327 lines
8.7 KiB
Markdown
327 lines
8.7 KiB
Markdown
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<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$/,
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loader: 'less-loader', // compiles Less to CSS
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},
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],
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},
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};
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```
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And run `webpack` via your preferred method.
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The `less-loader` requires [less](https://github.com/less/less.js) as [`peerDependency`](https://docs.npmjs.com/files/package.json#peerdependencies).
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Thus you are able to control the versions accurately.
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## Examples
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Chain the `less-loader` with the
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[`css-loader`](https://github.com/webpack-contrib/css-loader) and the
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[`style-loader`](https://github.com/webpack-contrib/style-loader) to immediately
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apply all styles to the DOM.
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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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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You can pass any Less specific options to the `less-loader` via [loader options](https://webpack.js.org/configuration/module/#rule-options-rule-query).
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See the [Less documentation](http://lesscss.org/usage/#command-line-usage-options)
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for all available options in dash-case. Since we're passing these options to
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Less programmatically, you need to pass them in camelCase here:
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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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strictMath: true,
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noIeCompat: 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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Unfortunately, Less doesn't map all options 1-by-1 to camelCase. When in doubt,
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[check their executable](https://github.com/less/less.js/blob/3.x/bin/lessc)
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and search for the dash-case option.
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### In production
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Usually, it's recommended to extract the style sheets into a dedicated file in
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production using the
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[MiniCssExtractPlugin](https://github.com/webpack-contrib/mini-css-extract-plugin).
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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
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and webpack's resolver. By default, webpack's resolver is used.
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#### webpack resolver
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webpack provides an
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[advanced mechanism to resolve files](https://webpack.js.org/configuration/resolve/).
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The `less-loader` applies a Less plugin that passes all queries to the webpack
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resolver. Thus you can import your Less modules from `node_modules`. Just
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prepend them with a `~` which tells webpack to look up the
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[`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
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home-directory. webpack needs to distinguish between `bootstrap` and
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`~bootstrap`, because CSS and Less files have no special syntax for importing
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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
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that exports valid Less code. Often, you will also want to set the `issuer`
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condition to ensure that this rule is only applied on imports originating from
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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$/,
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use: [
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{
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loader: 'js-to-less-loader',
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},
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],
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},
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],
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},
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};
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```
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#### Less resolver
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If you specify the `paths` option, the `less-loader` will not use webpack's
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resolver. Modules, that can't be resolved in the local folder, will be searched
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in the given `paths`. This is Less' default behavior. `paths` should be an array
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with absolute paths:
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**webpack.config.js**
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```js
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module.exports = {
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module: {
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rules: [
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{
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test: /\.less$/,
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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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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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In this case, all webpack features like importing non-Less files or aliasing
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won't work of course.
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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', options: {
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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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### Extracting style sheets
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Bundling CSS with webpack has some nice advantages like referencing images and
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fonts with hashed urls or
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[hot module replacement](https://webpack.js.org/concepts/hot-module-replacement/)
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in development. In production, on the other hand, it's not a good idea to apply
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your style sheets depending on JS execution. Rendering may be delayed or even a
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[FOUC](https://en.wikipedia.org/wiki/Flash_of_unstyled_content) might be visible.
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Thus it's often still better to have them as separate files in your final
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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
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specialized on the css-loader's output)
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- [MiniCssExtractPlugin](https://github.com/webpack-contrib/mini-css-extract-plugin)
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(more complex, but works in all use-cases)
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### Source maps
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To enable CSS source maps, you'll need to pass the `sourceMap` option to the
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`less-loader` _and_ the `css-loader`. Your `webpack.config.js` should look
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like this:
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**webpack.config.js**
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```javascript
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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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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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Also checkout the [sourceMaps example](https://github.com/webpack-contrib/less-loader/tree/master/examples/sourceMaps).
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If you want to edit the original Less files inside Chrome,
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[there's a good blog post](https://medium.com/@toolmantim/getting-started-with-css-sourcemaps-and-in-browser-sass-editing-b4daab987fb0).
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The blog post is about Sass but it also works for Less.
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### CSS modules gotcha
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There is a known problem with Less and
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[CSS modules](https://github.com/css-modules/css-modules) regarding relative
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file paths in `url(...)` statements.
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[See this issue for an explanation](https://github.com/webpack-contrib/less-loader/issues/109#issuecomment-253797335).
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## Contributing
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Please take a moment to read our contributing guidelines if you haven't yet done so.
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[CONTRIBUTING](./.github/CONTRIBUTING.md)
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## License
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[MIT](./LICENSE)
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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://dev.azure.com/webpack-contrib/less-loader/_apis/build/status/webpack-contrib.less-loader?branchName=master
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[tests-url]: https://dev.azure.com/webpack-contrib/less-loader/_build/latest?definitionId=2&branchName=master
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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
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