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clean-css is a fast and efficient CSS optimizer for Node.js platform and any modern browser.

According to tests it is one of the best available.

Table of Contents

Node.js version support

clean-css requires Node.js 4.0+ (tested on Linux, OS X, and Windows)

Install

npm install --save-dev clean-css

Use

var CleanCSS = require('clean-css');
var input = 'a{font-weight:bold;}';
var options = { /* options */ };
var output = new CleanCSS(options).minify(input);

Important: 4.0 breaking changes

clean-css 4.0 introduces some breaking changes:

What's new in version 4.1

clean-css 4.1 introduces the following changes / features:

What's new in version 4.2

clean-css 4.2 introduces the following changes / features:

Constructor options

clean-css constructor accepts a hash as a parameter with the following options available:

Compatibility modes

There is a certain number of compatibility mode shortcuts, namely:

Each of these modes is an alias to a fine grained configuration, with the following options available:

new CleanCSS({
  compatibility: {
    colors: {
      opacity: true // controls `rgba()` / `hsla()` color support
    },
    properties: {
      backgroundClipMerging: true, // controls background-clip merging into shorthand
      backgroundOriginMerging: true, // controls background-origin merging into shorthand
      backgroundSizeMerging: true, // controls background-size merging into shorthand
      colors: true, // controls color optimizations
      ieBangHack: false, // controls keeping IE bang hack
      ieFilters: false, // controls keeping IE `filter` / `-ms-filter`
      iePrefixHack: false, // controls keeping IE prefix hack
      ieSuffixHack: false, // controls keeping IE suffix hack
      merging: true, // controls property merging based on understandability
      shorterLengthUnits: false, // controls shortening pixel units into `pc`, `pt`, or `in` units
      spaceAfterClosingBrace: true, // controls keeping space after closing brace - `url() no-repeat` into `url()no-repeat`
      urlQuotes: false, // controls keeping quoting inside `url()`
      zeroUnits: true // controls removal of units `0` value
    },
    selectors: {
      adjacentSpace: false, // controls extra space before `nav` element
      ie7Hack: true, // controls removal of IE7 selector hacks, e.g. `*+html...`
      mergeablePseudoClasses: [':active', ...], // controls a whitelist of mergeable pseudo classes
      mergeablePseudoElements: ['::after', ...], // controls a whitelist of mergeable pseudo elements
      mergeLimit: 8191, // controls maximum number of selectors in a single rule (since 4.1.0)
      multiplePseudoMerging: true // controls merging of rules with multiple pseudo classes / elements (since 4.1.0)
    },
    units: {
      ch: true, // controls treating `ch` as a supported unit
      in: true, // controls treating `in` as a supported unit
      pc: true, // controls treating `pc` as a supported unit
      pt: true, // controls treating `pt` as a supported unit
      rem: true, // controls treating `rem` as a supported unit
      vh: true, // controls treating `vh` as a supported unit
      vm: true, // controls treating `vm` as a supported unit
      vmax: true, // controls treating `vmax` as a supported unit
      vmin: true // controls treating `vmin` as a supported unit
    }
  }
})

You can also use a string when setting a compatibility mode, e.g.

new CleanCSS({
  compatibility: 'ie9,-properties.merging' // sets compatibility to IE9 mode with disabled property merging
})

Fetch option

The fetch option accepts a function which handles remote resource fetching, e.g.

var request = require('request');
var source = '@import url(http://example.com/path/to/stylesheet.css);';
new CleanCSS({
  fetch: function (uri, inlineRequest, inlineTimeout, callback) {
    request(uri, function (error, response, body) {
      if (error) {
        callback(error, null);
      } else if (response && response.statusCode != 200) {
        callback(response.statusCode, null);
      } else {
        callback(null, body);
      }
    });
  }
}).minify(source);

This option provides a convenient way of overriding the default fetching logic if it doesn't support a particular feature, say CONNECT proxies.

Unless given, the default loadRemoteResource logic is used.

Formatting options

By default output CSS is formatted without any whitespace unless a format option is given. First of all there are two shorthands:

new CleanCSS({
  format: 'beautify' // formats output in a really nice way
})

and

new CleanCSS({
  format: 'keep-breaks' // formats output the default way but adds line breaks for improved readability
})

however format option also accept a fine-grained set of options:

new CleanCSS({
  format: {
    breaks: { // controls where to insert breaks
      afterAtRule: false, // controls if a line break comes after an at-rule; e.g. `@charset`; defaults to `false`
      afterBlockBegins: false, // controls if a line break comes after a block begins; e.g. `@media`; defaults to `false`
      afterBlockEnds: false, // controls if a line break comes after a block ends, defaults to `false`
      afterComment: false, // controls if a line break comes after a comment; defaults to `false`
      afterProperty: false, // controls if a line break comes after a property; defaults to `false`
      afterRuleBegins: false, // controls if a line break comes after a rule begins; defaults to `false`
      afterRuleEnds: false, // controls if a line break comes after a rule ends; defaults to `false`
      beforeBlockEnds: false, // controls if a line break comes before a block ends; defaults to `false`
      betweenSelectors: false // controls if a line break comes between selectors; defaults to `false`
    },
    breakWith: '\n', // controls the new line character, can be `'\r\n'` or `'\n'` (aliased as `'windows'` and `'unix'` or `'crlf'` and `'lf'`); defaults to system one, so former on Windows and latter on Unix
    indentBy: 0, // controls number of characters to indent with; defaults to `0`
    indentWith: 'space', // controls a character to indent with, can be `'space'` or `'tab'`; defaults to `'space'`
    spaces: { // controls where to insert spaces
      aroundSelectorRelation: false, // controls if spaces come around selector relations; e.g. `div > a`; defaults to `false`
      beforeBlockBegins: false, // controls if a space comes before a block begins; e.g. `.block {`; defaults to `false`
      beforeValue: false // controls if a space comes before a value; e.g. `width: 1rem`; defaults to `false`
    },
    wrapAt: false // controls maximum line length; defaults to `false`
  }
})

Inlining options

inline option whitelists which @import rules will be processed, e.g.

new CleanCSS({
  inline: ['local'] // default; enables local inlining only
})
new CleanCSS({
  inline: ['none'] // disables all inlining
})
// introduced in clean-css 4.1.0

new CleanCSS({
  inline: false // disables all inlining (alias to `['none']`)
})
new CleanCSS({
  inline: ['all'] // enables all inlining, same as ['local', 'remote']
})
new CleanCSS({
  inline: ['local', 'mydomain.example.com'] // enables local inlining plus given remote source
})
new CleanCSS({
  inline: ['local', 'remote', '!fonts.googleapis.com'] // enables all inlining but from given remote source
})

Optimization levels

The level option can be either 0, 1 (default), or 2, e.g.

new CleanCSS({
  level: 2
})

or a fine-grained configuration given via a hash.

Please note that level 1 optimization options are generally safe while level 2 optimizations should be safe for most users.

Level 0 optimizations

Level 0 optimizations simply means "no optimizations". Use it when you'd like to inline imports and / or rebase URLs but skip everything else.

Level 1 optimizations

Level 1 optimizations (default) operate on single properties only, e.g. can remove units when not required, turn rgb colors to a shorter hex representation, remove comments, etc

Here is a full list of available options:

new CleanCSS({
  level: {
    1: {
      cleanupCharsets: true, // controls `@charset` moving to the front of a stylesheet; defaults to `true`
      normalizeUrls: true, // controls URL normalization; defaults to `true`
      optimizeBackground: true, // controls `background` property optimizations; defaults to `true`
      optimizeBorderRadius: true, // controls `border-radius` property optimizations; defaults to `true`
      optimizeFilter: true, // controls `filter` property optimizations; defaults to `true`
      optimizeFont: true, // controls `font` property optimizations; defaults to `true`
      optimizeFontWeight: true, // controls `font-weight` property optimizations; defaults to `true`
      optimizeOutline: true, // controls `outline` property optimizations; defaults to `true`
      removeEmpty: true, // controls removing empty rules and nested blocks; defaults to `true`
      removeNegativePaddings: true, // controls removing negative paddings; defaults to `true`
      removeQuotes: true, // controls removing quotes when unnecessary; defaults to `true`
      removeWhitespace: true, // controls removing unused whitespace; defaults to `true`
      replaceMultipleZeros: true, // contols removing redundant zeros; defaults to `true`
      replaceTimeUnits: true, // controls replacing time units with shorter values; defaults to `true`
      replaceZeroUnits: true, // controls replacing zero values with units; defaults to `true`
      roundingPrecision: false, // rounds pixel values to `N` decimal places; `false` disables rounding; defaults to `false`
      selectorsSortingMethod: 'standard', // denotes selector sorting method; can be `'natural'` or `'standard'`, `'none'`, or false (the last two since 4.1.0); defaults to `'standard'`
      specialComments: 'all', // denotes a number of /*! ... */ comments preserved; defaults to `all`
      tidyAtRules: true, // controls at-rules (e.g. `@charset`, `@import`) optimizing; defaults to `true`
      tidyBlockScopes: true, // controls block scopes (e.g. `@media`) optimizing; defaults to `true`
      tidySelectors: true, // controls selectors optimizing; defaults to `true`,
      semicolonAfterLastProperty: false, // controls removing trailing semicolons in rule; defaults to `false` - means remove
      transform: function () {} // defines a callback for fine-grained property optimization; defaults to no-op
    }
  }
});

There is an all shortcut for toggling all options at the same time, e.g.

new CleanCSS({
  level: {
    1: {
      all: false, // set all values to `false`
      tidySelectors: true // turns on optimizing selectors
    }
  }
});

Level 2 optimizations

Level 2 optimizations operate at rules or multiple properties level, e.g. can remove duplicate rules, remove properties redefined further down a stylesheet, or restructure rules by moving them around.

Please note that if level 2 optimizations are turned on then, unless explicitely disabled, level 1 optimizations are applied as well.

Here is a full list of available options:

new CleanCSS({
  level: {
    2: {
      mergeAdjacentRules: true, // controls adjacent rules merging; defaults to true
      mergeIntoShorthands: true, // controls merging properties into shorthands; defaults to true
      mergeMedia: true, // controls `@media` merging; defaults to true
      mergeNonAdjacentRules: true, // controls non-adjacent rule merging; defaults to true
      mergeSemantically: false, // controls semantic merging; defaults to false
      overrideProperties: true, // controls property overriding based on understandability; defaults to true
      removeEmpty: true, // controls removing empty rules and nested blocks; defaults to `true`
      reduceNonAdjacentRules: true, // controls non-adjacent rule reducing; defaults to true
      removeDuplicateFontRules: true, // controls duplicate `@font-face` removing; defaults to true
      removeDuplicateMediaBlocks: true, // controls duplicate `@media` removing; defaults to true
      removeDuplicateRules: true, // controls duplicate rules removing; defaults to true
      removeUnusedAtRules: false, // controls unused at rule removing; defaults to false (available since 4.1.0)
      restructureRules: false, // controls rule restructuring; defaults to false
      skipProperties: [] // controls which properties won't be optimized, defaults to `[]` which means all will be optimized (since 4.1.0)
    }
  }
});

There is an all shortcut for toggling all options at the same time, e.g.

new CleanCSS({
  level: {
    2: {
      all: false, // sets all values to `false`
      removeDuplicateRules: true // turns on removing duplicate rules
    }
  }
});

Minify method

Once configured clean-css provides a minify method to optimize a given CSS, e.g.

var output = new CleanCSS(options).minify(source);

The output of the minify method is a hash with following fields:

console.log(output.styles); // optimized output CSS as a string
console.log(output.sourceMap); // output source map if requested with `sourceMap` option
console.log(output.errors); // a list of errors raised
console.log(output.warnings); // a list of warnings raised
console.log(output.stats.originalSize); // original content size after import inlining
console.log(output.stats.minifiedSize); // optimized content size
console.log(output.stats.timeSpent); // time spent on optimizations in milliseconds
console.log(output.stats.efficiency); // `(originalSize - minifiedSize) / originalSize`, e.g. 0.25 if size is reduced from 100 bytes to 75 bytes

The minify method also accepts an input source map, e.g.

var output = new CleanCSS(options).minify(source, inputSourceMap);

or a callback invoked when optimizations are finished, e.g.

new CleanCSS(options).minify(source, function (error, output) {
  // `output` is the same as in the synchronous call above
});

Promise interface

If you prefer clean-css to return a Promise object then you need to explicitely ask for it, e.g.

new CleanCSS({ returnPromise: true })
  .minify(source)
  .then(function (output) { console.log(output.styles); })
  .catch(function (error) { // deal with errors });

CLI utility

Clean-css has an associated command line utility that can be installed separately using npm install clean-css-cli. For more detailed information, please visit https://github.com/jakubpawlowicz/clean-css-cli.

FAQ

How to optimize multiple files?

It can be done either by passing an array of paths, or, when sources are already available, a hash or an array of hashes:

new CleanCSS().minify(['path/to/file/one', 'path/to/file/two']);
new CleanCSS().minify({
  'path/to/file/one': {
    styles: 'contents of file one'
  },
  'path/to/file/two': {
    styles: 'contents of file two'
  }
});
new CleanCSS().minify([
  {'path/to/file/one': {styles: 'contents of file one'}},
  {'path/to/file/two': {styles: 'contents of file two'}}
]);

Passing an array of hashes allows you to explicitly specify the order in which the input files are concatenated. Whereas when you use a single hash the order is determined by the traversal order of object properties - available since 4.1.0.

Important note - any @import rules already present in the hash will be resolved in memory.

How to process remote @imports correctly?

In order to inline remote @import statements you need to provide a callback to minify method as fetching remote assets is an asynchronous operation, e.g.:

var source = '@import url(http://example.com/path/to/remote/styles);';
new CleanCSS({ inline: ['remote'] }).minify(source, function (error, output) {
  // output.styles
});

If you don't provide a callback, then remote @imports will be left as is.

How to apply arbitrary transformations to CSS properties?

If clean-css doesn't perform a particular property optimization, you can use transform callback to apply it:

var source = '.block{background-image:url(/path/to/image.png)}';
var output = new CleanCSS({
  level: {
    1: {
      transform: function (propertyName, propertyValue, selector /* `selector` available since 4.2.0-pre */) {
        if (propertyName == 'background-image' && propertyValue.indexOf('/path/to') > -1) {
          return propertyValue.replace('/path/to', '../valid/path/to');
        }
      }
    }
  }
}).minify(source);

console.log(output.styles); # => .block{background-image:url(../valid/path/to/image.png)}

Note: returning false from transform callback will drop a property.

How to specify a custom rounding precision?

The level 1 roundingPrecision optimization option accept a string with per-unit rounding precision settings, e.g.

new CleanCSS({
  level: {
    1: {
      roundingPrecision: 'all=3,px=5'
    }
  }
}).minify(source)

which sets all units rounding precision to 3 digits except px unit precision of 5 digits.

How to keep a CSS fragment intact?

Note: available in the current master, to be released in 4.2.0.

Wrap the CSS fragment in special comments which instruct clean-css to preserve it, e.g.

.block-1 {
  color: red
}
/* clean-css ignore:start */
.block-special {
  color: transparent
}
/* clean-css ignore:end */
.block-2 {
  margin: 0
}

Optimizing this CSS will result in the following output:

.block-1{color:red}
.block-special {
  color: transparent
}
.block-2{margin:0}

How to preserve a comment block?

Use the /*! notation instead of the standard one /*:

/*!
  Important comments included in optimized output.
*/

How to rebase relative image URLs?

clean-css will handle it automatically for you in the following cases:

How to work with source maps?

To generate a source map, use sourceMap: true option, e.g.:

new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory })
  .minify(source, function (error, output) {
    // access output.sourceMap for SourceMapGenerator object
    // see https://github.com/mozilla/source-map/#sourcemapgenerator for more details
});

You can also pass an input source map directly as a 2nd argument to minify method:

new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory })
  .minify(source, inputSourceMap, function (error, output) {
    // access output.sourceMap to access SourceMapGenerator object
    // see https://github.com/mozilla/source-map/#sourcemapgenerator for more details
});

or even multiple input source maps at once:

new CleanCSS({ sourceMap: true, rebaseTo: pathToOutputDirectory }).minify({
  'path/to/source/1': {
    styles: '...styles...',
    sourceMap: '...source-map...'
  },
  'path/to/source/2': {
    styles: '...styles...',
    sourceMap: '...source-map...'
  }
}, function (error, output) {
  // access output.sourceMap as above
});

How to apply level 1 & 2 optimizations at the same time?

Using the hash configuration specifying both optimization levels, e.g.

new CleanCSS({
  level: {
    1: {
      all: true,
      normalizeUrls: false
    },
    2: {
      restructureRules: true
    }
  }
})

will apply level 1 optimizations, except url normalization, and default level 2 optimizations with rule restructuring.

What level 2 optimizations do?

All level 2 optimizations are dispatched here, and this is what they do:

How to use clean-css with build tools?

There is a number of 3rd party plugins to popular build tools:

How to use clean-css from web browser?

Contributing

See CONTRIBUTING.md.

How to get started?

First clone the sources:

git clone git@github.com:jakubpawlowicz/clean-css.git

then install dependencies:

cd clean-css
npm install

then use any of the following commands to verify your copy:

npm run bench # for clean-css benchmarks (see [test/bench.js](https://github.com/jakubpawlowicz/clean-css/blob/master/test/bench.js) for details)
npm run browserify # to create the browser-ready clean-css version
npm run check # to lint JS sources with [JSHint](https://github.com/jshint/jshint/)
npm test # to run all tests

Acknowledgments

Sorted alphabetically by GitHub handle:

License

clean-css is released under the MIT License.