URLResolver.js

import http from 'http';
import https from 'https';

const fetchUrl = url => {
	return new Promise((resolve, reject) => {
		const parsedUrl = new URL(url);
		const h = (parsedUrl.protocol === 'https:') ? https : http;

		h.get(url, res => {
			if (res.statusCode >= 300 && res.statusCode < 400 && res.headers.location) { // redirect url
				fetchUrl(res.headers.location).then(buffer => {
					resolve(buffer);
				}, result => {
					reject(result);
				});
				return;
			}

			const ok = res.statusCode >= 200 && res.statusCode < 300;
			if (!ok) {
				reject(new TypeError(`Failed to fetch (status code: ${res.statusCode}, url: "${url}")`));
			}

			const chunks = [];
			res.on('end', () => resolve(Buffer.concat(chunks)));
			res.on('data', d => chunks.push(d));
		}).on('error', reject);
	});
};

class URLResolver {
	constructor(fs) {
		this.fs = fs;
		this.resolving = {};
	}

	resolve(url) {
		if (!this.resolving[url]) {
			this.resolving[url] = new Promise((resolve, reject) => {
				if (url.toLowerCase().indexOf('https://') === 0 || url.toLowerCase().indexOf('http://') === 0) {
					fetchUrl(url).then(buffer => {
						this.fs.writeFileSync(url, buffer);
						resolve();
					}, result => {
						reject(result);
					});
				} else {
					// cannot be resolved
					resolve();
				}
			});
		}

		return this.resolving[url];
	}

	resolved() {
		return new Promise((resolve, reject) => {
			Promise.all(Object.values(this.resolving)).then(() => {
				resolve();
			}, result => {
				reject(result);
			});
		});
	}

}

export default URLResolver;