Create emoji flag (not only in JS, the logic is the same for others)

createEmojiFlag.js

/* jshint esversion: 6,-W097, -W040, browser: true, expr: true, undef: true */
/**
 * Flags emojis are just concation of two emoji letters (see [1]).
 * Inspired by [2].
 * 
 * - [1] https://apps.timwhitlock.info/emoji/tables/iso3166
 * - [2] https://twitter.com/rauschma/status/1491189507261333505
 * 
 * @param {string|string[]} country Two letters (e.g. 'cs', 'us', …)
 * @returns {string}
 * @example
 * console.log(createEmojiFlag("cz")); //🇨🇿
 */
function createEmojiFlag([ l1, l2 ]){
    const a= "a".codePointAt(0) - 0x1F1E6;
    return [ l1, l2 ].map(l=> String.fromCodePoint(l.toLowerCase().codePointAt(0) - a)).join("");
}