generate-sw.js

const fs = require('fs');
const path = require('path');
const globby = require('globby');
const loadJsonFile = require('load-json-file');

const dotNext = path.resolve(__dirname, './.next');
const target = path.resolve(__dirname, './.next/service-worker.js');

function bundles(app) {
  return new Promise((resolve, reject) => {
    fs.readdir(`${dotNext}/bundles/pages`, (err, files) => {
      if (err) {
        resolve(app);
      }

      if (files) {
        const root = `/_next/${app.buildId}/page`;
        app.precaches = app.precaches.concat(
          files.filter(file => file !== 'index.js').map(file => {
            // req /_next/22321e97-8895-48db-b915-82e255f3faa8/new
            return path.join(root, file.replace(/.js$/, ''));
          })
        );
      }

      resolve(app);
    });
  });
}

function chunks(app) {
  return new Promise((resolve, reject) => {
    fs.readdir(`${dotNext}/chunks`, (err, files) => {
      if (err) {
        resolve(app);
      }

      if (files) {
        const root = `/_next/webpack/chunks`;
        app.precaches = app.precaches.concat(
          files.filter(file => /\.js$/.test(file)).map(file => {
            // req /_next/webpack/chunks/22321e97-8895-48db-b915-82e255f3faa8.js
            return path.join(root, file);
          })
        );
      }

      resolve(app);
    });
  });
}

function app() {
  const app = {
    buildId: fs.readFileSync(`${dotNext}/BUILD_ID`, 'utf8'),
    precaches: [],
  };

  return loadJsonFile(`${dotNext}/build-stats.json`).then(stats => {
    Object.keys(stats).map(src => {
      // /_next/9265fa769281943ee96625770433e573/app.js
      app.precaches.push(`/_next/${stats[src].hash}/${src}`);
    });

    return app;
  });
}

const swSnippet = precache =>
  `importScripts('https://unpkg.com/workbox-sw@0.0.2')
const workboxSW = new WorkboxSW({clientsClaim: true})
// set precahe listed item
workboxSW.precache(${precache})
// cache very first page by sw
workboxSW.router.registerRoute(
  '/',
  workboxSW.strategies.staleWhileRevalidate()
)
`;

app().then(chunks).then(bundles).then(app => {
  fs.writeFileSync(
    target,
    swSnippet(JSON.stringify(app.precaches, null, 2)),
    'utf8'
  );
});