sw.js

 self.addEventListener('fetch', function(event) {
  var request = event.request;
  if (request.method === "POST") {
    event.respondWith(
          // Try to POST form data to server
          fetch(event.request)
          .catch(function() {
          // If it doesn't work, post a message to reassure user
          self.clients.matchAll().then(function (clients){
            clients.forEach(function(client){
              client.postMessage({
                msg: "Post unsuccessful! Server will be updated when connection is re-established.",
                url: event.request.url
              });
            });
          });
        })
          )}
})