web-vitals
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Easily measure performance metrics in JavaScript
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JavaScript
/*
* 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 { LCPEntryManager } from './lib/LCPEntryManager.js';
import { onBFCacheRestore } from './lib/bfcache.js';
import { bindReporter } from './lib/bindReporter.js';
import { doubleRAF } from './lib/doubleRAF.js';
import { getActivationStart } from './lib/getActivationStart.js';
import { getVisibilityWatcher } from './lib/getVisibilityWatcher.js';
import { initMetric } from './lib/initMetric.js';
import { initUnique } from './lib/initUnique.js';
import { observe } from './lib/observe.js';
import { runOnce } from './lib/runOnce.js';
import { whenActivated } from './lib/whenActivated.js';
import { whenIdleOrHidden } from './lib/whenIdleOrHidden.js';
/** Thresholds for LCP. See https://web.dev/articles/lcp#what_is_a_good_lcp_score */
export const LCPThresholds = [2500, 4000];
/**
* Calculates the [LCP](https://web.dev/articles/lcp) value for the current page and
* calls the `callback` function once the value is ready (along with the
* relevant `largest-contentful-paint` performance entry used to determine the
* value). The reported value is a `DOMHighResTimeStamp`.
*
* If the `reportAllChanges` configuration option is set to `true`, the
* `callback` function will be called any time a new `largest-contentful-paint`
* performance entry is dispatched, or once the final value of the metric has
* been determined.
*/
export const onLCP = (onReport, opts = {}) => {
whenActivated(() => {
const visibilityWatcher = getVisibilityWatcher();
let metric = initMetric('LCP');
let report;
const lcpEntryManager = initUnique(opts, LCPEntryManager);
const handleEntries = (entries) => {
// If reportAllChanges is set then call this function for each entry,
// otherwise only consider the last one.
if (!opts.reportAllChanges) {
entries = entries.slice(-1);
}
for (const entry of entries) {
lcpEntryManager._processEntry(entry);
// Only report if the page wasn't hidden prior to LCP.
if (entry.startTime < visibilityWatcher.firstHiddenTime) {
// The startTime attribute returns the value of the renderTime if it is
// not 0, and the value of the loadTime otherwise. The activationStart
// reference is used because LCP should be relative to page activation
// rather than navigation start if the page was prerendered. But in cases
// where `activationStart` occurs after the LCP, this time should be
// clamped at 0.
metric.value = Math.max(entry.startTime - getActivationStart(), 0);
metric.entries = [entry];
report();
}
}
};
const po = observe('largest-contentful-paint', handleEntries);
if (po) {
report = bindReporter(onReport, metric, LCPThresholds, opts.reportAllChanges);
// Ensure this logic only runs once, since it can be triggered from
// any of three different event listeners below.
const stopListening = runOnce(() => {
handleEntries(po.takeRecords());
po.disconnect();
report(true);
});
// Stop listening after input or visibilitychange.
// Note: while scrolling is an input that stops LCP observation, it's
// unreliable since it can be programmatically generated.
// See: https://github.com/GoogleChrome/web-vitals/issues/75
for (const type of ['keydown', 'click', 'visibilitychange']) {
// Wrap the listener in an idle callback so it's run in a separate
// task to reduce potential INP impact.
// https://github.com/GoogleChrome/web-vitals/issues/383
addEventListener(type, () => whenIdleOrHidden(stopListening), {
capture: true,
once: true,
});
}
// Only report after a bfcache restore if the `PerformanceObserver`
// successfully registered.
onBFCacheRestore((event) => {
metric = initMetric('LCP');
report = bindReporter(onReport, metric, LCPThresholds, opts.reportAllChanges);
doubleRAF(() => {
metric.value = performance.now() - event.timeStamp;
report(true);
});
});
}
});
};