Can Javascript do a Caesar cipher both ways in one line?

tried to create a function that does caesar cipher (alphabet shift by key value). Wanted to do it in least lines possible (one line?). Was able to create a function:

const alphabet = "abcdefghijklmnopqrstuvwxyz".split('')
let regExp = /[a-zA-Z]/;

function cypher(input, key){
    return input.split('').map((a) => (regExp.test(a)) ? alphabet[(alphabet.indexOf(a) + key) % alphabet.length] : a).join('')
}

then I figured out that I can use the same function to decipher the code, except with reversed alphabet

const alphabetReverse = alphabet.reverse();

function decypher(input, key){
    return input.split('').map((a) => (regExp.test(a)) ? alphabetReverse[(alphabetReverse.indexOf(a) + key) % alphabetReverse.length] : a).join('')

}

then, I tried to do function that does both (if key < 0 it decyphers):

function cypher(input, key){
    let alphabet = "abcdefghijklmnopqrstuvwxyz".split(''); // 26 long, 0 to 25
    if(key < 0){
        alphabet = alphabet.reverse();
        key = -key;
    }
    return input.split('').map((a) => (regExp.test(a)) ? alphabet[(alphabet.indexOf(a) + key) % alphabet.length] : a).join('')
}

I cant figure out a way to do both-ways function in an elegant way like just one-way.
(Sorry if i’m missing something, this is more of an learning question, I’m really new to programming, like 2-3 weeks.

Tried creating both-ways function in one line, couldnt do it.