LESS Loader

npm install --save-dev less-loader less

Use less-loader in tandem with css-loader & style-loader to add LESS support for webpack.

Use the loader either via your webpack config, CLI or inline.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.less$/,
        use: [
          'style-loader',
          { loader: 'css-loader', options: { importLoaders: 1 } },
          'less-loader'
        ]
      }
    ]
  }
}

In your application

import css from 'file.less';

webpack --module-bind 'less=style-loader!css-loader!less-loader'

In your application

import css from 'file.less';

In your application

import css from 'style-loader!css-loader!less-loader!./file.less';

You can pass any LESS specific options to less-loader via loader options or query parameters.

See the LESS documentation for all available options. LESS translates dash-case to camelCase. Certain options which take values (e.g. lessc --modify-var="a=b") are better handled with the JSON Syntax

{
  test: /\.less$/,
  use: [
    'style-loader',
    { loader: 'css-loader', options: { importLoaders: 1 } },
    { loader: 'less-loader', options: { strictMath: true, noIeCompat: true } }
	]
}

In order to use plugins, simply set the options.lessPlugins-option on your webpack.config.js.

const CleanCSSPlugin = require('less-plugin-clean-css');

{
  test: /\.less$/,
  use: [
    'style-loader',
    { loader: 'css-loader', options: { importLoaders: 1 } },
    {
      loader: 'less-loader',
      options: { lessPlugins: [ new CleanCSSPlugin({ advanced: true }) ] }
    }
}

webpack provides an advanced mechanism to resolve files. The less-loader stubs less' fileLoader and passes all queries to the webpack resolving engine. Thus you can import your less-modules from node_modules. Just prepend them with a ~ which tells webpack to look-up the modules

@import "~bootstrap/less/bootstrap";

It's important to only prepend it with ~, because ~/ resolves to the home-directory. webpack needs to distinguish between bootstrap and ~bootstrap because css- and less-files have no special syntax for importing relative files. Writing @import "file" is the same as @import "./file";

Also please note that for CSS modules, relative file paths do not work as expected. Please see this issue for the explanation.

Because of browser limitations, sourcemaps are only available in conjunction with the extract-text-webpack-plugin. Use that plugin to extract the CSS code from the generated JS bundle into a separate file (which improves performance because JS and CSS are loaded in parallel).

webpack.config.js

const ExtractTextPlugin = require('extract-text-webpack-plugin');

module.exports = {
  // must be 'source-map' or 'inline-source-map'
  devtool: 'source-map',
  module: {
    rules: [
      {
        test: /\.less$/,
        use: ExtractTextPlugin.extract({
          fallbackLoader: 'style-loader',
          use: [
            // activate source maps via loader query
            {
              loader: 'css-loader',
              options: { sourceMap: true, importLoaders: 1 }
            },
            {
              loader: 'less-loader',
              options: { sourceMap: true }
            }
          ]
        })
      }
    ]
  },
  plugins: [
    // extract CSS into separate file
    new ExtractTextPlugin('app.bundle.css')
  ]
}

If you want to view the original LESS files inside Chrome and even edit it, there's a good blog post. Checkout test/sourceMap for a running example. Make sure to serve the content with an HTTP server.

Don't hesitate to create a pull request. Every contribution is appreciated. In development you can start the tests by calling npm test.

The tests are basically just comparing the generated css with a reference css-file located under test/css. You can easily generate a reference css-file by calling node test/helpers/generateCss.js <less-file-without-less-extension>. It passes the less-file to less and writes the output to the test/css-folder.

Johannes Ewald