window.performance usable metrics

window.performance.es6

let performance = window.performance;
let perfSupport = !!performance;

/**
 * Receive Serve, DomComplete and PageLoaded timings
 * @returns {Object}
 */
export function laodTimes() {
  if (!perfSupport) {
    return;
  }
  let t = performance.timing;
  let navi = performance.navigation;
  return {
    redirectCount: navi.redirectCount,
    //loadType: navi.type, // 0:user action(typing, link), 1:reload, 2: history move
    latency: t.responseEnd - t.fetchStart,
    serverTime: t.responseEnd - t.requestStart,
    domComplete: t.domComplete - t.responseEnd,
    pageLoad: t.loadEventEnd - t.responseEnd
  };
}

// This information could be stored using sendBeacon()
try {
  navigator.sendBeacon('/analytics', JSON.stringify(laodTimes()));
} catch {}