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core(largest-contentful-paint-element): add LCP savings #15178

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Aug 15, 2023
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p10
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adamraine committed Aug 11, 2023
commit d1d7423046fcbd2691df4418b2e5d93e9b85a537
18 changes: 12 additions & 6 deletions core/audits/largest-contentful-paint-element.js
Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,8 @@

import {Audit} from './audit.js';
import * as i18n from '../lib/i18n/i18n.js';
import {LargestContentfulPaint} from '../computed/metrics/largest-contentful-paint.js';
import {LargestContentfulPaint as LargestContentfulPaintComputed} from '../computed/metrics/largest-contentful-paint.js';
import LargestContentfulPaint from './metrics/largest-contentful-paint.js';
import {LCPBreakdown} from '../computed/metrics/lcp-breakdown.js';
import {Sentry} from '../lib/sentry.js';

Expand Down Expand Up @@ -125,19 +126,21 @@

const elementTable = this.makeElementTable(artifacts);
if (!elementTable) {
return {
score: null,
notApplicable: true,
metricSavings: {LCP: 0},
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I've generally been of the mindset that we should avoid adding any of the usual stuff to audits in the notApplicable case. It's not a hard and fast rule, but when looking at an LHR, you generally shouldn't have to wonder if a value on an audit is important. The whole thing has been marked not applicable; it's not that there's no savings to be had, it's that this audit is irrelevant for this particular page load.

(the only exception might be debugdata if we ever want to pipe through information on why something was marked n/a to http archive or wherever, but I don't know if that's ever come up before)

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True, but I was planning for metricSavings to eventually replace our current metric applicability lists. Removing metricSavings when an audit is N/A means that it will always be hidden when the user filters to any metric.

Not necessarily a bad UX, but it's also not how we do the filters right now. Knowing that an audit can affect a certain metric even when it's N/A can be useful information IMO.

};
}

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core/audits/largest-contentful-paint-element.js#L129-L134

Added lines #L129 - L134 were not covered by tests

const items = [elementTable];
let displayValue;
let metricLcp = 0;

try {
const {timing: metricLcp} =
await LargestContentfulPaint.request(metricComputationData, context);
const lcpResult =
await LargestContentfulPaintComputed.request(metricComputationData, context);
metricLcp = lcpResult.timing;
displayValue = str_(i18n.UIStrings.ms, {timeInMs: metricLcp});

const phaseTable = await this.makePhaseTable(metricLcp, metricComputationData, context);
Expand All @@ -152,14 +155,17 @@

const details = Audit.makeListDetails(items);

// Conceptually, this doesn't make much sense as "savings" for this audit since there isn't anything to "fix".
// However, this audit will always be useful when improving LCP and that should be reflected in our impact calculations.
const idealLcp = LargestContentfulPaint.defaultOptions[context.settings.formFactor].scoring.p10;
const lcpSavings = Math.max(0, metricLcp - idealLcp);

return {
score: 1,
displayValue,
details,
metricSavings: {
// Conceptually, this doesn't make much sense as "savings" for this audit since there isn't anything to "fix".
// However, this audit will always be useful when improving LCP and that should be reflected in our impact calculations.
LCP: metricLcp || 0,
LCP: lcpSavings,
},
};
}
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Original file line number Diff line number Diff line change
Expand Up @@ -103,7 +103,7 @@ describe('Performance: largest-contentful-paint-element audit', () => {
expect(auditResult.score).toEqual(1);
expect(auditResult.notApplicable).toBeUndefined();
expect(auditResult.displayValue).toBeDisplayString('5,800\xa0ms');
expect(auditResult.metricSavings).toEqual({LCP: 5804});
expect(auditResult.metricSavings).toEqual({LCP: 3304}); // 5804 - 2500 (p10 mobile)
expect(auditResult.details.items).toHaveLength(2);
expect(auditResult.details.items[0].items).toHaveLength(1);
expect(auditResult.details.items[0].items[0].node.path).toEqual('1,HTML,3,BODY,5,DIV,0,HEADER');
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