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99 lines
4.7 KiB
99 lines
4.7 KiB
2 months ago
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/*
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* Copyright 2020 Google LLC
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* https://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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import { onBFCacheRestore } from './lib/bfcache.js';
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import { bindReporter } from './lib/bindReporter.js';
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import { doubleRAF } from './lib/doubleRAF.js';
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import { getActivationStart } from './lib/getActivationStart.js';
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import { getVisibilityWatcher } from './lib/getVisibilityWatcher.js';
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import { initMetric } from './lib/initMetric.js';
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import { observe } from './lib/observe.js';
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import { onHidden } from './lib/onHidden.js';
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import { runOnce } from './lib/runOnce.js';
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import { whenActivated } from './lib/whenActivated.js';
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/** Thresholds for LCP. See https://web.dev/lcp/#what-is-a-good-lcp-score */
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export const LCPThresholds = [2500, 4000];
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const reportedMetricIDs = {};
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/**
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* Calculates the [LCP](https://web.dev/lcp/) value for the current page and
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* calls the `callback` function once the value is ready (along with the
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* relevant `largest-contentful-paint` performance entry used to determine the
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* value). The reported value is a `DOMHighResTimeStamp`.
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*
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* If the `reportAllChanges` configuration option is set to `true`, the
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* `callback` function will be called any time a new `largest-contentful-paint`
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* performance entry is dispatched, or once the final value of the metric has
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* been determined.
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*/
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export const onLCP = (onReport, opts) => {
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// Set defaults
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opts = opts || {};
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whenActivated(() => {
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const visibilityWatcher = getVisibilityWatcher();
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let metric = initMetric('LCP');
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let report;
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const handleEntries = (entries) => {
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const lastEntry = entries[entries.length - 1];
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if (lastEntry) {
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// Only report if the page wasn't hidden prior to LCP.
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if (lastEntry.startTime < visibilityWatcher.firstHiddenTime) {
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// The startTime attribute returns the value of the renderTime if it is
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// not 0, and the value of the loadTime otherwise. The activationStart
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// reference is used because LCP should be relative to page activation
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// rather than navigation start if the page was prerendered. But in cases
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// where `activationStart` occurs after the LCP, this time should be
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// clamped at 0.
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metric.value = Math.max(lastEntry.startTime - getActivationStart(), 0);
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metric.entries = [lastEntry];
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report();
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}
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}
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};
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const po = observe('largest-contentful-paint', handleEntries);
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if (po) {
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report = bindReporter(onReport, metric, LCPThresholds, opts.reportAllChanges);
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const stopListening = runOnce(() => {
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if (!reportedMetricIDs[metric.id]) {
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handleEntries(po.takeRecords());
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po.disconnect();
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reportedMetricIDs[metric.id] = true;
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report(true);
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}
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});
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// Stop listening after input. Note: while scrolling is an input that
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// stops LCP observation, it's unreliable since it can be programmatically
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// generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
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['keydown', 'click'].forEach((type) => {
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// Wrap in a setTimeout so the callback is run in a separate task
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// to avoid extending the keyboard/click handler to reduce INP impact
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// https://github.com/GoogleChrome/web-vitals/issues/383
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addEventListener(type, () => setTimeout(stopListening, 0), true);
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});
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onHidden(stopListening);
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// Only report after a bfcache restore if the `PerformanceObserver`
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// successfully registered.
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onBFCacheRestore((event) => {
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metric = initMetric('LCP');
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report = bindReporter(onReport, metric, LCPThresholds, opts.reportAllChanges);
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doubleRAF(() => {
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metric.value = performance.now() - event.timeStamp;
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reportedMetricIDs[metric.id] = true;
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report(true);
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});
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});
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}
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});
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};
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