109 lines
5.0 KiB
JavaScript
109 lines
5.0 KiB
JavaScript
/*
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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 { initMetric } from './lib/initMetric.js';
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import { observe } from './lib/observe.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 { onHidden } from './lib/onHidden.js';
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import { runOnce } from './lib/runOnce.js';
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import { onFCP } from './onFCP.js';
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/** Thresholds for CLS. See https://web.dev/articles/cls#what_is_a_good_cls_score */
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export const CLSThresholds = [0.1, 0.25];
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/**
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* Calculates the [CLS](https://web.dev/articles/cls) value for the current page and
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* calls the `callback` function once the value is ready to be reported, along
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* with all `layout-shift` performance entries that were used in the metric
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* value calculation. The reported value is a `double` (corresponding to a
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* [layout shift score](https://web.dev/articles/cls#layout_shift_score)).
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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 as soon as the value is initially
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* determined as well as any time the value changes throughout the page
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* lifespan.
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*
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* _**Important:** CLS should be continually monitored for changes throughout
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* the entire lifespan of a page—including if the user returns to the page after
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* it's been hidden/backgrounded. However, since browsers often [will not fire
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* additional callbacks once the user has backgrounded a
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* page](https://developer.chrome.com/blog/page-lifecycle-api/#advice-hidden),
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* `callback` is always called when the page's visibility state changes to
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* hidden. As a result, the `callback` function might be called multiple times
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* during the same page load._
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*/
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export const onCLS = (onReport, opts) => {
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// Set defaults
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opts = opts || {};
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// Start monitoring FCP so we can only report CLS if FCP is also reported.
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// Note: this is done to match the current behavior of CrUX.
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onFCP(runOnce(() => {
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let metric = initMetric('CLS', 0);
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let report;
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let sessionValue = 0;
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let sessionEntries = [];
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const handleEntries = (entries) => {
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entries.forEach((entry) => {
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// Only count layout shifts without recent user input.
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if (!entry.hadRecentInput) {
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const firstSessionEntry = sessionEntries[0];
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const lastSessionEntry = sessionEntries[sessionEntries.length - 1];
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// If the entry occurred less than 1 second after the previous entry
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// and less than 5 seconds after the first entry in the session,
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// include the entry in the current session. Otherwise, start a new
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// session.
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if (sessionValue &&
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entry.startTime - lastSessionEntry.startTime < 1000 &&
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entry.startTime - firstSessionEntry.startTime < 5000) {
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sessionValue += entry.value;
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sessionEntries.push(entry);
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}
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else {
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sessionValue = entry.value;
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sessionEntries = [entry];
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}
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}
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});
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// If the current session value is larger than the current CLS value,
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// update CLS and the entries contributing to it.
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if (sessionValue > metric.value) {
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metric.value = sessionValue;
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metric.entries = sessionEntries;
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report();
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}
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};
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const po = observe('layout-shift', handleEntries);
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if (po) {
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report = bindReporter(onReport, metric, CLSThresholds, opts.reportAllChanges);
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onHidden(() => {
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handleEntries(po.takeRecords());
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report(true);
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});
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// Only report after a bfcache restore if the `PerformanceObserver`
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// successfully registered.
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onBFCacheRestore(() => {
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sessionValue = 0;
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metric = initMetric('CLS', 0);
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report = bindReporter(onReport, metric, CLSThresholds, opts.reportAllChanges);
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doubleRAF(() => report());
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});
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// Queue a task to report (if nothing else triggers a report first).
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// This allows CLS to be reported as soon as FCP fires when
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// `reportAllChanges` is true.
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setTimeout(report, 0);
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}
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}));
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};
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