web-vitals/src/onLCP.ts

Easily measure performance metrics in JavaScript

/*
 * 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 {LCPEntryManager} from './lib/LCPEntryManager.js';
import {getBFCacheRestoreTime, onBFCacheRestore} from './lib/bfcache.js';
import {bindReporter} from './lib/bindReporter.js';
import {doubleRAF} from './lib/doubleRAF.js';
import {getActivationStart} from './lib/getActivationStart.js';
import {checkSoftNavsEnabled, storeSoftNavEntry} from './lib/softNavs.js';
import {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
import {initMetric} from './lib/initMetric.js';
import {initUnique} from './lib/initUnique.js';
import {observe} from './lib/observe.js';
import {whenActivated} from './lib/whenActivated.js';
import {whenIdleOrHidden} from './lib/whenIdleOrHidden.js';
import type {
  LCPMetric,
  Metric,
  MetricRatingThresholds,
  ReportOpts,
} from './types.js';

/** Thresholds for LCP. See https://web.dev/articles/lcp#what_is_a_good_lcp_score */
export const LCPThresholds: MetricRatingThresholds = [2500, 4000];

/**
 * Calculates the [LCP](https://web.dev/articles/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`.
 *
 * If the `reportAllChanges` configuration option is set to `true`, the
 * `callback` function will be called any time a new `largest-contentful-paint`
 * performance entry is dispatched, or once the final value of the metric has
 * been determined.
 */
export const onLCP = (
  onReport: (metric: LCPMetric) => void,
  opts: ReportOpts = {},
) => {
  // As InteractionContentfulPaint entries used by soft navs can emit after
  // LCP is finalized, we need a flag to know to ignore them.
  let isFinalized = false;
  const softNavsEnabled = checkSoftNavsEnabled(opts);

  whenActivated(() => {
    let visibilityWatcher = getVisibilityWatcher();
    let metric = initMetric('LCP');
    let report: ReturnType<typeof bindReporter>;

    const lcpEntryManager = initUnique(opts, LCPEntryManager);

    const initNewLCPMetric = (
      navigation?: Metric['navigationType'],
      navigationId?: number,
      navigationInteractionId?: number,
      navigationURL?: string,
      navigationStartTime?: number,
    ) => {
      metric = initMetric(
        'LCP',
        -1,
        navigation,
        navigationId,
        navigationInteractionId,
        navigationURL,
        navigationStartTime,
      );
      report = bindReporter(
        onReport,
        metric,
        LCPThresholds,
        opts.reportAllChanges,
      );
      // Reset the finalized flag
      isFinalized = false;
      // If it's a soft nav, then need to reset the visibilityWatcher
      if (navigation === 'soft-navigation') {
        visibilityWatcher = getVisibilityWatcher(true);
      }
    };

    const handleSoftNavEntry = (entry: PerformanceSoftNavigation) => {
      if (lcpEntryManager._softNavigationEntryMap && entry.navigationId) {
        storeSoftNavEntry(lcpEntryManager._softNavigationEntryMap, entry);
      }

      if (!isFinalized) report(true);
      initNewLCPMetric(
        'soft-navigation',
        entry.navigationId,
        entry.interactionId,
        entry.name,
        entry.startTime,
      );
      // Soft Navs should contain the largest paint until now, so handle that
      // as if it just happened, then listen for more.
      // It can however be null in rare circumstances
      // (see https://github.com/GoogleChrome/web-vitals/issues/725)
      const largestInteractionContentfulPaint =
        entry.getLargestInteractionContentfulPaint?.();
      if (largestInteractionContentfulPaint) {
        handleEntries([largestInteractionContentfulPaint]);
      }
    };

    const handleEntries = (
      entries: (
        | LargestContentfulPaint
        | InteractionContentfulPaint
        | PerformanceSoftNavigation
      )[],
    ) => {
      // If reportAllChanges is set or soft navs is enabled then call this
      // function for each entry, otherwise only consider the last one.
      if (!opts.reportAllChanges && !softNavsEnabled) {
        entries = entries.slice(-1);
      }

      for (const entry of entries) {
        if (!entry) continue;

        if (entry.entryType === 'soft-navigation') {
          handleSoftNavEntry(entry as PerformanceSoftNavigation);
          continue;
        }

        let value = 0;
        let metricEntries: LargestContentfulPaint[] = [];
        let renderTime = entry.startTime;
        if (entry.entryType === 'largest-contentful-paint') {
          // 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. But in cases
          // where `activationStart` occurs after the LCP, this time should be
          // clamped at 0.
          value = Math.max(entry.startTime - getActivationStart(), 0);

          lcpEntryManager._processEntry(entry as LargestContentfulPaint);
          metricEntries = [entry as LargestContentfulPaint];
        } else if (entry.entryType === 'interaction-contentful-paint') {
          const ICPEntry = entry as InteractionContentfulPaint;
          // InteractionContentfulPaints should only happen after a
          // PerformanceSoftNavigation so the metric should have been set
          // with a non-zero navigationId mapping to a soft nav.
          if (!metric.navigationId) continue;

          // Ignore interactions not for this soft nav
          // (either paints that have bled into this interaction or paints when
          // we should have already finalized)
          if (
            'interactionId' in ICPEntry &&
            ICPEntry.interactionId != metric.navigationInteractionId
          ) {
            continue;
          }

          renderTime = ICPEntry.largestContentfulPaint?.renderTime || 0;

          // Paints should never be less than 0 but add cap just in case
          value = Math.max(renderTime - entry.startTime, 0);

          if (ICPEntry.largestContentfulPaint) {
            lcpEntryManager._processEntry(ICPEntry.largestContentfulPaint);
            metricEntries = [ICPEntry.largestContentfulPaint];
          }
        }

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

    const types = ['largest-contentful-paint'] as (
      | 'largest-contentful-paint'
      | 'interaction-contentful-paint'
      | 'soft-navigation'
    )[];
    if (softNavsEnabled) {
      types.push('interaction-contentful-paint', 'soft-navigation');
    }
    const po = observe(types, handleEntries);

    if (po) {
      report = bindReporter(
        onReport,
        metric,
        LCPThresholds,
        opts.reportAllChanges,
      );

      const finalizeEventTypes = ['keydown', 'click', 'visibilitychange'];

      const finalizeLCP = (event: Event) => {
        if (event.isTrusted && !isFinalized) {
          // Wrap the listener in an idle callback so it's run in a separate
          // task to reduce potential INP impact.
          // https://github.com/GoogleChrome/web-vitals/issues/383
          const metricIdToFinalize = metric.id;
          whenIdleOrHidden(() => {
            if (!isFinalized) {
              if (!softNavsEnabled) {
                // Do some clean up since these won't be needed anymore.
                po!.disconnect();
                for (const type of finalizeEventTypes) {
                  removeEventListener(type, finalizeLCP, {capture: true});
                }
              }
              // As this is in a whenIdleOrHidden check, whether we're still
              // on the metric you meant to finalize, and ignore if we've moved
              // on in the meantime.
              if (metricIdToFinalize === metric.id) {
                isFinalized = true;
                report(true);
              }
            }
          });
        }
      };

      // Finalize the current LCP after input or visibilitychange.
      // Although the browser will automatically stop emitting entries in these
      // cases, we don't know it's finalized, so we track to allow early report.
      // Note: while scrolling is an input that stops LCP observation, it's
      // unreliable since it can be programmatically generated.
      // See: https://github.com/GoogleChrome/web-vitals/issues/75
      for (const type of finalizeEventTypes) {
        addEventListener(type, finalizeLCP, {
          capture: true,
        });
      }

      // Only report after a bfcache restore if the `PerformanceObserver`
      // successfully registered.
      onBFCacheRestore((event) => {
        initNewLCPMetric(
          'back-forward-cache',
          metric.navigationId,
          metric.navigationInteractionId,
          metric.navigationURL,
          getBFCacheRestoreTime(),
        );
        report = bindReporter(
          onReport,
          metric,
          LCPThresholds,
          opts.reportAllChanges,
        );

        doubleRAF(() => {
          metric.value = performance.now() - event.timeStamp;
          isFinalized = true;
          report(true);
        });
      });
    }
  });
};