svelte.config.js

const sass = require('node-sass');

// related: https://github.com/UnwrittenFun/svelte-vscode/issues/1

module.exports = {
	preprocess: {
		style: async ({ content, attributes }) => {
			if (!['text/sass', 'text/scss'].some(attributes.type) && !['sass', 'scss'].some(attributes.lang)) return;

			return new Promise((resolve, reject) => {
				sass.render(
					{
						data: content,
						sourceMap: true,
						outFile: 'x', // this is necessary, but is ignored
					},
					(err, result) => {
						if (err) return reject(err);

						resolve({
							code: result.css.toString(),
							map: result.map.toString(),
						});
					},
				);
			});
		},
	},
};