Solve Caesar Cyphers

CaesarSolver.js

function CaesarSolver(str) {
    let alphabet = 'abcdefghijklmnopqrstuvwxyz';
    str = str.toLowerCase();
    let words = str.split(/\s+/);
    let permutations = [];
    for (let j = 0; j < alphabet.length; j++) {
        let list = [];
        for (let i = 0; i < words.length; i++) {
            let word = "";
            let letters = words[i].split('');
            for (let k = 0; k < letters.length; k++) {
                word += alphabet[(alphabet.indexOf(letters[k]) + j)%alphabet.length];
            }
            list.push(word);
        }
        permutations.push(list.join(' '));
    }
    return permutations;
}