ROT18 Implementation in JavaScript

rot18.js

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());
	}
}
const rot18 = input => input.replace(/\w/gu, char => charMap.get(char) ?? char);