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73 lines
3.2 KiB
73 lines
3.2 KiB
2 months ago
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/*
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* Copyright 2022 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 { getLoadState } from '../lib/getLoadState.js';
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import { getSelector } from '../lib/getSelector.js';
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import { onCLS as unattributedOnCLS } from '../onCLS.js';
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const getLargestLayoutShiftEntry = (entries) => {
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return entries.reduce((a, b) => (a && a.value > b.value ? a : b));
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};
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const getLargestLayoutShiftSource = (sources) => {
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return sources.find((s) => s.node && s.node.nodeType === 1) || sources[0];
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};
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const attributeCLS = (metric) => {
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if (metric.entries.length) {
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const largestEntry = getLargestLayoutShiftEntry(metric.entries);
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if (largestEntry && largestEntry.sources && largestEntry.sources.length) {
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const largestSource = getLargestLayoutShiftSource(largestEntry.sources);
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if (largestSource) {
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metric.attribution = {
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largestShiftTarget: getSelector(largestSource.node),
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largestShiftTime: largestEntry.startTime,
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largestShiftValue: largestEntry.value,
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largestShiftSource: largestSource,
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largestShiftEntry: largestEntry,
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loadState: getLoadState(largestEntry.startTime),
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};
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return;
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}
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}
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}
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// Set an empty object if no other attribution has been set.
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metric.attribution = {};
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};
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/**
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* Calculates the [CLS](https://web.dev/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/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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unattributedOnCLS(((metric) => {
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attributeCLS(metric);
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onReport(metric);
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}), opts);
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};
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