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/*
* Copyright 2022 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 {getLoadState} from '../lib/getLoadState.js';
import {getSelector} from '../lib/getSelector.js';
import {onCLS as unattributedOnCLS} from '../onCLS.js';
import {
CLSAttribution,
CLSMetric,
CLSMetricWithAttribution,
ReportOpts,
} from '../types.js';
const getLargestLayoutShiftEntry = (entries: LayoutShift[]) => {
return entries.reduce((a, b) => (a && a.value > b.value ? a : b));
};
const getLargestLayoutShiftSource = (sources: LayoutShiftAttribution[]) => {
return sources.find((s) => s.node && s.node.nodeType === 1) || sources[0];
};
const attributeCLS = (metric: CLSMetric): CLSMetricWithAttribution => {
// Use an empty object if no other attribution has been set.
let attribution: CLSAttribution = {};
if (metric.entries.length) {
const largestEntry = getLargestLayoutShiftEntry(metric.entries);
if (largestEntry && largestEntry.sources && largestEntry.sources.length) {
const largestSource = getLargestLayoutShiftSource(largestEntry.sources);
if (largestSource) {
attribution = {
largestShiftTarget: getSelector(largestSource.node),
largestShiftTime: largestEntry.startTime,
largestShiftValue: largestEntry.value,
largestShiftSource: largestSource,
largestShiftEntry: largestEntry,
loadState: getLoadState(largestEntry.startTime),
};
}
}
}
// Use Object.assign to set property to keep tsc happy.
const metricWithAttribution: CLSMetricWithAttribution = Object.assign(
metric,
{attribution},
);
return metricWithAttribution;
};
/**
* Calculates the [CLS](https://web.dev/articles/cls) value for the current page and
* calls the `callback` function once the value is ready to be reported, along
* with all `layout-shift` performance entries that were used in the metric
* value calculation. The reported value is a `double` (corresponding to a
* [layout shift score](https://web.dev/articles/cls#layout_shift_score)).
*
* If the `reportAllChanges` configuration option is set to `true`, the
* `callback` function will be called as soon as the value is initially
* determined as well as any time the value changes throughout the page
* lifespan.
*
* _**Important:** CLS should be continually monitored for changes throughout
* the entire lifespan of a page—including if the user returns to the page after
* it's been hidden/backgrounded. However, since browsers often [will not fire
* additional callbacks once the user has backgrounded a
* page](https://developer.chrome.com/blog/page-lifecycle-api/#advice-hidden),
* `callback` is always called when the page's visibility state changes to
* hidden. As a result, the `callback` function might be called multiple times
* during the same page load._
*/
export const onCLS = (
onReport: (metric: CLSMetricWithAttribution) => void,
opts?: ReportOpts,
) => {
unattributedOnCLS((metric: CLSMetric) => {
const metricWithAttribution = attributeCLS(metric);
onReport(metricWithAttribution);
}, opts);
};
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