Caesar's cipher, the shift cipher

caesar-cypher.js

const shift = (cypherShift, alphabet, messageChar) => {
	const charCode = (ch) => ch.toLowerCase().charCodeAt(0);
	
	let begin = charCode(alphabet.from); 
	let end = charCode(alphabet.to);
	[begin, end] = (begin > end) ? [end, begin] : [begin, end];

	const alphabetLength = end - begin; 
	const shiftLength = cypherShift % alphabetLength;
	if (alphabetLength === 0) return messageChar;

	let result = charCode(messageChar);
	if (result > end || result < begin) return messageChar;

	result += shiftLength;
	if (result > end) {
		result = begin - 1 + result - end;
	} else if (result < begin) {
		result = end - 1 + begin - result;
	}
	return String.fromCharCode(result);
}

const cypher = (cypherShift, alphabet, message) => {
	const shiftCurry = (messageChar) => shift(cypherShift, alphabet, messageChar);
	return message
		.toLowerCase()
		.split('')
		.map(shiftCurry)
		.join('');
}

const decypher = (cypherShift, alphabet, message) => {
	return cypher(-cypherShift, alphabet, message);
}

const CYPHER_SHIFT = 3;
const message = 'Good sense is the most equally distributed';
const A = 'a', Z = 'z';

const myCaesarCypher = cypher(CYPHER_SHIFT, {from: A, to: Z}, message);
console.log(message);
console.log(myCaesarCypher);