ES6 script for creating web site snapshots in .webarchive format (WIP)

webarchive.js

const bplist = require('bplist-creator'),
    request = require('request'),
    xpath = require('xpath'),
    dom = require('xmldom').DOMParser,
    url = require('url'),
    fs = require('fs');
    

const USER_AGENT = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/603.1.30 (KHTML, like Gecko) Version/10.1 Safari/603.1.30',
    ACCEPT_HEADERS = {
        "User-Agent": USER_AGENT,
        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
        "Accept-Language": "en-us",
        "Accept-Encoding": "gzip"
    },
    XPATH_PATTERNS = ['//img/@src', '//img/@data-src', '//img/@data-src-retina', '//img/@data-cid', '//script/@src', "//link[@rel='stylesheet']/@href"];


function requestAsync(url) {
    return new Promise((resolve, reject) => {
        var req = request({
            followAllRedirects: true,    
            url: url,
            headers: ACCEPT_HEADERS,
            gzip: true,
            encoding: null
        }, (err, response, body) => {
            if (err) return reject(err, response, body);
            resolve({
                "url": url,
                "headers": response.headers,
                "body": body
            });
        });
    });
};

async function getParallel(urls) {
    try {
        return await Promise.all(urls.map(requestAsync));
    } catch (err) {
        console.error(err);
    }
}

var target = "http://help.websiteos.com/websiteos/example_of_a_simple_html_page.htm";

var target = "https://theverge.com";

requestAsync(target).then((result) => {
    if (result.headers['content-type'].includes("text/html")) {
        var doc = new dom().parseFromString(String(result.body)), 
            urls = XPATH_PATTERNS.map((pattern) => {
                var nodes = xpath.select(pattern, doc);
                return nodes.map((x) => {return url.resolve(target, x.value)});
            }).reduce((a,b) => a.concat(b))
             .filter((x) => x.substring(0,4) ==="http") // fetchable only
             .filter((v,i,a) => a.indexOf(v) === i); // unique
  
         getParallel(urls).then((results) => {
            var webarchive = {
                'WebMainResource': {
                    "WebResourceData": result.body,
                    "WebResourceMIMEType": "text/html",
                    "WebResourceURL": target                            
                },
                'WebSubresources': results.map((r) => {
                    return {
                        "WebResourceData": r.body,
                        "WebResourceMIMEType": r.headers['content-type'],
                        "WebResourceURL": r.url
                    }
                })
            }
            fs.writeFile("test.webarchive", bplist(webarchive),  "binary", (err) => console.log(err));
        }).catch((reason) => console.log(reason));
    }
}).catch((reason) => console.log(reason));