sass-loader

Compiles Sass to CSS.
Use the css-loader or the raw-loader to turn it into a JS module and the ExtractTextPlugin to extract it into a separate file.

Looking for the webpack 1 loader? Check out the archive/webpack-1 branch.

Install

npm install sass-loader node-sass webpack --save-dev

The sass-loader requires node-sass and webpack as peerDependency. Thus you are able to control the versions accurately.

Examples

Chain the sass-loader with the css-loader and the style-loader to immediately apply all styles to the DOM.

// webpack.config.js
module.exports = {
	...
    module: {
        rules: [{
            test: /\.scss$/,
            use: [{
                loader: "style-loader" // creates style nodes from JS strings
            }, {
                loader: "css-loader" // translates CSS into CommonJS
            }, {
                loader: "sass-loader" // compiles Sass to CSS
            }]
        }]
    }
};

You can also pass options directly to node-sass by specifying an options property like this:

// webpack.config.js
module.exports = {
	...
    module: {
        rules: [{
            test: /\.scss$/,
            use: [{
                loader: "style-loader"
            }, {
                loader: "css-loader"
            }, {
                loader: "sass-loader",
                options: {
                    includePaths: ["absolute/path/a", "absolute/path/b"]
                }
            }]
        }]
    }
};

See node-sass for all available Sass options.

In production

Usually, it's recommended to extract the stylesheets into a dedicated file in production using the ExtractTextPlugin. This way your styles are not dependent on JavaScript:

const ExtractTextPlugin = require("extract-text-webpack-plugin");

const extractSass = new ExtractTextPlugin({
    filename: "[name].[contenthash].css",
    disable: process.env.NODE_ENV === "development"
});

module.exports = {
	...
    module: {
        rules: [{
            test: /\.scss$/,
            loader: extractSass.extract({
                loader: [{
                    loader: "css-loader"
                }, {
                    loader: "sass-loader"
                }],
                // use style-loader in development
                fallbackLoader: "style-loader"
            })
        }]
    },
    plugins: [
        extractSass
    ]
};

Usage

Imports

webpack provides an advanced mechanism to resolve files. The sass-loader uses node-sass' custom importer feature to pass all queries to the webpack resolving engine. Thus you can import your Sass modules from node_modules. Just prepend them with a ~ to tell webpack that this is not a relative import:

@import "~bootstrap/css/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 Sass files have no special syntax for importing relative files. Writing @import "file" is the same as @import "./file";

Environment variables

If you want to prepend Sass code before the actual entry file, you can simply set the data option. In this case, the sass-loader will not override the data option but just append the entry's content. This is especially useful when some of your Sass variables depend on the environment:

{
    loader: "sass-loader",
    options: {
        data: "$env: " + process.env.NODE_ENV + ";"
    }
}

Problems with `url(...)`

Since Sass/libsass does not provide url rewriting, all linked assets must be relative to the output.

If you're just generating CSS without passing it to the css-loader, it must be relative to your web root.
If you pass the generated CSS on to the css-loader, all urls must be relative to the entry-file (e.g. main.scss).

More likely you will be disrupted by this second issue. It is natural to expect relative references to be resolved against the .scss file in which they are specified (like in regular .css files). Thankfully there are a two solutions to this problem:

Add the missing url rewriting using the resolve-url-loader. Place it directly after the sass-loader in the loader chain.
Library authors usually provide a variable to modify the asset path. bootstrap-sass for example has an $icon-font-path. Check out this working bootstrap example.

Extracting stylesheets

Bundling CSS with webpack has some nice advantages like referencing images and fonts with hashed urls or hot module replacement in development. In production, on the other hand, it's not a good idea to apply your stylesheets depending on JS execution. Rendering may be delayed or even a FOUC might be visible. Thus it's often still better to have them as separate files in your final production build.

There are two possibilties to extract a stylesheet from the bundle:

extract-loader (simpler, but specialized on the css-loader's output)
extract-text-webpack-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 sass-loader and the css-loader. Your webpack.config.js should look like this:

module.exports = {
    ...
    devtool: "source-map", // any "source-map"-like devtool is possible
    module: {
        rules: [{
            test: /\.scss$/,
            use: [{
                loader: "style-loader"
            }, {
                loader: "css-loader", options: {
                    sourceMap: true
                }
            }, {
                loader: "sass-loader", options: {
                    sourceMap: true
                }
            }]
        }]
    }
};