rollup.config.js

import svelte from "rollup-plugin-svelte";
import preprocess from "svelte-preprocess";
import resolve from "@rollup/plugin-node-resolve";
import commonjs from "@rollup/plugin-commonjs";
import babel from "@rollup/plugin-babel";
import replace from "@rollup/plugin-replace";
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.env.NODE_ENV": JSON.stringify("production"),
    }),
    svelte({
      // help the parser understand language features you intent to use, typescript support goes here
      // https://github.com/sveltejs/svelte-preprocess#what-is-it
      preprocess: preprocess({
        babel: {
          presets: [
            [
              "@babel/preset-env",
              {
                loose: true,
                // No need for babel to resolve modules
                modules: false,
                targets: {
                  // ! Very important. Target es6+
                  esmodules: true,
                },
              },
            ],
          ],
          plugins: ["@babel/plugin-proposal-optional-chaining"],
        },
      }),
      // 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/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(),

    babel({
      extensions: [".js", ".mjs", ".html", ".svelte"],
      babelHelpers: "runtime",
      exclude: ["node_modules/@babel/**"],
      presets: [
        [
          "@babel/preset-env",
          {
            targets: "> 0.25%, not dead",
          },
        ],
      ],
      plugins: [
        "@babel/plugin-syntax-dynamic-import",
        "@babel/plugin-proposal-optional-chaining",
        [
          "@babel/plugin-transform-runtime",
          {
            useESModules: true,
          },
        ],
      ],
    }),

    // 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 server;

  function toExit() {
    if (server) server.kill(0);
  }

  return {
    writeBundle() {
      if (server) return;
      server = require("child_process").spawn("npm", ["run", "start", "--", "--dev"], {
        stdio: ["ignore", "inherit", "inherit"],
        shell: true,
      });

      process.on("SIGTERM", toExit);
      process.on("exit", toExit);
    },
  };
}