[![npm][npm]][npm-url] [![node][node]][node-url] [![deps][deps]][deps-url] [![tests][tests]][tests-url] [![cover][cover]][cover-url] [![chat][chat]][chat-url] [![size][size]][size-url] # less-loader A Less loader for webpack. Compiles Less to CSS. ## Getting Started To begin, you'll need to install `less` and `less-loader`: ```console npm install less less-loader --save-dev ``` or ```console yarn add -D less less-loader ``` or ```console pnpm add -D less less-loader ``` Then add the loader to your `webpack` config. For example: **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ // compiles Less to CSS "style-loader", "css-loader", "less-loader", ], }, ], }, }; ``` And run `webpack` via your preferred method. ## Options - **[`lessOptions`](#lessoptions)** - **[`additionalData`](#additionalData)** - **[`sourceMap`](#sourcemap)** - **[`webpackImporter`](#webpackimporter)** - **[`implementation`](#implementation)** ### `lessOptions` Type: ```ts type lessOptions = import('less').options | ((loaderContext: LoaderContext) => import('less').options}) ``` Default: `{ relativeUrls: true }` You can pass any Less specific options to the `less-loader` through the `lessOptions` property in the [loader options](https://webpack.js.org/configuration/module/#rule-options-rule-query). See the [Less documentation](http://lesscss.org/usage/#command-line-usage-options) for all available options in dash-case. Since we're passing these options to Less programmatically, you need to pass them in camelCase here: #### `object` Use an object to pass options through to Less. **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ { loader: "style-loader", }, { loader: "css-loader", }, { loader: "less-loader", options: { lessOptions: { strictMath: true, }, }, }, ], }, ], }, }; ``` #### `function` Allows setting the options passed through to Less based off of the loader context. ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { lessOptions: (loaderContext) => { // More information about available properties https://webpack.js.org/api/loaders/ const { resourcePath, rootContext } = loaderContext; const relativePath = path.relative(rootContext, resourcePath); if (relativePath === "styles/foo.less") { return { paths: ["absolute/path/c", "absolute/path/d"], }; } return { paths: ["absolute/path/a", "absolute/path/b"], }; }, }, }, ], }, ], }, }; ``` ### `additionalData` Type: ```ts type additionalData = | string | ((content: string, loaderContext: LoaderContext) => string); ``` Default: `undefined` Prepends/Appends `Less` code to the actual entry file. In this case, the `less-loader` will not override the source but just **prepend** the entry's content. This is especially useful when some of your Less variables depend on the environment: > Since you're injecting code, this will break the source mappings in your entry file. Often there's a simpler solution than this, like multiple Less entry files. #### `string` ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { additionalData: `@env: ${process.env.NODE_ENV};`, }, }, ], }, ], }, }; ``` #### `function` ##### `Sync` ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { additionalData: (content, loaderContext) => { // More information about available properties https://webpack.js.org/api/loaders/ const { resourcePath, rootContext } = loaderContext; const relativePath = path.relative(rootContext, resourcePath); if (relativePath === "styles/foo.less") { return "@value: 100px;" + content; } return "@value: 200px;" + content; }, }, }, ], }, ], }, }; ``` ##### `Async` ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { additionalData: async (content, loaderContext) => { // More information about available properties https://webpack.js.org/api/loaders/ const { resourcePath, rootContext } = loaderContext; const relativePath = path.relative(rootContext, resourcePath); if (relativePath === "styles/foo.less") { return "@value: 100px;" + content; } return "@value: 200px;" + content; }, }, }, ], }, ], }, }; ``` ### `sourceMap` Type: ```ts type sourceMap = boolean; ``` Default: depends on the `compiler.devtool` value By default generation of source maps depends on the [`devtool`](https://webpack.js.org/configuration/devtool/) option. All values enable source map generation except `eval` and `false` value. **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", { loader: "css-loader", options: { sourceMap: true, }, }, { loader: "less-loader", options: { sourceMap: true, }, }, ], }, ], }, }; ``` ### `webpackImporter` Type: ```ts type webpackImporter = boolean; ``` Default: `true` Enables/Disables the default `webpack` importer. This can improve performance in some cases. Use it with caution because aliases and `@import` at-rules starting with `~` will not work. **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { webpackImporter: false, }, }, ], }, ], }, }; ``` ### `implementation` Type: ```ts type implementation = object | string; ``` > less-loader compatible with Less 3 and 4 versions The special `implementation` option determines which implementation of Less to use. Overrides the locally installed `peerDependency` version of `less`. **This option is only really useful for downstream tooling authors to ease the Less 3-to-4 transition.** #### `object` **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { implementation: require("less"), }, }, ], }, ], }, }; ``` #### `string` **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ "style-loader", "css-loader", { loader: "less-loader", options: { implementation: require.resolve("less"), }, }, ], }, ], }, }; ``` ## Examples ### Normal usage Chain the `less-loader` with the [`css-loader`](https://github.com/webpack-contrib/css-loader) and the [`style-loader`](https://github.com/webpack-contrib/style-loader) to immediately apply all styles to the DOM. **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ { loader: "style-loader", // creates style nodes from JS strings }, { loader: "css-loader", // translates CSS into CommonJS }, { loader: "less-loader", // compiles Less to CSS }, ], }, ], }, }; ``` Unfortunately, Less doesn't map all options 1-by-1 to camelCase. When in doubt, [check their executable](https://github.com/less/less.js/blob/3.x/bin/lessc) and search for the dash-case option. ### Source maps To enable sourcemaps for CSS, you'll need to pass the `sourceMap` property in the loader's options. If this is not passed, the loader will respect the setting for webpack source maps, set in `devtool`. **webpack.config.js** ```js module.exports = { devtool: "source-map", // any "source-map"-like devtool is possible module: { rules: [ { test: /\.less$/i, use: [ "style-loader", { loader: "css-loader", options: { sourceMap: true, }, }, { loader: "less-loader", options: { sourceMap: true, }, }, ], }, ], }, }; ``` If you want to edit the original Less files inside Chrome, [there's a good blog post](https://medium.com/@toolmantim/getting-started-with-css-sourcemaps-and-in-browser-sass-editing-b4daab987fb0). The blog post is about Sass but it also works for Less. ### In production Usually, it's recommended to extract the style sheets into a dedicated file in production using the [MiniCssExtractPlugin](https://github.com/webpack-contrib/mini-css-extract-plugin). This way your styles are not dependent on JavaScript. ### Imports First we try to use built-in `less` resolve logic, then `webpack` resolve logic (aliases and `~`). #### Webpack Resolver `webpack` provides an [advanced mechanism to resolve files](https://webpack.js.org/configuration/resolve/). `less-loader` applies a Less plugin that passes all queries to the webpack resolver if `less` could not resolve `@import`. Thus you can import your Less modules from `node_modules`. ```css @import "bootstrap/less/bootstrap"; ``` Using `~` is deprecated and can be removed from your code (**we recommend it**), but we still support it for historical reasons. Why you can removed it? The loader will first try to resolve `@import` as relative, if it cannot be resolved, the loader will try to resolve `@import` inside [`node_modules`](https://webpack.js.org/configuration/resolve/#resolve-modules). Just prepend them with a `~` which tells webpack to look up the [`modules`](https://webpack.js.org/configuration/resolve/#resolve-modules). ```css @import "~bootstrap/less/bootstrap"; ``` Default resolver options can be modified by [`resolve.byDependency`](https://webpack.js.org/configuration/resolve/#resolvebydependency): **webpack.config.js** ```js module.exports = { devtool: "source-map", // any "source-map"-like devtool is possible module: { rules: [ { test: /\.less$/i, use: ["style-loader", "css-loader", "less-loader"], }, ], }, resolve: { byDependency: { // More options can be found here https://webpack.js.org/configuration/resolve/ less: { mainFiles: ["custom"], }, }, }, }; ``` 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";` #### Less Resolver If you specify the `paths` option, modules will be searched in the given `paths`. This is `less` default behavior. `paths` should be an array with absolute paths: **webpack.config.js** ```js module.exports = { module: { rules: [ { test: /\.less$/i, use: [ { loader: "style-loader", }, { loader: "css-loader", }, { loader: "less-loader", options: { lessOptions: { paths: [path.resolve(__dirname, "node_modules")], }, }, }, ], }, ], }, }; ``` ### Plugins In order to use [plugins](http://lesscss.org/usage/#plugins), simply set the `plugins` option like this: **webpack.config.js** ```js const CleanCSSPlugin = require('less-plugin-clean-css'); module.exports = { ... { loader: 'less-loader', options: { lessOptions: { plugins: [ new CleanCSSPlugin({ advanced: true }), ], }, }, }, ... }; ``` > ℹ️ Access to the [loader context](https://webpack.js.org/api/loaders/#the-loader-context) inside the custom plugin can be done using the `pluginManager.webpackLoaderContext` property. ```js module.exports = { install: function (less, pluginManager, functions) { functions.add("pi", function () { // Loader context is available in `pluginManager.webpackLoaderContext` return Math.PI; }); }, }; ``` ### Extracting style sheets Bundling CSS with webpack has some nice advantages like referencing images and fonts with hashed urls or [hot module replacement](https://webpack.js.org/concepts/hot-module-replacement/) in development. In production, on the other hand, it's not a good idea to apply your style sheets depending on JS execution. Rendering may be delayed or even a [FOUC](https://en.wikipedia.org/wiki/Flash_of_unstyled_content) might be visible. Thus it's often still better to have them as separate files in your final production build. There are two possibilities to extract a style sheet from the bundle: - [`extract-loader`](https://github.com/peerigon/extract-loader) (simpler, but specialized on the css-loader's output) - [`MiniCssExtractPlugin`](https://github.com/webpack-contrib/mini-css-extract-plugin) (more complex, but works in all use-cases) ### CSS modules gotcha There is a known problem with Less and [CSS modules](https://github.com/css-modules/css-modules) regarding relative file paths in `url(...)` statements. [See this issue for an explanation](https://github.com/webpack-contrib/less-loader/issues/109#issuecomment-253797335). ## Contributing Please take a moment to read our contributing guidelines if you haven't yet done so. [CONTRIBUTING](./.github/CONTRIBUTING.md) ## License [MIT](./LICENSE) [npm]: https://img.shields.io/npm/v/less-loader.svg [npm-url]: https://npmjs.com/package/less-loader [node]: https://img.shields.io/node/v/less-loader.svg [node-url]: https://nodejs.org [deps]: https://david-dm.org/webpack-contrib/less-loader.svg [deps-url]: https://david-dm.org/webpack-contrib/less-loader [tests]: https://github.com/webpack-contrib/less-loader/workflows/less-loader/badge.svg [tests-url]: https://github.com/webpack-contrib/less-loader/actions [cover]: https://codecov.io/gh/webpack-contrib/less-loader/branch/master/graph/badge.svg [cover-url]: https://codecov.io/gh/webpack-contrib/less-loader [chat]: https://img.shields.io/badge/gitter-webpack%2Fwebpack-brightgreen.svg [chat-url]: https://gitter.im/webpack/webpack [size]: https://packagephobia.now.sh/badge?p=less-loader [size-url]: https://packagephobia.now.sh/result?p=less-loader