Nice website for doing the diff:

http://railsdiff.org/4.2.10/5.0.0

class PostsController < ApplicationController
  ...  
  def index    
    respond_to do |format|
      format.html
      format.turbo_stream { @pagy, @posts = pagy_countless(Post.all) }    
    end  
  end
  ...

Notification prompting can only be done from a user gesture.

async function startServiceWorker() {
  const vapidPublicKey = document.head
    .querySelector('meta[name="vapid_public_key"]')
    .getAttribute('content');
  await registerServiceWorker();

  const subscription = await getPushSubscription(vapidPublicKey);
  if (!subscription) return;

  await sendSubscriptionToServer(subscription);
}