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[![npm][npm]][npm-url]
[![node][node]][node-url]
[![deps][deps]][deps-url]
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[![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-loader`:

```console
$ npm install less-loader --save-dev
```

Then add the loader to your `webpack` config. For example:

**webpack.config.js**

```js
module.exports = {
  module: {
    rules: [
      {
        test: /\.less$/,
        loader: 'less-loader', // compiles Less to CSS
      },
    ],
  },
};
```

And run `webpack` via your preferred method.

The `less-loader` requires [less](https://github.com/less/less.js) as [`peerDependency`](https://docs.npmjs.com/files/package.json#peerdependencies).
Thus you are able to control the versions accurately.

## Examples

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$/,
        use: [
          {
            loader: 'style-loader', // creates style nodes from JS strings
          },
          {
            loader: 'css-loader', // translates CSS into CommonJS
          },
          {
            loader: 'less-loader', // compiles Less to CSS
          },
        ],
      },
    ],
  },
};
```

You can pass any Less specific options to the `less-loader` via [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:

**webpack.config.js**

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

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.

### 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

Starting with `less-loader` 4, you can now choose between Less' builtin resolver
and webpack's resolver. By default, webpack's resolver is used.

#### webpack resolver

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).

```css
@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";`

##### Non-Less imports

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:

```js
// webpack.config.js
module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        issuer: /\.less$/,
        use: [
          {
            loader: 'js-to-less-loader',
          },
        ],
      },
    ],
  },
};
```

#### Less resolver

If you specify the `paths` option, the `less-loader` will not use webpack's
resolver. Modules, that can't be resolved in the local folder, 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$/,
        use: [
          {
            loader: 'style-loader',
          },
          {
            loader: 'css-loader',
          },
          {
            loader: 'less-loader',
            options: {
              paths: [path.resolve(__dirname, 'node_modules')],
            },
          },
        ],
      },
    ],
  },
};
```

In this case, all webpack features like importing non-Less files or aliasing
won't work of course.

### Plugins

In order to use [plugins](http://lesscss.org/usage/#plugins), simply set the
`plugins` option like this:

```js
// webpack.config.js
const CleanCSSPlugin = require('less-plugin-clean-css');

module.exports = {
  ...
    {
      loader: 'less-loader', options: {
        plugins: [
          new CleanCSSPlugin({ advanced: true })
        ]
      }
    }]
  ...
};
```

### 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)

### Source maps

To enable CSS source maps, you'll need to pass the `sourceMap` option to the
`less-loader` _and_ the `css-loader`. Your `webpack.config.js` should look
like this:

**webpack.config.js**

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

Also checkout the [sourceMaps example](https://github.com/webpack-contrib/less-loader/tree/master/examples/sourceMaps).

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.

### 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://dev.azure.com/webpack-contrib/less-loader/_apis/build/status/webpack-contrib.less-loader?branchName=master
[tests-url]: https://dev.azure.com/webpack-contrib/less-loader/_build/latest?definitionId=2&branchName=master
[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
