS tree. Its children can be only AtRule or Rule nodes in rules property.

You can create new root by shortcut:

var root = postcss.root();

Method toString() will stringify all current CSS:

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

AtRule Node

@charset 'utf-8';

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

@media print {
    img { display: none }
}

AtRule has two own properties: name and params.

As you see, some at-rules don’t contain any children (like @charset or @import), some of at-rules can contain only declarations (like @font-face or @page), but most of them can contain rules and nested at-rules (like @media, @keyframes and others).

Parser select AtRule content type by its name. If you create AtRule node manually, it will detect own content type with new child type on first append or other add method call:

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 new at-rule by shortcut:

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

Rule Node

a {
    color: black;
}

Rule node has selector property and contains Declaration children in decls property.

You can miss Declaration constructor in append and other insert methods:

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

Property semicolon marks does last declaration in rule has 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 new rule by shortcut:

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

Declaration Node

color: black

Declaration node has prop and value properties.

You can create new declaration by shortcut:

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

Or use short form in append() and other add methods:

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