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perf.js from Philip Walton's video A deep dive into optimizing LCP
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new PerformanceObserver((list) => { | |
const lcpEntry = list.getEntries().at(-1); | |
if (!lcpEntry.url) { | |
// In case LCP is a <p> or <h1> etc (not a resource w url) | |
console.log('LCP: ', lcpEntry.startTime, lcpEntry.element); | |
console.log('LCP element outerHTML: ', lcpEntry.element.outerHTML); | |
return; | |
} | |
const navEntry = performance.getEntriesByType('navigation')[0]; | |
const resEntries = performance.getEntriesByType('resource'); | |
const lcpResEntry = resEntries.filter((e) => e.name === lcpEntry.url)[0]; | |
const docTTFB = navEntry.responseStart; | |
const lcpRequestStart = Math.max(docTTFB, | |
lcpResEntry ? lcpResEntry.requestStart : 0); | |
const lcpResponseEnd = Math.max(lcpRequestStart, | |
lcpResEntry ? lcpResEntry.responseEnd : 0); | |
const lcpRenderTime = Math.max(lcpResponseEnd, | |
lcpEntry ? lcpEntry.startTime : 0); | |
console.log('LCP: ', lcpRenderTime, lcpEntry.element); | |
console.log('document_ttfb', docTTFB); | |
console.log('resource_load_delay', lcpRequestStart - docTTFB); | |
console.log('resource_load_time', lcpResponseEnd - lcpRequestStart); | |
console.log('element_render_delay', lcpRenderTime - lcpResponseEnd); | |
performance.measure('document_ttfb', | |
{start: 0, end: docTTFB}); | |
performance.measure('resource_load_delay', // Stagger time to force a new row. | |
{start: docTTFB - 0.01, end: lcpRequestStart - 0.01}); | |
performance.measure('resource_load_time', | |
{start: lcpRequestStart, end: lcpResponseEnd}); | |
performance.measure('element_render_delay', // Stagger time to force a new row. | |
{start: lcpResponseEnd - 0.01, end: lcpRenderTime - 0.01}); | |
}).observe({type: 'largest-contentful-paint', buffered: true}); |
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Revision 2: Improved it by adding support for non-resource LCP element. See revision 1 if you want Philip Waltons original.