Preview Wikipedia links of an HTML document (via the `title` attribute)

wkpd.js

/**
 * @file Preview Wikipedia links 📖🔗👀
 *
 * @see <https://en.wikipedia.org/api/rest_v1/>
 *
 * @version 2020-11
 * @since   2018-08
 * @author  <https://paul.fragara.com/#me>
 * @license CC0-1.0
 *  The author has dedicated all rights to this software to the public domain.
 *  This software is distributed without any warranty.
 */
"use strict";

/**
 * Handle the few special cases in Wikipedia language/host names.
 *
 * @see <https://meta.wikimedia.org/wiki/List_of_Wikipedias#Nonstandard_language_codes>
 * @see <https://www.ietf.org/rfc/bcp/bcp47.txt>
 *
 * @param {!string} languageCode  (Human) language, in Wikipedia naming
 * @return {!string} A corresponding BCP 47 language tag
 */
function bcp47FromWikipedia(languageCode) {
	switch (languageCode.toLowerCase()) {
	case "simple":       return "en";
	case "nrm":          return "nrf";
	case "bat-smg":      return "sgs";
	case "roa-rup":      return "rup";
	case "fiu-vro":      return "vro";
	case "zh-yue":       return "yue";
	case "zh-min-nan":   return "nan";
	case "zh-classical": return "lzh";
	}
	return languageCode;
}

/**
 * Query Wikipedia to put a summary, in the ‘title’ attribute,
 * of any HTMLAnchorElement pointing to one of its articles.
 *
 * @see <https://www.mediawiki.org/wiki/Specs/Summary/1.3.0>
 * @see <https://en.wikipedia.org/api/rest_v1/#/Page%20content/get_page_summary__title_>
 *
 * @param {!Iterable<Element>=} elts  Link elements, non-Wikipedia href get filtered
 * @param {!Map<string, Promise<string>>=} memo  (Previously recorded) fetched summaries
 * @param {RegExp=} regexp  Matches Wikipedia URLs, extracts language & article name (“slug”)
 * @param {number=} reqMax  Max API calls; Wikimedia asks for ≤ 200 requests per second
 * @param {!RequestInit=} reqParams  Fetch parameters
 * @return {!Map<string, Promise<string>>} (Updated) fetched summaries
 */
function titleWikipediaAnchors(
	elts = document.getElementsByTagName("a"),
	memo = new Map,
	regexp = /^https?:\/\/([a-z\-]{2,64})\.wikipedia\.org\/wiki\/([^?#]+)/i,
	reqMax = 100,
	reqParams = {
		"headers": {
			"Accept": "application/json; charset=utf-8; "
				+ 'profile="https://www.mediawiki.org/wiki/Specs/Summary/1.4.2"',
		},
		"cache": "force-cache",  // HTTP-expired data is totally OK
		"credentials": "omit",
		"mode": "cors",
		// Wikimedia asks to “Set a unique User-Agent […] to contact you quickly.”
		// By using CORS, the ‘Origin’ header points to us, the embedding site.
	}
) {
	for (const elt of elts) {
		if (elt.title !== "") continue;

		const parts = elt.href.match(regexp);
		if (parts === null) continue;

		const [ , lang, slug] = parts;
		if (!elt.matches(`:lang(${bcp47FromWikipedia(lang)})`)) continue;

		const url = `https://${lang}.wikipedia.org/api/rest_v1/page/summary/`
			  + slug.replace(/\//g, "%2F");

		if (!memo.has(url)) {
			if (memo.size >= reqMax) continue;

			memo.set(url, fetch(url, reqParams)
					 .then(resp => resp.json())
					 .then(json => (json.titles ? json.titles.normalized : slug)
						   + (json.description ? ` (${json.description})` : "")));
		}
		memo.get(url).then(txt => { elt.title = txt; });
	}
	return memo;
}

console.log("wkpd: fetched", titleWikipediaAnchors());