Determine if resource was cached in browser using RT

was-served-from-browser-cache.js

function wasServedFromBrowserCache(url) {
  const {transferSize, decodedBodySize, duration} = performance.getEntriesByName(url)[0]
  
  // if we transferred bytes, it must not be a cache hit
  // (will return false for 304 Not Modified)
  if (transferSize > 0) return false;

  // if the body size is non-zero, it must mean this is a
  // ResourceTiming2 browser, this was same-origin or TAO,
  // and transferSize was 0, so it was in the cache
  if (decodedBodySize > 0) return true;

  // fall back to duration checking (non-RT2 or cross-origin)
  return duration < 30;
}