callbacks_async_scrape.js

// callbacks_async_scrape.js (tested with node 11.3)
const http = require('http');
const https = require('https');

const fetchUrl = (url, onSuccess, onError) => {
    console.time(`fetchUrl(${url})`);
    (url.indexOf('https') === 0 ? https : http).get(url, resp => {
        let html = '';
        resp.on('data', chunk => html += chunk);
        resp.on('end', () => {
            console.timeEnd(`fetchUrl(${url})`);
            onSuccess(html);
        });
    }).on('error', onError);
}

const scrapeData = html => {
    const re = /href="([^"]+)"/g;
    const hrefs = [];
    let m;
    while (m = re.exec(html)) hrefs.push(m[1]);
    return hrefs;
};

const urls = [
    "http://neverssl.com/",
    "https://www.ietf.org/rfc/rfc2616.txt",
    "https://en.wikipedia.org/wiki/Asynchronous_I/O",
];
const extactedData = {};
let nUrlsProcessed = 0;

console.time("elapsed");
for (url of urls) {
    fetchUrl(
        url,
        html => {
            nUrlsProcessed++;
            extactedData[url] = scrapeData(html);
            if (nUrlsProcessed <= 0) {
                console.log("> extracted data:", extactedData)
                console.timeEnd("elapsed");
            }
        },
        () => nUrlsProcessed++
    );
}