A css linter for immutable selectors
Clone or download
Latest commit 1179af1 Jul 22, 2016



Build Status js-standard-style

Best practices suggest avoiding overriding styles from vendor libraries to prevent unwanted side effects. Base library styles should not be altered – or as Harry Roberts describes, base styles should be treated as Immutable CSS.

See the interactive web app.


npm install --save immutable-css


immutableCss.processFiles(immutableSourceCss, customCss, options) takes two stylesheet paths, and ensures the custom CSS doesn't override any selectors contained within the immutable source. This is typically best when comparing vendor CSS (Bootstrap, Tachyons, Basscss, etc.) to your app's customizations.

var immutableCss = require('immutable-css')

immutableCss.processFiles('css/vendor.css', 'css/app.css')
// => [...]

immutableCss.processGlob(cssGlob, options) takes a glob that matches CSS files and ensures that no stylesheet overrides selectors contained within another. This is useful to ensure that CSS partials aren't mixing concerns by mutating selectors contained within another file.

var immutableCss = require('immutable-css')

immutableCss.processGlob('src/css/**/*.css', { verbose: true })

Using with PostCSS

Immutable CSS detects mutations among files by leveraging PostCSS sourcemaps. It is also best used as a PostCSS plugin in tandem with postcss-import and postcss-reporter.

var fs = require('fs')
var postcss = require('postcss')
var import = require('postcss-import')
var reporter = require('postcss-reporter')
var immutableCss = require('immutable-css')

var css = fs.readFileSync('styles.css', 'utf8')

var mutations = postcss([import(), immutableCss(), reporter()])
                  .process(css, { from: 'styles.css' })

Using with Gulp

var gulp = require('gulp')
var postcss = require('gulp-postcss')
var import = require('postcss-import')
var reporter = require('postcss-reporter')
var immutableCss = require('immutable-css')

gulp.task('immutable', function () {
  var processors = [
    // If you want Immutable CSS to halt the gulp pipline if there are any warnings
    // then set throwError to true
    reporter({clearMessages: true, throwError: false})


@import 'basscss';

.button {}
.left {}
.something-else {}


⚠  .button was mutated 2 times
[line 93, col 1]: /css/basscss.css
[line 3, col 1]: /css/custom.css
⚠  .left was mutated 2 times
[line 291, col 1]: /css/basscss.css
[line 4, col 1]: /css/custom.css


  • strict (Boolean): Whether class mutations are allowed in the same file. Default: false.
  • ignoredClasses (Array): List of classes to ignore for mutation violations.
    Ex: ['.some-mutable-class']
  • immutableClasses (Array): List of classes to check against.
    Ex: ['.button', '.foobar']
  • immutablePrefixes (Array): List of prefix regexes that are immutable.
    Ex: [/\.u\-/, /\.util\-/]
  • callback (Function): Callback that receives a mutations object.
    Ex: function (mutations) { console.log(mutations) }
  • verbose (Boolean): Whether mutations are logged (defaults to true with PostCSS).

Using the callback

Immutable CSS accepts an optional callback, which returns the mutations hash. The key is the mutated class name, the value is an array of mutating filenames.

  immutableCss({ ignoredClasses: ['.button'] }, function(mutations) {
    // => { '.foobar': [] }
]).process(css, { from: cssFile })

Using the immutable-css-cli

npm i -g immutable-css-cli
immutable-css css/main.css
⚠  .button was mutated 2 times
[line 93, col 1]: /css/_basscss.css
[line 11, col 1]: /css/_custom.css
⚠  .left was mutated 2 times
[line 291, col 1]: /css/_basscss.css
[line 15, col 1]: /css/_custom.css



Related Reading




  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

Crafted with <3 by @jxnblk & @4lpine.

This package was initially generated with yeoman and the p generator.