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largest-contentful-paint-element.js
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largest-contentful-paint-element.js
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/**
* @license
* Copyright 2020 Google LLC
* SPDX-License-Identifier: Apache-2.0
*/
import log from 'lighthouse-logger';
import {Audit} from './audit.js';
import * as i18n from '../lib/i18n/i18n.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';
const UIStrings = {
/** Descriptive title of a diagnostic audit that provides the element that was determined to be the Largest Contentful Paint. */
title: 'Largest Contentful Paint element',
/** Description of a Lighthouse audit that tells the user that the element shown was determined to be the Largest Contentful Paint. */
description: 'This is the largest contentful element painted within the viewport. ' +
'[Learn more about the Largest Contentful Paint element](https://developer.chrome.com/docs/lighthouse/performance/lighthouse-largest-contentful-paint/)',
/** Label for a column in a data table; entries will be the name of a phase in the Largest Contentful Paint (LCP) metric. */
columnPhase: 'Phase',
/** Label for a column in a data table; entries will be the percent of Largest Contentful Paint (LCP) that a phase covers. */
columnPercentOfLCP: '% of LCP',
/** Label for a column in a data table; entries will be the amount of time spent in a phase in the Largest Contentful Paint (LCP) metric. */
columnTiming: 'Timing',
/** Table item value for the Time To First Byte (TTFB) phase of the Largest Contentful Paint (LCP) metric. */
itemTTFB: 'TTFB',
/** Table item value for the load delay phase of the Largest Contentful Paint (LCP) metric. */
itemLoadDelay: 'Load Delay',
/** Table item value for the load time phase of the Largest Contentful Paint (LCP) metric. */
itemLoadTime: 'Load Time',
/** Table item value for the render delay phase of the Largest Contentful Paint (LCP) metric. */
itemRenderDelay: 'Render Delay',
};
const str_ = i18n.createIcuMessageFn(import.meta.url, UIStrings);
class LargestContentfulPaintElement extends Audit {
/**
* @return {LH.Audit.Meta}
*/
static get meta() {
return {
id: 'largest-contentful-paint-element',
title: str_(UIStrings.title),
description: str_(UIStrings.description),
scoreDisplayMode: Audit.SCORING_MODES.METRIC_SAVINGS,
guidanceLevel: 1,
supportedModes: ['navigation'],
requiredArtifacts:
['traces', 'TraceElements', 'devtoolsLogs', 'GatherContext', 'settings', 'URL'],
};
}
/**
* @param {LH.Artifacts} artifacts
* @return {LH.Audit.Details.Table|undefined}
*/
static makeElementTable(artifacts) {
const lcpElement = artifacts.TraceElements
.find(element => element.traceEventType === 'largest-contentful-paint');
if (!lcpElement) return;
/** @type {LH.Audit.Details.Table['headings']} */
const headings = [
{key: 'node', valueType: 'node', label: str_(i18n.UIStrings.columnElement)},
];
const lcpElementDetails = [{node: Audit.makeNodeItem(lcpElement.node)}];
return Audit.makeTableDetails(headings, lcpElementDetails);
}
/**
* @param {number} metricLcp
* @param {LH.Artifacts.MetricComputationDataInput} metricComputationData
* @param {LH.Audit.Context} context
* @return {Promise<LH.Audit.Details.Table>}
*/
static async makePhaseTable(metricLcp, metricComputationData, context) {
const {ttfb, loadStart, loadEnd} = await LCPBreakdown.request(metricComputationData, context);
let loadDelay = 0;
let loadTime = 0;
let renderDelay = metricLcp - ttfb;
if (loadStart && loadEnd) {
loadDelay = loadStart - ttfb;
loadTime = loadEnd - loadStart;
renderDelay = metricLcp - loadEnd;
}
const results = [
{phase: str_(UIStrings.itemTTFB), timing: ttfb},
{phase: str_(UIStrings.itemLoadDelay), timing: loadDelay},
{phase: str_(UIStrings.itemLoadTime), timing: loadTime},
{phase: str_(UIStrings.itemRenderDelay), timing: renderDelay},
].map(result => {
const percent = 100 * result.timing / metricLcp;
const percentStr = `${percent.toFixed(0)}%`;
return {...result, percent: percentStr};
});
/** @type {LH.Audit.Details.Table['headings']} */
const headings = [
{key: 'phase', valueType: 'text', label: str_(UIStrings.columnPhase)},
{key: 'percent', valueType: 'text', label: str_(UIStrings.columnPercentOfLCP)},
{key: 'timing', valueType: 'ms', label: str_(UIStrings.columnTiming)},
];
return Audit.makeTableDetails(headings, results);
}
/**
* @param {LH.Artifacts} artifacts
* @param {LH.Audit.Context} context
* @return {Promise<LH.Audit.Product>}
*/
static async audit(artifacts, context) {
const trace = artifacts.traces[Audit.DEFAULT_PASS];
const devtoolsLog = artifacts.devtoolsLogs[Audit.DEFAULT_PASS];
const gatherContext = artifacts.GatherContext;
const metricComputationData = {trace, devtoolsLog, gatherContext,
settings: context.settings, URL: artifacts.URL};
const elementTable = this.makeElementTable(artifacts);
if (!elementTable) {
return {
score: null,
notApplicable: true,
metricSavings: {LCP: 0},
};
}
const items = [elementTable];
let displayValue;
let metricLcp = 0;
try {
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);
items.push(phaseTable);
} catch (err) {
Sentry.captureException(err, {
tags: {audit: this.meta.id},
level: 'error',
});
log.error(this.meta.id, err.message);
}
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: lcpSavings ? 0 : 1,
scoreDisplayMode: lcpSavings ? undefined : Audit.SCORING_MODES.INFORMATIVE,
displayValue,
details,
metricSavings: {
LCP: lcpSavings,
},
};
}
}
export default LargestContentfulPaintElement;
export {UIStrings};