web-perf.js

// (c) Netflix UI Engineering
// https://www.youtube.com/watch?v=V8oTJ8OZ5S0
// window.performance.timing

// time to Interactive (TTI)
// domInteractive - requestStart

function getTimeToInteractive() {
  const requestStart = window.performance.timing.requestStart;
  const domInteractive = window.performance.timing.domInteractive;

  return domInteractive - requestStart;
}

// Resource Timing
function getResourceEntries() {
  return window.performance.getEntriesByType('resource') || [];
}

function getNavigationTiming() {
  const navs = window.performance.getEntriesByType("navigation");
  
  return navs && navs[0];
}

function getRoundTripLength() {
  const nav = getNavigationTiming();
  
  if (!nav) {
    return undefined;
  }
  
  return nav.responseStart - nav.fetchStart;
}

// will output
const willOutput = {
  // ...
  name: 'https://cdn.netflix.com/myBundle.js',
  durartion: 57.734999999,
  transferSize: 21790
  // ...
};