ROT18 Implementation in JavaScript

rot18.js

const rot18 = (() => {
	const forward = "ABCDEFGHIJKLM01234";
	const reverse = "NOPQRSTUVWXYZ56789";
	const charMap = new Map();
	for (let loopIndex = 0; loopIndex < 18; loopIndex++) {
		const key = forward[loopIndex];
		const val = reverse[loopIndex];
		charMap.set(key, val);
		charMap.set(val, key);
		if (/\D/.test(key)) {
			charMap.set(key.toLowerCase(), val.toLowerCase());
			charMap.set(val.toLowerCase(), key.toLowerCase());
		}
	}
	return input => input.replace(/\w/gu, char => charMap.get(char) ?? char);
})();