Simple Google Lighthouse tool to create and compare reports from the console.

lh.js

const lighthouse = require('lighthouse');
const chromeLauncher = require('chrome-launcher');
const argv = require('yargs').argv;
const url = require('url');
const fs = require('fs');
const glob = require('glob');
const path = require('path');

const launchChromeAndRunLighthouse = (url) => {
	return chromeLauncher.launch().then(chrome => {
		const opts =  {
			port: chrome.port
		}
		return lighthouse(url, opts).then(results => {
			return chrome.kill().then(() => {
				return {
					js: results.lhr,
					json: results.report
				}
			});
		});
	});
}

const getContents = (pathStr) => {
	const output = fs.readFileSync(pathStr, 'utf8', (err, results) => {
		return results;
	});
	return JSON.parse(output);
};

const compareReports = (from, to) => {
	const metricFilter = [
		'first-contentful-paint',
		'first-meaningful-paint',
		'speed-index',
		'estimated-input-latency',
		'total-blocking-time',
		'max-potential-fid',
		'time-to-first-byte',
		'first-cpu-idle',
		'interactive'
	]

	const calcPercentageDiff = (from, to) => {
		const per = ((to - from) / from) * 100;
		return Math.round(per * 100) / 100;
	};

	for(let auditObj in from['audits']) {
		if(metricFilter.includes(auditObj)) {
			const percentageDiff = calcPercentageDiff(
				from['audits'][auditObj].numericValue,
				to['audits'][auditObj].numericValue
			);

			let logColor = '\x1b[37m';
			const log = (() => {
				if(Math.sign(percentageDiff) === 1) {
					logColor = "\x1b[31m";
					return `${percentageDiff.toString().replace('-','') + '%'} slower`;
				}
				else if(Math.sign(percentageDiff) === 0) {
					return 'unchanged';
				}
				else {
					logColor = "\x1b[32m";
					return `${percentageDiff.toString().replace('-','') + '%'} faster`;
				}
			})();
			console.log(logColor, `${from['audits'][auditObj].title} is ${log}`);
		}
	}
}

if(argv.from && argv.to) {
	compareReports(
		getContents(argv.from + '.json'),
		getContents(argv.to + '.json')
	);
}
else if(argv.url) {
	const urlObj = new URL(argv.url);
	let dirName = urlObj.host.replace('www.','');
	if(urlObj.pathname !== '/') {
		dirName = dirName + urlObj.pathname.replace(/\//g,'_');
	}

	if(!fs.existsSync(dirName)) {
	    fs.mkdirSync(dirName);
	}

	launchChromeAndRunLighthouse(argv.url).then(results => {
		const prevReports = glob(`${dirName}/*.json`, {
			sync: true
		});

		if(prevReports.length) {
			dates = [];
			for(report in prevReports) {
				dates.push(new Date(path.parse(prevReports[report]).name.replace(/_/g, ':')));
			}
			const max = dates.reduce(function(a, b) {
				return Math.max(a, b);
			});
			const recentReport = new Date(max).toISOString();

			const recentReportContents = getContents(dirName + '/' + recentReport.replace(/:/g, '_') + '.json');

			compareReports(recentReportContents, results.js);
		}

		fs.writeFile(`${dirName}/${results.js['fetchTime'].replace(/:/g, '_')}.json`, results.json, (err) => {
			if (err) throw err;
		});
	});
}
else {
	throw "You haven't passed a URL to Lighthouse";
}