Code Coverage in Puppetter

codeCover.js

// npm install puppeteer
const puppeteer = require('puppeteer');
/* variables to allow for user-agent / screensize choice 
example use: URL='https://tamethebots.com' useragent='Mozilla/5.0 (Linux; Android 6.0.1; Nexus 5X Build/MMB29P) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.96 Mobile Safari/537.36 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)' viewportw=412 viewporth=732 node codeCover.js
*/
const url = process.env.URL || 'https://www.example.com';
const useragent = process.env.useragent || 'Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)';
const viewportw = process.env.viewportw || 1920;
const viewporth = process.env.viewporth || 1696;

(async () => {
    const browser = await puppeteer.launch();
    const page = await browser.newPage();
    await page.setUserAgent(useragent);
    await page.setViewport({
        width: parseInt(viewportw),
        height: parseInt(viewporth)
    });

    // start collecting coverage
    await Promise.all([page.coverage.startJSCoverage(), page.coverage.startCSSCoverage()]);
    try {
        // load page and await until idle.
        await page.goto(url, {
            waitUntil: 'networkidle0'
        });

        // Stops collecting
        const [coverageJScript, coverageCss] = await Promise.all([
            page.coverage.stopJSCoverage(),
            page.coverage.stopCSSCoverage()
        ]);

        // Work out Used & Unused 
        const calcUsed = (type, coverage) =>
            coverage.map(({
                url,
                ranges,
                text
            }) => {
                let usedBytes = 0;
                ranges.forEach(range => (usedBytes += range.end - range.start - 1));
                if (parseInt(text.length - usedBytes) > 0) {
                    return {
                        url,
                        type,
                        usedBytes,
                        totalBytes: text.length,
                        unusedBytes: parseInt(text.length - usedBytes),
                        unusedPercent: parseFloat((parseFloat(text.length - usedBytes) / text.length * 100).toFixed(2))
                    };
                } else {
                    return {
                        url,
                        type,
                        usedBytes,
                        totalBytes: text.length,
                        unusedBytes: parseInt(text.length - usedBytes),
                        unusedPercent: 0
                    };
                }
            });
        let outputArray = [
            ...calcUsed('js', coverageJScript),
            ...calcUsed('css', coverageCss)
        ]
        console.log(JSON.stringify(outputArray));
        await browser.close();
    } catch (err) {
        console.log('Error loading page:', err);
        await browser.close();
    }
})();