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

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merged 6 commits into from
Aug 15, 2023
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core(largest-contentful-paint-element): add LCP savings #15178

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5f5077e
core(largest-contentful-paint-element): add LCP savings
adamraine Jun 15, 2023
5a5be83
comment
adamraine Jun 15, 2023
c456dfa
Merge branch 'main' into lcp-elem-savings
adamraine Aug 11, 2023
d1d7423
p10
adamraine Aug 11, 2023
1c2eb68
Merge branch 'main' into lcp-elem-savings
adamraine Aug 14, 2023
97ec846
Merge branch 'main' into lcp-elem-savings
adamraine Aug 15, 2023
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25 changes: 21 additions & 4 deletions core/audits/largest-contentful-paint-element.js
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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 @@ -124,14 +125,22 @@
settings: context.settings, URL: artifacts.URL};

const elementTable = this.makeElementTable(artifacts);
if (!elementTable) return {score: null, notApplicable: true};
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 @@ -146,10 +155,18 @@

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: {
LCP: lcpSavings,
},
};
}
}
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2 changes: 2 additions & 0 deletions core/test/audits/largest-contentful-paint-element-test.js
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Expand Up @@ -103,6 +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: 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');
Expand Down Expand Up @@ -148,6 +149,7 @@ describe('Performance: largest-contentful-paint-element audit', () => {
expect(auditResult.score).toEqual(null);
expect(auditResult.notApplicable).toEqual(true);
expect(auditResult.displayValue).toBeUndefined();
expect(auditResult.metricSavings).toEqual({LCP: 0});
expect(auditResult.details).toBeUndefined();
});

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