JS Caesar Cypher

caesar-cypher.js

const letters = 'abcdefghijklmnopqrstuvwxyz';

let circleBackAlphabet = new Proxy({ letters }, {
	get: (obj, index) => {
		let ltrs = obj.letters;
		while(index > ltrs.length-1) {
			index = index - ltrs.length;
		}
		return ltrs[index];
	}
});

function caesarCypher(str, x) {
	str = str.toLowerCase();
	return [...str].map(c => {
		let i = letters.indexOf(c);
		if (i === -1) return c;
		return circleBackAlphabet[i+x];
	}).join('');
}

// ----

console.log('  5 =>', circleBackAlphabet[5]);
console.log(' 25 =>', circleBackAlphabet[25]);
console.log(' 37 =>', circleBackAlphabet[37]);
console.log('284 =>', circleBackAlphabet[284]);

console.log('Hello world! (x = 2) =>', caesarCypher('Hello world!', 2));
console.log('Lorem ipsum dolor sit amet (x = 735) =>', caesarCypher('Lorem ipsum dolor sit amet', 735));
console.log('!1349;:#€ (x = 4) =>', caesarCypher('!1349;:#€', 4));