552 lines
15 KiB
Markdown
552 lines
15 KiB
Markdown
<div align="center">
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<a href="https://github.com/webpack/webpack">
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<img width="200" height="200" src="https://webpack.js.org/assets/icon-square-big.svg">
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</a>
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</div>
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[![npm][npm]][npm-url]
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[![node][node]][node-url]
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[![deps][deps]][deps-url]
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[![tests][tests]][tests-url]
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[![cover][cover]][cover-url]
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[![chat][chat]][chat-url]
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[![size][size]][size-url]
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# less-loader
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A Less loader for webpack. Compiles Less to CSS.
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## Getting Started
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To begin, you'll need to install `less-loader`:
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```console
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$ npm install less-loader --save-dev
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```
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Then add the loader to your `webpack` config. For example:
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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loader: 'less-loader', // compiles Less to CSS
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},
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],
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},
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};
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```
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And run `webpack` via your preferred method.
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## Options
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| Name | Type | Default | Description |
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| :-------------------------------------: | :------------------: | :----------------------: | :----------------------------------------------- |
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| **[`lessOptions`](#lessoptions)** | `{Object\|Function}` | `{ relativeUrls: true }` | Options for Less. |
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| **[`prependData`](#prependdata)** | `{String\|Function}` | `undefined` | Prepends Less code before the actual entry file. |
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| **[`appendData`](#appenddata)** | `{String\|Function}` | `undefined` | Prepends Less code after the actual entry file. |
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| **[`sourceMap`](#sourcemap)** | `{Boolean}` | `compiler.devtool` | Enables/Disables generation of source maps. |
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| **[`implementation`](#implementation)** | `{Object}` | `less` | Setup Less implementation to use. |
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### `lessOptions`
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Type: `Object|Function`
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Default: `{ relativeUrls: true }`
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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:
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#### `Object`
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Use an object to pass options through to Less.
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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{
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loader: 'style-loader',
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},
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{
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loader: 'css-loader',
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},
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{
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loader: 'less-loader',
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options: {
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lessOptions: {
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strictMath: true,
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},
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},
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},
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],
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},
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],
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},
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};
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```
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#### `Function`
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Allows setting the options passed through to Less based off of the loader context.
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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'css-loader',
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{
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loader: 'less-loader',
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options: {
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lessOptions: (loaderContext) => {
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// More information about available properties https://webpack.js.org/api/loaders/
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const { resourcePath, rootContext } = loaderContext;
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const relativePath = path.relative(rootContext, resourcePath);
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if (relativePath === 'styles/foo.less') {
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return {
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paths: ['absolute/path/c', 'absolute/path/d'],
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};
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}
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return {
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paths: ['absolute/path/a', 'absolute/path/b'],
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};
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},
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},
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},
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],
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},
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],
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},
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};
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```
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### `prependData`
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Type: `String|Function`
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Default: `undefined`
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Prepends `Less` code before the actual entry file.
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This is especially useful when some of your Less variables depend on the environment:
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> ℹ 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.
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#### `String`
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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'css-loader',
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{
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loader: 'less-loader',
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options: {
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prependData: `@env: ${process.env.NODE_ENV};`,
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},
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},
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],
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},
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],
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},
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};
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```
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#### `Function`
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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'css-loader',
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{
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loader: 'less-loader',
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options: {
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prependData: (loaderContext) => {
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// More information about available properties https://webpack.js.org/api/loaders/
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const { resourcePath, rootContext } = loaderContext;
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const relativePath = path.relative(rootContext, resourcePath);
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if (relativePath === 'styles/foo.less') {
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return '@value: 100px;';
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}
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return '@value: 200px;';
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},
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},
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},
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],
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},
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],
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},
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};
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```
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### `appendData`
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Type: `String|Function`
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Default: `undefined`
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AppendData `Less` code after the actual entry file.
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This can be useful when you need to rewrite some of your Less variables.:
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> ℹ 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.
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#### `String`
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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'css-loader',
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{
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loader: 'less-loader',
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options: {
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appendData: `@env: ${process.env.NODE_ENV};`,
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},
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},
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],
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},
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],
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},
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};
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```
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#### `Function`
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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'css-loader',
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{
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loader: 'less-loader',
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options: {
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appendData: (loaderContext) => {
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// More information about available properties https://webpack.js.org/api/loaders/
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const { resourcePath, rootContext } = loaderContext;
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const relativePath = path.relative(rootContext, resourcePath);
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if (relativePath === 'styles/foo.less') {
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return '@value: 100px;';
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}
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return '@value: 200px;';
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},
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},
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},
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],
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},
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],
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},
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};
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```
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### `sourceMap`
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Type: `Boolean`
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Default: depends on the `compiler.devtool` value
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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.
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/i,
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use: [
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'style-loader',
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{
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loader: 'css-loader',
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options: {
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sourceMap: true,
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},
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},
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{
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loader: 'less-loader',
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options: {
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sourceMap: true,
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},
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},
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],
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},
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],
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},
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};
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```
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### `implementation`
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Type: `Object`
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Default: `less`
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> ⚠ less-loader compatible with Less version 3 only
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The special `implementation` option determines which implementation of Less to use.
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The `implementation` options either accepts `less` as a module.
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This is useful if you want to use Less with a smaller version. Do not forget that then you must install your own version of Less.
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For example, to use custom Less implementation, you'd pass:
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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'css-loader',
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{
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loader: 'less-loader',
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options: {
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implementation: require('less'),
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},
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},
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],
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},
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],
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},
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};
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```
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## Examples
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### Normal usage
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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.
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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{
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loader: 'style-loader', // creates style nodes from JS strings
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},
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{
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loader: 'css-loader', // translates CSS into CommonJS
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},
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{
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loader: 'less-loader', // compiles Less to CSS
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},
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],
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},
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],
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},
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};
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```
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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.
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### Source maps
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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`.
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**webpack.config.js**
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```javascript
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module.exports = {
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devtool: 'source-map', // any "source-map"-like devtool is possible
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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'style-loader',
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{
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loader: 'css-loader',
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options: {
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sourceMap: true,
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},
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},
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{
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loader: 'less-loader',
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options: {
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sourceMap: true,
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},
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},
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],
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},
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],
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},
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};
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```
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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.
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### In production
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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.
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### Imports
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Starting with `less-loader` 4, you can now choose between Less' builtin resolver and webpack's resolver. By default, webpack's resolver is used.
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#### webpack resolver
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webpack provides an [advanced mechanism to resolve files](https://webpack.js.org/configuration/resolve/). The `less-loader` applies a Less plugin that passes all queries to the webpack resolver. Thus you can import your Less modules from `node_modules`. Just prepend them with a `~` which tells webpack to look up the [`modules`](https://webpack.js.org/configuration/resolve/#resolve-modules).
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```css
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@import '~bootstrap/less/bootstrap';
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```
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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";`
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##### Non-Less imports
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Using webpack's resolver, you can import any file type. You just need a loader that exports valid Less code. Often, you will also want to set the `issuer` condition to ensure that this rule is only applied on imports originating from Less files:
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```js
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// webpack.config.js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.js$/,
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issuer: /\.less$/,
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use: [
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{
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loader: 'js-to-less-loader',
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},
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],
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},
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],
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},
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};
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```
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#### Less resolver
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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:
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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use: [
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{
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loader: 'style-loader',
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},
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{
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loader: 'css-loader',
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},
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{
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loader: 'less-loader',
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options: {
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lessOptions: {
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paths: [path.resolve(__dirname, 'node_modules')],
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},
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},
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},
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],
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},
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],
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},
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};
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```
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### Plugins
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In order to use [plugins](http://lesscss.org/usage/#plugins), simply set the
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`plugins` option like this:
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```js
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// webpack.config.js
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const CleanCSSPlugin = require('less-plugin-clean-css');
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module.exports = {
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...
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{
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loader: 'less-loader',
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options: {
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lessOptions: {
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plugins: [
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new CleanCSSPlugin({ advanced: true }),
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],
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},
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},
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},
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...
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};
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```
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### Extracting style sheets
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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.
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There are two possibilities to extract a style sheet from the bundle:
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- [`extract-loader`](https://github.com/peerigon/extract-loader) (simpler, but specialized on the css-loader's output)
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- [MiniCssExtractPlugin](https://github.com/webpack-contrib/mini-css-extract-plugin) (more complex, but works in all use-cases)
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### CSS modules gotcha
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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).
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|
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## Contributing
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||
|
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Please take a moment to read our contributing guidelines if you haven't yet done so.
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|
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[CONTRIBUTING](./.github/CONTRIBUTING.md)
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||
|
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## License
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||
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[MIT](./LICENSE)
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|
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[npm]: https://img.shields.io/npm/v/less-loader.svg
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[npm-url]: https://npmjs.com/package/less-loader
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[node]: https://img.shields.io/node/v/less-loader.svg
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[node-url]: https://nodejs.org
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[deps]: https://david-dm.org/webpack-contrib/less-loader.svg
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[deps-url]: https://david-dm.org/webpack-contrib/less-loader
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[tests]: https://github.com/webpack-contrib/less-loader/workflows/less-loader/badge.svg
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[tests-url]: https://github.com/webpack-contrib/less-loader/actions
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[cover]: https://codecov.io/gh/webpack-contrib/less-loader/branch/master/graph/badge.svg
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[cover-url]: https://codecov.io/gh/webpack-contrib/less-loader
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[chat]: https://img.shields.io/badge/gitter-webpack%2Fwebpack-brightgreen.svg
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[chat-url]: https://gitter.im/webpack/webpack
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[size]: https://packagephobia.now.sh/badge?p=less-loader
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[size-url]: https://packagephobia.now.sh/result?p=less-loader
|