getLCP.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';
import { getActivationStart } from './lib/getActivationStart';
import { getVisibilityWatcher } from './lib/getVisibilityWatcher';
import { initMetric } from './lib/initMetric';
import { observe } from './lib/observe';
import { onHidden } from './lib/onHidden';
import type { LCPMetric, ReportCallback } from './types';

const reportedMetricIDs: Record<string, boolean> = {};

/**
 * Calculates the [LCP](https://web.dev/lcp/) value for the current page and
 * calls the `callback` function once the value is ready (along with the
 * relevant `largest-contentful-paint` performance entry used to determine the
 * value). The reported value is a `DOMHighResTimeStamp`.
 */
export const onLCP = (onReport: ReportCallback): void => {
  const visibilityWatcher = getVisibilityWatcher();
  const metric = initMetric('LCP');
  let report: ReturnType<typeof bindReporter>;

  const handleEntries = (entries: LCPMetric['entries']): void => {
    const lastEntry = entries[entries.length - 1] as LargestContentfulPaint;
    if (lastEntry) {
      // The startTime attribute returns the value of the renderTime if it is
      // not 0, and the value of the loadTime otherwise. The activationStart
      // reference is used because LCP should be relative to page activation
      // rather than navigation start if the page was prerendered.
      const value = Math.max(lastEntry.startTime - getActivationStart(), 0);

      // Only report if the page wasn't hidden prior to LCP.
      if (value < visibilityWatcher.firstHiddenTime) {
        metric.value = value;
        metric.entries = [lastEntry];
        report();
      }
    }
  };

  const po = observe('largest-contentful-paint', handleEntries);

  if (po) {
    report = bindReporter(onReport, metric);

    const stopListening = (): void => {
      if (!reportedMetricIDs[metric.id]) {
        handleEntries(po.takeRecords() as LCPMetric['entries']);
        po.disconnect();
        reportedMetricIDs[metric.id] = true;
        report(true);
      }
    };

    // Stop listening after input. Note: while scrolling is an input that
    // stop LCP observation, it's unreliable since it can be programmatically
    // generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
    ['keydown', 'click'].forEach(type => {
      addEventListener(type, stopListening, { once: true, capture: true });
    });

    onHidden(stopListening, true);
  }
};