Example of building only sass with webpack for legacy project

webpack.config.js

const isProduction = process.env.NODE_ENV === 'production';

module.exports = {
    context: __dirname,
    entry: {
        styles: ['./sass/main.scss']
    },
    output: {
        filename: 'build.css'
    },
    module: {
        loaders: [{
            test: /\.scss$/,
            loader: isProduction
                ? 'css-loader!sass-loader'
                : 'css-loader?sourceMap!sass-loader?outputStyle=expanded&sourceMap=true&sourceMapContents=true'
        }]
    }
};

It puts a JS code to the build.css file, doesn't it?

I know this a bit old, but I had the same problem a few months ago of a JS file being generated for every CSS and I solved it using this plugin: https://github.com/fqborges/webpack-fix-style-only-entries#readme. This plugin removes the unnecessary JavaScript file.

Here is the version I made from @jakeguti's suggestion.
https://www.dotnetthailand.com/to-other-languages/typescript-for-c-sharp-developers/webpack-configuration-for-sass-only