sass-extract

Extract structured variables from your sass files with no effort. Have all your style variables defined in style files, while being able to use them in javascript for things that cannot be styled with css such as complex visualisations or other dynamic content.

If you are using webpack make sure to also check out the sass-extract-loader.

Demo of sass-extract using the sass-extract-loader

Installation
API
Extract options
Variable context
Data types
Plugins
What is sass-extract?
Requirements
Contributing
- Running tests

Installation

npm install --save node-sass sass-extract

Note that the node-sass compiler have to be installed as it is a peer dependency of sass-extract.

API

The API is deliberately kept very similar to that of node-sass. This is because sass-extract can be used as a replacement that will add variable extraction as an additional feature to compiling the sass into css.

render(compileOptions, extractOptions)

An augmented version of the node-sass render function that in addition to rendering css also extract sass variables into rendered.vars.

See node-sass for documentation of the compileOptions object.

To be able to extract variables across multiple files using the @import directive you need to provide either file or includePaths for import lookups.

const sassExtract = require('sass-extract');

sassExtract.render({
  file: 'path/to/my/styles.scss'
})
.then(rendered => {
  console.log(rendered.vars);
  console.log(rendered.css.toString());
});

renderSync(compileOptions, extractOptions)

A synchronous version of the render function.

const sassExtract = require('sass-extract');

const rendered = sassExtract.renderSync({
  file: 'path/to/my/styles.scss'
});

console.log(rendered.vars);
console.log(rendered.css.toString());

extract(rendered, { compileOptions, extractOptions })

Extract variables for a rendered sass files.

See node-sass for documentation of the compileOptions object.

Generally you will pass the same compileOptions to both node-sass for rendering and sass-extract for extraction.

To be able to extract variables across multiple files using the @import directive you need to provide either file or includePaths for import lookups.

const sass = require('node-sass');
const sassExtract = require('sass-extract');

const rendered = sass.renderSync({
  file: 'path/to/my/styles.scss'
});

sassExtract.extract(rendered, {
  file: 'path/to/my/styles.scss'
})
.then(vars => {
  console.log(vars);
});

extractSync(rendered, { compileOptions, extractOptions })

A synchronous version of the extract function.

const sass = require('node-sass');
const sassExtract = require('sass-extract');

const rendered = sass.renderSync({
  file: 'path/to/my/styles.scss'
});

const vars = sassExtract.extractSync(rendered, {
  file: 'path/to/my/styles.scss'
});

console.log(vars);

Extract options

Can be provided in all render or extract methods.

extractOptions.plugins: []: Provide any plugins to be used during the extraction. Provide a string '<plugin>' for the node module or bundled plugin name, or alternatively an object { plugin: '<plugin>', options: {} } to provide any options to the plugin. Note: A plugin instance can be passed directly instead of a plugin name which allows importing the plugin separately

Variable context

Sass features both global and local variables.

// style.scss

$globalVariable1: 123px; // global

div {
  $localVariable1: red; // Local
  $globalVariable2: 1em !global; // global
}

The extracted variables returned from sass-extract are namespaced by the context where they are declared, so global variables will be placed in vars.global;

Global variables

A global variable is accessible from anywhere within that file, as well as from files that @import the file where the variable is declared. A variable is considered global when it is declared outside of any selector, or if the annotation !global is included after the variable expression.

// styleA.scss
$a: 123px;
@import './styleB.scss';

// styleB.scss
$b: 456px;

// extracted variables of styleA.scss
{
  global: {
    $a: {/*...*/},
    $b: {/*...*/}
  }
}

Local variables

A local variable is only accessible from within the selector where it is declared and from children to that selector.

Currently sass-extract supports extraction of global variables only. Local variables based on selectors and APIs to utilize them is on the roadmap. Global variables is however covers the most likely use cases of this tool at this point, and thus made sense to be the fundamental first feature.

Overrides

Variables in sass can be overridden any number of times and across multiple files when using @import. A variable can only have one final value within its context, but intermediate values can be assigned to separate variables in order to be retained.

sass-extract will always extract the final computed value of a variable, no matter the number of overrides. This means however that variables can have multiple different expressions and be specified in multiple files, while still always having one value.

// styleA.scss
$a: 123px;
$b: $a;
@import './styleB.scss';

// styleB.scss
$a: 456px;

// extracted variables of styleA.scss
{
  global: {
    $a: { value: 456 },
    $b: { value: 123 }
  }
}

Data types

Sass has a few different data types that a variable can have. These are detected by sass-extract automatically and the structure of the result of a variable value will be adapted to the type of the variables. Below follows descriptions for how each type of variable is extracted.

General variable value structure

Each variable extracted is available under its context by a key which is the variable name as specified in the sass file.

// style.scss
$myVariable: 123px;

// Corresponding variable result object
{
  global: {
    $myVariable: {
      type: 'SassNumber',
      sources: [ 'path/to/style.scss' ],
      value: 123, // Extracted value
      unit: 'px', // Data type specific metadata
      declarations: [
        expression: '123px',
        flags: { default: false, global: false },
        in: 'path/to/style.scss',
        position: { cursor: 0, line: 1, column: 0 }
      ]
    }
  }
}

Each of the variable results will have the same general structure and potentially additional type specific fields.

Field	Description
`type`	A string describing the data type of the extracted variables
`sources`	An array of all file paths where this variables is declared
`declarations`	An array of declarations of the variable
`declarations[i].expression`	The raw expression of the variable definition
`declarations[i].flags`	Describes any present flags such as `!default` or `!global`
`declarations[i].in`	The file where the declaration was found
`declarations[i].position`	The exact position of the declaration in the file

Note that sources and expressions are both arrays, see Overrides for details about this.

SassString

$variable: 'string';

{
  type: 'SassString',
  value: 'string'
}

SassBoolean

$variable: true;

{
  type: 'SassBoolean',
  value: true
}

SassNull

$variable: null;

{
  type: 'SassNull',
  value: null
}

SassNumber

SassNumbers contains both the extracted number and their unit

$variable: 123px;

{
  type: 'SassNumber',
  value: 123,
  unit: 'px'
}

SassColor

SassColors contains extracted colors in both rgba and hex formats

$variable: #FF0000;

{
  type: 'SassColor',
  value: {
    r: 255,
    g: 0,
    b: 0,
    a: 1,
    hex: '#FF0000'
  }
}

Or alternatively

$variable: rgba(0, 255, 0, 0.5);

{
  type: 'SassColor',
  value: {
    r: 0,
    g: 255,
    b: 0,
    a: 0.5,
    hex: '#00FF00'
  }
}

SassList

SassLists contains recursive types as an array

$variable: 1px solid black;

{
  type: 'SassList',
  separator: ' ',
  value: [
    {
      type: 'SassNumber',
      value: 1,
      unit: 'px'
    },
    {
      type: 'SassString',
      value: 'solid'
    },
    {
      type: 'SassColor',
      value: {
        r: 0,
        g: 0,
        b: 0,
        a: 1,
        hex: '#000000'
      }
    }
  ]
}

Or when it's comma separated

$variable: tahoma, arial;

{
  type: 'SassList',
  separator: ',',
  value: [
    {
      type: 'SassString',
      value: 'tahoma'
    },
    {
      type: 'SassString',
      value: 'arial'
    }
  ]
}

SassMap

SassMaps contains recursive types as an object with matching field names

$variable: ( width: 10em, height: 5em );

{
  type: 'SassMap',
  value: {
    width: {
      type: 'SassNumber',
      value: 10,
      unit: 'em'
    },
    height: {
      type: 'SassNumber',
      value: 5,
      unit: 'em'
    }
  }
}

Plugins

sass-extract has plugin support which can be used to modify the behavior and output of the extraction process.

A plugin is a simple object containing a run(options) function. This will be called at each extraction run. The run function should return a plugin interface with a function for each stage of the process where the plugin want to be injected. Each hook can return a modified version of its input which will modify the final output. Plugins are run in order as a pipeline, and one plugin can modify the input of a plugin later in the chain. Plugins can store state between stages within the run function as it is called once for each extraction run. Plugins provided to the plugin through render or extract APIs will be passed as the only argument to the run function.

For examples see the bundled plugins in src/plugins.

Pluggable stages

postValue({ value, sassValue }) => { value, sassValue }: Executed for each extracted variable value with the computed value as well as the raw value provided by sass. The returned value object can be modified in order to change the output of that variable
postExtract(extractedVariables) => extractedVariables: Executed at the end of an extraction with the complete result. The return extractedVariables object can be modified in order to change the final result of the extraction

Bundled plugins

There are some bundled plugins that comes with the library. To use them simply require('sass-extract/lib/plugins/<plugin>') and add them to the plugin array of the extraction options, or specify them by module name such as { plugins: ['minimal'] }.

Plugin	Description
`serialize`	Get a serialized variant of each variable instead of a deconstructed object. E.g. `123px` is extracted as `{ value: 123px }` instead of the default `{ value: 123, unit: 'px' }`
`compact`	Remove all metadata about variables and only output the actual value for each variable
`minimal`	Combines serialize and compact to create a small serialized representation of the extracted variables
`filter`	Filter the results based on a combination of property names and/or variable types

Bundled plugin options

`filter`

Option field	Description
`except.props = ['$my-prop']`	Will remove any properties with a name in the provided array
`except.types = ['SassNumber']`	Will remove any properties with a type in the provided array
`only.props = ['$my-prop']`	Will remove any properties with a name not in the provided array
`only.types = ['SassNumber']`	Will remove any properties with a type not in the provided array

Note: Empty arrays are ignored and treated as no filtering should be applied for that selection

What is sass-extract?

sass-extract is a tool that compiles sass files into a JSON object containing its variables and their computed values. This can be very useful when you need to style something that cannot be styled by css, or when you need to know details about the styles in your javascript.

It is built on top of node-sass which is using the performant libsass compiler. sass-extract is using native features in libsass in order to extract computed variables from your stylesheet that will be identical to the values in the generated css. This means you can expect to get the correct extracted value of a variable like $myVariable: $multiplier * 200px / 10, even if the $multipler variable is a variable defined in a separate imported sass file.

The purpose of this library is to give you the option to keep all of your style information in style files as expected. Maybe you are rendering on a canvas and cannot use css for styling the content, then this library will help you out.

There are other solutions to the same problem such as placing some of your style variables in a JSON file an including them into the sass, but with sass-extract you can skip that additional complexity of separate files and preprocessing and just get the variables directly from the sass itself.