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Cumulative Layout Shift (CLS) with Puppeteer
const puppeteer = require('puppeteer');
const devices = require('puppeteer/DeviceDescriptors');
const Good3G = {
'offline': false,
'downloadThroughput': 1.5 * 1024 * 1024 / 8,
'uploadThroughput': 750 * 1024 / 8,
'latency': 40
};
const phone = devices['Nexus 5X'];
function calcJank() {
window.cumulativeLayoutShiftScore = 0;
const observer = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
if (!entry.hadRecentInput) {
console.log("New observer entry for cls: " + entry.value);
window.cumulativeLayoutShiftScore += entry.value;
}
}
});
observer.observe({type: 'layout-shift', buffered: true});
document.addEventListener('visibilitychange', () => {
if (document.visibilityState === 'hidden') {
observer.takeRecords();
observer.disconnect();
console.log('CLS:', window.cumulativeLayoutShiftScore);
}
});
}
async function getCLS(url) {
const browser = await puppeteer.launch({
args: ['--no-sandbox'],
timeout: 10000
});
try {
const page = await browser.newPage();
const client = await page.target().createCDPSession();
// throttle network and CPU as needed, see e.g. here:
// https://michaljanaszek.com/blog/test-website-performance-with-puppeteer
await client.send('Network.enable');
await client.send('ServiceWorker.enable');
await client.send('Network.emulateNetworkConditions', Good3G);
await client.send('Emulation.setCPUThrottlingRate', { rate: 4 });
await page.emulate(phone);
// inject a function with the code from https://web.dev/cls/#measure-cls-in-javascript
await page.evaluateOnNewDocument(calcJank);
await page.goto(url, { waitUntil: 'networkidle2', timeout: 60000});
//await page.evaluate(calcJank);
let cls = await page.evaluate(function() {return window.cumulativeLayoutShiftScore;});
return cls;
browser.close();
} catch (error) {
console.log(error);
browser.close();
}
}
getCLS("https://www.thinkwithgoogle.com").then(cls => console.log("CLS is: " + cls));
@Ah089

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@Ah089 Ah089 commented Apr 19, 2020

Ny

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