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 {onBFCacheRestore} from './lib/bfcache.js';
import {bindReporter} from './lib/bindReporter.js';
import {doubleRAF} from './lib/doubleRAF.js';
import {getActivationStart} from './lib/getActivationStart.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 {runOnce} from './lib/runOnce.js';
import {whenActivated} from './lib/whenActivated.js';
import {whenIdle} from './lib/whenIdle.js';
import {LCPMetric, 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];

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

/**
 * 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,
) => {
  // Set defaults
  opts = opts || {};

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

    const handleEntries = (entries: LCPMetric['entries']) => {
      // If reportAllChanges is set then call this function for each entry,
      // otherwise only consider the last one.
      if (!opts!.reportAllChanges) {
        entries = entries.slice(-1);
      }

      entries.forEach((entry) => {
        // Only report if the page wasn't hidden prior to LCP.
        if (entry.startTime < visibilityWatcher.firstHiddenTime) {
          // 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.
          metric.value = Math.max(entry.startTime - getActivationStart(), 0);
          metric.entries = [entry];
          report();
        }
      });
    };

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

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

      const stopListening = runOnce(() => {
        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
      // stops LCP observation, it's unreliable since it can be programmatically
      // generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
      ['keydown', 'click'].forEach((type) => {
        // Wrap in a setTimeout so the callback is run in a separate task
        // to avoid extending the keyboard/click handler to reduce INP impact
        // https://github.com/GoogleChrome/web-vitals/issues/383
        addEventListener(type, () => whenIdle(stopListening), {
          once: true,
          capture: true,
        });
      });

      onHidden(stopListening);

      // Only report after a bfcache restore if the `PerformanceObserver`
      // successfully registered.
      onBFCacheRestore((event) => {
        metric = initMetric('LCP');
        report = bindReporter(
          onReport,
          metric,
          LCPThresholds,
          opts!.reportAllChanges,
        );

        doubleRAF(() => {
          metric.value = performance.now() - event.timeStamp;
          reportedMetricIDs[metric.id] = true;
          report(true);
        });
      });
    }
  });
};