onTTFB.ts

/*
 * Copyright 2020 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import {bindReporter} from './lib/bindReporter.js';
import {initMetric} from './lib/initMetric.js';
import {onBFCacheRestore} from './lib/bfcache.js';
import {getNavigationEntry} from './lib/getNavigationEntry.js';
import {ReportCallback, ReportOpts} from './types.js';
import { getActivationStart } from './lib/getActivationStart.js';


/**
 * Runs in the next task after the page is done loading and/or prerendering.
 * @param callback
 */
const whenReady = (callback: () => void) => {
  if (document.prerendering) {
    addEventListener('prerenderingchange', () => whenReady(callback), true);
  } else if (document.readyState !== 'complete') {
    addEventListener('load', () => whenReady(callback), true);
  } else {
    // Queue a task so the callback runs after `loadEventEnd`.
    setTimeout(callback, 0);
  }
}

export const onTTFB = (onReport: ReportCallback, opts?: ReportOpts) => {
  // Set defaults
  opts = opts || {};

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

  whenReady(() => {
    const navEntry = 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(), 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);
    }
  });

  onBFCacheRestore((event) => {
    metric = initMetric('TTFB');
    report = bindReporter(onReport, metric, opts!.reportAllChanges);
    metric.value = performance.now() - event.timeStamp;
    report(true);
  });
};