onTTFB = (onReport, opts) => {
  // Set defaults
  // eslint-disable-next-line no-param-reassign
  opts = opts || {};

  // https://web.dev/ttfb/#what-is-a-good-ttfb-score
  // const thresholds = [800, 1800];

  const metric = initMetric.initMetric('TTFB');
  const report = bindReporter.bindReporter(onReport, metric, opts.reportAllChanges);

  whenReady(() => {
    const navEntry = getNavigationEntry.getNavigationEntry() ;

    if (navEntry) {
      // The activationStart reference is used because TTFB should be
      // relative to page activation rather than navigation start if the
      // page was prerendered. But in cases where `activationStart` occurs
      // after the first byte is received, this time should be clamped at 0.
      metric.value = Math.max(navEntry.responseStart - getActivationStart.getActivationStart(), 0);

      // In some cases the value reported is negative or is larger
      // than the current page time. Ignore these cases:
      // https://github.com/GoogleChrome/web-vitals/issues/137
      // https://github.com/GoogleChrome/web-vitals/issues/162
      if (metric.value < 0 || metric.value > performance.now()) return;

      metric.entries = [navEntry];

      report(true);
    }
  });
}
Calculates the [TTFB](https://web.dev/time-to-first-byte/) value for the
current page and calls the `callback` function once the page has loaded,
along with the relevant `navigation` performance entry used to determine the
value. The reported value is a `DOMHighResTimeStamp`.

Note, this function waits until after the page is loaded to call `callback`
in order to ensure all properties of the `navigation` entry are populated.
This is useful if you want to report on other metrics exposed by the
[Navigation Timing API](https://w3c.github.io/navigation-timing/). For
example, the TTFB metric starts from the page's [time
origin](https://www.w3.org/TR/hr-time-2/#sec-time-origin), which means it
includes time spent on DNS lookup, connection negotiation, network latency,
and server processing time.