rollup.config.js

import svelte from 'rollup-plugin-svelte'
import resolve from '@rollup/plugin-node-resolve'
import commonjs from '@rollup/plugin-commonjs'
import livereload from 'rollup-plugin-livereload'
import { terser } from 'rollup-plugin-terser'
import sveltePreprocessPostcss from 'svelte-preprocess-postcss'
import postcss from 'rollup-plugin-postcss'
import autoPreprocess from 'svelte-preprocess'
import path  from 'path';

const stylePreprocessor = sveltePreprocessPostcss({useConfigFile: true})
const production = !process.env.ROLLUP_WATCH;
const postcssOptions = () => ({
  extensions: ['.scss', '.sass'],
  extract: false,
  minimize: true,
  use: [
    ['sass', {
      includePaths: [
        './src/theme',
        './node_modules',
        // This is only needed because we're using a local module. :-/
        // Normally, you would not need this line.
        path.resolve(__dirname, '..', 'node_modules')
      ]
    }]
  ]
});

export default {
	input: 'src/main.js',
	output: {
		sourcemap: true,
		format: 'iife',
		name: 'app',
		file: 'public/build/bundle.js'
	},
	plugins: [
		svelte({
			// enable run-time checks when not in production
			dev: !production,
			// we'll extract any component CSS out into
			// a separate file - better for performance
			css: css => {
				css.write('public/build/bundle.css');
			},
			preprocess: {
      style: stylePreprocessor
    },
      
		}),

		// If you have external dependencies installed from
		// npm, you'll most likely need these plugins. In
		// some cases you'll need additional configuration -
		// consult the documentation for details:
		// https://github.com/rollup/plugins/tree/master/packages/commonjs
		resolve({
			browser: true,
			dedupe: ['svelte']
		}),
		commonjs(),

		// In dev mode, call `npm run start` once
		// the bundle has been generated
		!production && serve(),
		postcss(postcssOptions()),
		// Watch the `public` directory and refresh the
		// browser on changes when not in production
		!production && livereload('public'),

		// If we're building for production (npm run build
		// instead of npm run dev), minify
		production && terser()
	],
	watch: {
		clearScreen: false
	}
};

function serve() {
	let started = false;

	return {
		writeBundle() {
			if (!started) {
				started = true;

				require('child_process').spawn('npm', ['run', 'start', '--', '--dev'], {
					stdio: ['ignore', 'inherit', 'inherit'],
					shell: true
				});
			}
		}
	};
}