fes.js/packages/fes-plugin-sass/src/index.js

export default (api) => {
    const {
        utils
    } = api;

    api.describe({
        key: 'sass',
        config: {
            schema(joi) {
                return joi.object({
                    implementation: joi.any(),
                    sassOptions: joi.object(),
                    prependData: joi.alternatives(joi.string(), joi.func()),
                    sourceMap: joi.boolean(),
                    webpackImporter: joi.boolean()
                });
            },
            default: {}
        }
    });

    api.chainWebpack((memo, { createCSSRule }) => {
        createCSSRule({
            lang: 'sass',
            test: /\.(sass|scss)(\?.*)?$/,
            loader: require.resolve('sass-loader'),
            options: utils.deepmerge(
                {
                    implementation: require('sass')
                },
                api.config.sass || {}
            )
        });
        return memo;
    });
};