including eslint() so at buildtime all code has to pass .eslintrc.js

rollup.config.js

// @ts-nocheck //TODO: enable remove once there are typings in place
// TODO: uncomment all three eslint() occurrences once eslint warnings and errors are fixed
// for info on emitCSS etc. https://github.com/rollup/rollup-plugin-svelte#extracting-css and https://svelte.dev/docs#svelte_preprocess

import babel from 'rollup-plugin-babel';
import commonjs from 'rollup-plugin-commonjs';
import config from 'sapper/config/rollup.js';
import getPreprocessor from 'svelte-preprocess';
import path from 'path';
import postcss from 'rollup-plugin-postcss';
import replace from 'rollup-plugin-replace';
import resolve from 'rollup-plugin-node-resolve';
import svelte from 'rollup-plugin-svelte';
import { eslint } from 'rollup-plugin-eslint';
import { terser } from 'rollup-plugin-terser';
import pkg from './package.json';

const mode = process.env.NODE_ENV;
const dev = mode === 'development';
const legacy = !!process.env.SAPPER_LEGACY_BUILD;

class SvelteExtractor {
  static extract(content) {
    return content.match(/[A-Za-z0-9-_:/]+/g) || [];
  }
}

const postcssPlugins = (purgecss = false) =>
  [
    require('postcss-import')(),
    require('postcss-url')(),
    require('tailwindcss')('./tailwind.config.js'),
    require('autoprefixer')({
      overrideBrowserslist: 'last 1 version',
    }),
    // Do not purge the CSS in dev mode to be able to play with classes in the browser dev-tools.
    purgecss &&
      require('@fullhuman/postcss-purgecss')({
        content: [
          './src/**/*.html',
          './src/**/*.svelte',
          './src/**/*.css',
          './static/**/*.css',
        ],
        extractors: [
          {
            extractor: SvelteExtractor,

            // Specify the file extensions to include when scanning for
            // class names.
            extensions: ['svelte', 'css', 'html'],
          },
        ],
        fontFace: true,
        // Whitelist selectors to stop Purgecss from removing them from your CSS.
        whitelist: ['html', 'body'],
      }),
    purgecss &&
      require('cssnano')({
        preset: 'default',
      }),
  ].filter(Boolean);

const preprocess = getPreprocessor({
  transformers: {
    postcss: {
      plugins: postcssPlugins(), // Don't need purgecss because Svelte handle unused css for you.
    },
  },
});

const onwarn = (warning, onwarn) =>
  (warning.code === 'CIRCULAR_DEPENDENCY' &&
    /[/\\]@sapper[/\\]/.test(warning.message)) ||
  onwarn(warning);
const dedupe = importee =>
  importee === 'svelte' || importee.startsWith('svelte/');

export default {
  client: {
    input: config.client.input(),
    output: config.client.output(),
    plugins: [
      // eslint(),
      replace({
        'process.browser': true,
        'process.env.NODE_ENV': JSON.stringify(mode),
      }),
      svelte({
        dev,
        hydratable: true,
        emitCss: false,
        preprocess,
      }),
      resolve({
        browser: true,
        dedupe,
      }),
      commonjs(),
      legacy &&
        babel({
          extensions: ['.js', '.mjs', '.html', '.svelte'],
          runtimeHelpers: true,
          exclude: ['node_modules/@babel/**'],
          presets: [
            [
              '@babel/preset-env',
              {
                targets: '> 0.25%, not dead',
              },
            ],
          ],
          plugins: [
            '@babel/plugin-syntax-dynamic-import',
            [
              '@babel/plugin-transform-runtime',
              {
                useESModules: true,
              },
            ],
          ],
        }),
      !dev &&
        terser({
          module: true,
        }),
    ],
    onwarn,
  },

  server: {
    input: config.server.input(),
    output: config.server.output(),
    plugins: [
      // eslint(),
      replace({
        'process.browser': false,
        'process.env.NODE_ENV': JSON.stringify(mode),
      }),
      svelte({
        generate: 'ssr',
        dev,
        preprocess,
      }),
      resolve({
        dedupe,
      }),
      commonjs(),
      postcss({
        plugins: postcssPlugins(!dev),
        extract: path.resolve(__dirname, './static/global.css'),
      }),
    ],
    external: Object.keys(pkg.dependencies).concat(
      require('module').builtinModules ||
        Object.keys(process.binding('natives'))
    ),
    onwarn,
  },

  serviceworker: {
    input: config.serviceworker.input(),
    output: config.serviceworker.output(),
    plugins: [
      // eslint(),
      resolve(),
      replace({
        'process.browser': true,
        'process.env.NODE_ENV': JSON.stringify(mode),
      }),
      commonjs(),
      !dev && terser(),
    ],
    onwarn,
  },
};