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 replace from "@rollup/plugin-replace";
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: {
					isProd: production
				}
			})
		}),

		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(),

		// Development
		// Serve once
		!production && serve(),

		// Watch `public` directory and refresh the browser for changes
		!production && livereload("public"),

		// Production
		// 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
				});
			}
		}
	};
}