Simple Service Worker to respond with cached request

sw.js

(function () {
  'use strict'
  self.addEventListener('fetch', event => {
    let request = event.request
    if (request.method !== 'GET') {
      return
    }
    event.respondWith(
        caches.match(request)
            .then(response => {
              return response || fetch(request)
            })
    )
  })
})()