less-loader

What is less-loader?

The less-loader npm package is a loader for webpack that allows you to preprocess Less files into CSS. This enables developers to write styles in Less, a dynamic preprocessor style language that extends CSS with dynamic behavior such as variables, mixins, operations, and functions, and compile them into native CSS files that can be loaded into a web page.

What are less-loader's main functionalities?

Compiling Less to CSS

This feature allows you to compile Less files into CSS. The code sample shows how to configure webpack to use less-loader along with css-loader and style-loader to process `.less` files.

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

Importing Other Less or CSS Files

less-loader supports importing other Less or CSS files within a Less file. This feature enables code reusability and modularization. The code sample demonstrates importing a 'library' file and using a variable from it.

@import 'library';

.body {
  color: @baseColor;
}

Customizing Less Options

This feature allows the customization of Less compiler options through less-loader. The code sample demonstrates how to enable strict math and disable IE compatibility mode by configuring less-loader options in webpack.

module.exports = {
  module: {
    rules: [{
      test: /\.less$/,
      use: [
        'style-loader',
        'css-loader',
        {
          loader: 'less-loader',
          options: {
            lessOptions: {
              strictMath: true,
              noIeCompat: true
            }
          }
        }
      ]
    }]
  }
};

Other packages similar to less-loader

sass-loader

sass-loader is similar to less-loader but for compiling Sass/SCSS files into CSS. While less-loader focuses on Less files, sass-loader provides functionality for Sass, another popular CSS preprocessor language. Both loaders serve a similar purpose of integrating CSS preprocessing into webpack's build process.

stylus-loader

stylus-loader is designed for processing Stylus files into CSS, similar to how less-loader works with Less files. Stylus is another CSS preprocessor that offers syntax and features comparable to Less and Sass. The choice between less-loader, sass-loader, and stylus-loader typically depends on the specific preprocessor language a project uses.

postcss-loader

postcss-loader is a loader that allows you to use PostCSS, a tool for transforming CSS with JavaScript plugins. Unlike less-loader which is specific to Less, postcss-loader is more flexible and can be used to apply a wide range of transformations and optimizations to CSS files, making it a powerful tool in a developer's workflow.

README

less loader for webpack

Usage

Documentation: Using loaders

var css = require("!raw!less!./file.less");
// => returns compiled css code from file.less, resolves imports
var css = require("!css!less!./file.less");
// => returns compiled css code from file.less, resolves imports and url(...)s

Use in tandem with the style-loader to add the css rules to your document:

require("!style!css!less!./file.less");

webpack config

module.exports = {
  module: {
    loaders: [
      {
        test: /\.less$/,
        loader: "style-loader!css-loader!less-loader"
      }
    ]
  }
};

Then you only need to write: require("./file.less")

webpack config options

You can pass LESS specific configuration options through to the render function via loader parameters.

Acceptable config options that can be appended to the loader as parameters are:

paths, optimization, filename, strictImports, syncImport, dumpLineNumbers, relativeUrls, rootpath, compress, cleancss, cleancssOptions, ieCompat, strictMath, strictUnits, urlArgs, sourceMap, sourceMapFilename, sourceMapURL, sourceMapBasepath, sourceMapRootpath, outputSourceFiles'

module.exports = {
  module: {
    loaders: [
      {
        test: /\.less$/,
        loader: "style-loader!css-loader!less-loader?strictMath&cleancss"
      }
    ]
  }
};

Note on imports

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 or bower_components. Just prepend them with a ~ which tells webpack to look-up the modulesDirectories

@import "~bootstrap/less/bootstrap";

It's important to only prepend it with ~, because ~/ resolves to the home-directory. webpack needs to distinguish bootstrap from ~bootstrap because css- and less-files have no special syntax for importing relative files:

@import "file";

is the same as

@import "./file";

Contribution

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.

License

MIT (http://www.opensource.org/licenses/mit-license.php)

Changelog

12.1.0 (2024-01-19)

Features

• added the lessLogAsWarnOrErr option (#536) (3c4e6e0)