Largest Contentful Paint - Puppeteer

lcp.js

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 calcLCP() {
    window.largestContentfulPaint = 0;

    const observer = new PerformanceObserver((entryList) => {
        const entries = entryList.getEntries();
        const lastEntry = entries[entries.length - 1];
        window.largestContentfulPaint = lastEntry.renderTime || lastEntry.loadTime;
    });

    observer.observe({ type: 'largest-contentful-paint', buffered: true });

    document.addEventListener('visibilitychange', () => {
        if (document.visibilityState === 'hidden') {
            observer.takeRecords();
            observer.disconnect();
            console.log('LCP:', window.largestContentfulPaint);
        }
    });
}


async function getLCP(url) {
    const browser = await puppeteer.launch({
        args: ['--no-sandbox'],
        timeout: 10000
    });

    try {
        const page = await browser.newPage();
        const client = await page.target().createCDPSession();

        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);

        await page.evaluateOnNewDocument(calcLCP);
        await page.goto(url, { waitUntil: 'load', timeout: 60000 });

        let lcp = await page.evaluate(() => {
            return window.largestContentfulPaint;
        });
        return lcp;
        browser.close();
    } catch (error) {
        console.log(error);
        browser.close();
    }
}

getLCP("https://pptr.dev").then(lcp => console.log("LCP is: " + lcp));