rollup.config.js

import svelte from 'rollup-plugin-svelte';
import replace from '@rollup/plugin-replace';
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';

const production = !process.env.ROLLUP_WATCH;

export default {
    input: 'src/main.js',
    output: {
        sourcemap: true,
        format: 'iife',
        name: 'app',
        file: 'public/build/bundle.js',
    },
    plugins: [
        replace({
            process: JSON.stringify({
                env: {
                    NODE_ENV: production ? 'production' : 'development',
                    isProd: production,
                },
            }),
            __SVELTER_DEV__: true
        }),

        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');
            },
        }),      

        // 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(),

        // 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,
                });
            }
        },
    };
}