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Refactor the stopListening() function to only run once #548

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merged 2 commits into from
Oct 18, 2024
Merged

Refactor the stopListening() function to only run once #548

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403cb79
Refactor the stopListening function
philipwalton Oct 15, 2024
10e42db
Remove the redundant reportedMetricIDs check
philipwalton Oct 16, 2024
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30 changes: 11 additions & 19 deletions src/onLCP.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -29,8 +29,6 @@ import {LCPMetric, MetricRatingThresholds, ReportOpts} from './types.js';
/** Thresholds for LCP. See https://web.dev/articles/lcp#what_is_a_good_lcp_score */
export const LCPThresholds: MetricRatingThresholds = [2500, 4000];

const reportedMetricIDs: Record<string, boolean> = {};

/**
* 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
Expand Down Expand Up @@ -84,28 +82,23 @@ export const onLCP = (
opts!.reportAllChanges,
);

// Ensure this logic only runs once, and wrap it in an idle callback
// so the callback is run in a separate task to avoid extending the
// keyboard/click handler to reduce INP impact.
// https://github.com/GoogleChrome/web-vitals/issues/383
const stopListening = () =>
whenIdleOrHidden(
runOnce(() => {
if (!reportedMetricIDs[metric.id]) {
handleEntries(po!.takeRecords() as LCPMetric['entries']);
po!.disconnect();
reportedMetricIDs[metric.id] = true;
report(true);
}
}),
);
// 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() as LCPMetric['entries']);
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']) {
addEventListener(type, () => stopListening(), {
// 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,
});
Expand All @@ -124,7 +117,6 @@ export const onLCP = (

doubleRAF(() => {
metric.value = performance.now() - event.timeStamp;
reportedMetricIDs[metric.id] = true;
report(true);
});
});
Expand Down