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 { string } from 'rollup-plugin-string'
import css from 'rollup-plugin-css-only'

const production = !process.env.ROLLUP_WATCH

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)
    }
  }
}

export default [
  // Main app config
  {
    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('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(),

      css({ output: 'public/build/vendor.css' }),

      // 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
    }
  },

  // Worker config
  {
    input: 'src/bundler/index.js',
    output: {
      sourcemap: true,
      format: 'esm',
      name: 'app',
      file: 'public/workers/bundler.js'
    },
    plugins: [
      resolve({
        browser: true,
        dedupe: ['svelte']
      }),
      commonjs(),
      string({
        // Required to be specified
        include: '**/*.txt'

        // Undefined by default
        // exclude: []
      })
    ],
    watch: {
      clearScreen: false
    }
  }
]