title: JS: check if hidden image is still in browser's cache?
body: <p>Our web-page contains a 360 angle product viewer based on 36 product images. Only one image is showed at a time, all other are hidden. All images are served from AWS S3 bucket with no cache directive (and that is how it should be). On the first start JS plugin shows preloader for 36 images and
  everything works perfect after loading.</p>
  
  <p>Problem comes when the web tab (tested only in chrome) remains open for a long time (several hours) when user works in another tabs. Those hidden images are removing from the cache, and JS script reloads all of them again and 360 drug looks odd (not smooth). After browser loads all absent images it starts to work smooth again and after few hours of inactivity it repeats again.</p>
  
  <p>This behaviour is what we expect, but we want somehow to check if hidden images are not cached anymore to invoke preloader.</p>
  
  <p>I searched the web and stackoverflow for the answer, but all other cache related question are not answering my question, like "check if image is cached after reopening browser or cache". My question is: how to check if hidden image is still in cache?</p>
  
  <p>Example code appreciated. Thanks!</p>
  
  <p>PS - I know how to enable cache headers for images delivered from S3 and that is not an option.</p>
  
  <p>PSS - Creating 1px image holders is not an option too.</p>