onFID.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 {onBFCacheRestore} from './lib/bfcache.js';
import {bindReporter} from './lib/bindReporter.js';
import {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
import {initMetric} from './lib/initMetric.js';
import {observe} from './lib/observe.js';
import {onHidden} from './lib/onHidden.js';
import {firstInputPolyfill, resetFirstInputPolyfill} from './lib/polyfills/firstInputPolyfill.js';
import {FIDMetric, FirstInputPolyfillCallback, ReportCallback, ReportOpts} from './types.js';

/**
 * Calculates the [FID](https://web.dev/fid/) value for the current page and
 * calls the `callback` function once the value is ready, along with the
 * relevant `first-input` performance entry used to determine the value. The
 * reported value is a `DOMHighResTimeStamp`.
 *
 * _**Important:** since FID is only reported after the user interacts with the
 * page, it's possible that it will not be reported for some page loads._
 */
export const onFID = (onReport: ReportCallback, opts?: ReportOpts) => {
  // Set defaults
  opts = opts || {};

  // https://web.dev/fid/#what-is-a-good-fid-score
  const thresholds = [100, 300];

  const visibilityWatcher = getVisibilityWatcher();
  let metric = initMetric('FID');
  let report: ReturnType<typeof bindReporter>;

  const handleEntry = (entry: PerformanceEventTiming) => {
    // Only report if the page wasn't hidden prior to the first input.
    if (entry.startTime < visibilityWatcher.firstHiddenTime) {
      metric.value = entry.processingStart - entry.startTime;
      metric.entries.push(entry);
      report(true);
    }
  }

  const handleEntries = (entries: FIDMetric['entries']) => {
    (entries as PerformanceEventTiming[]).forEach(handleEntry);
  }

  const po = observe('first-input', handleEntries);
  report = bindReporter(onReport, metric, thresholds, opts.reportAllChanges);

  if (po) {
    onHidden(() => {
      handleEntries(po.takeRecords() as FIDMetric['entries']);
      po.disconnect();
    }, true);
  }

  if (window.__WEB_VITALS_POLYFILL__) {
    console.warn('The web-vitals "base+polyfill" build is deprecated. See: https://bit.ly/3aqzsGm');

    // Prefer the native implementation if available,
    if (!po) {
      window.webVitals.firstInputPolyfill(handleEntry as FirstInputPolyfillCallback)
    }
    onBFCacheRestore(() => {
      metric = initMetric('FID');
      report = bindReporter(
          onReport, metric, thresholds, opts!.reportAllChanges);

      window.webVitals.resetFirstInputPolyfill();
      window.webVitals.firstInputPolyfill(handleEntry as FirstInputPolyfillCallback);
    });
  } else {
    // Only monitor bfcache restores if the browser supports FID natively.
    if (po) {
      onBFCacheRestore(() => {
        metric = initMetric('FID');
        report = bindReporter(
            onReport, metric, thresholds, opts!.reportAllChanges);

        resetFirstInputPolyfill();
        firstInputPolyfill(handleEntry as FirstInputPolyfillCallback);
      });
    }
  }
};