web-vitals
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Easily measure performance metrics in JavaScript
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text/typescript
/*
* 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 {initMetric} from './lib/initMetric.js';
import {observe} from './lib/observe.js';
import {bindReporter} from './lib/bindReporter.js';
import {doubleRAF} from './lib/doubleRAF.js';
import {onHidden} from './lib/onHidden.js';
import {runOnce} from './lib/runOnce.js';
import {onFCP} from './onFCP.js';
import {CLSMetric, MetricRatingThresholds, ReportOpts} from './types.js';
/** Thresholds for CLS. See https://web.dev/articles/cls#what_is_a_good_cls_score */
export const CLSThresholds: MetricRatingThresholds = [0.1, 0.25];
/**
* 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: CLSMetric) => void,
opts?: ReportOpts,
) => {
// Set defaults
opts = opts || {};
// Start monitoring FCP so we can only report CLS if FCP is also reported.
// Note: this is done to match the current behavior of CrUX.
onFCP(
runOnce(() => {
let metric = initMetric('CLS', 0);
let report: ReturnType<typeof bindReporter>;
let sessionValue = 0;
let sessionEntries: LayoutShift[] = [];
const handleEntries = (entries: LayoutShift[]) => {
entries.forEach((entry) => {
// Only count layout shifts without recent user input.
if (!entry.hadRecentInput) {
const firstSessionEntry = sessionEntries[0];
const lastSessionEntry = sessionEntries[sessionEntries.length - 1];
// If the entry occurred less than 1 second after the previous entry
// and less than 5 seconds after the first entry in the session,
// include the entry in the current session. Otherwise, start a new
// session.
if (
sessionValue &&
entry.startTime - lastSessionEntry.startTime < 1000 &&
entry.startTime - firstSessionEntry.startTime < 5000
) {
sessionValue += entry.value;
sessionEntries.push(entry);
} else {
sessionValue = entry.value;
sessionEntries = [entry];
}
}
});
// If the current session value is larger than the current CLS value,
// update CLS and the entries contributing to it.
if (sessionValue > metric.value) {
metric.value = sessionValue;
metric.entries = sessionEntries;
report();
}
};
const po = observe('layout-shift', handleEntries);
if (po) {
report = bindReporter(
onReport,
metric,
CLSThresholds,
opts!.reportAllChanges,
);
onHidden(() => {
handleEntries(po.takeRecords() as CLSMetric['entries']);
report(true);
});
// Only report after a bfcache restore if the `PerformanceObserver`
// successfully registered.
onBFCacheRestore(() => {
sessionValue = 0;
metric = initMetric('CLS', 0);
report = bindReporter(
onReport,
metric,
CLSThresholds,
opts!.reportAllChanges,
);
doubleRAF(() => report());
});
// Queue a task to report (if nothing else triggers a report first).
// This allows CLS to be reported as soon as FCP fires when
// `reportAllChanges` is true.
setTimeout(report, 0);
}
}),
);
};