Simple Caesar Decryption in JavaScript

julius.js

function caesar(str, increment) {
  const letters = 'abcdefghijklmnopqrstuvwxyz';
  return str
    .toLowerCase()
    .split('')
    .map(c => {
      const index = letters.indexOf(c);
      if (index === -1) return c;
      let newIndex  = index + increment;
      if (newIndex < 0) {
        newIndex = letters.length - 1;
      } else if (newIndex >= letters.length) {
        newIndex = newIndex- letters.length;
      }
      return letters[newIndex];
    })
    .join('');
}

function allCaesar(str) {
  for (let i = 0; i < 26; i++) {
    console.log(caesar(str, i));
  }
}

allCaesar('pbqr vf neg. Nanepul fubhyq gnxr bire gur jbeyq. wninfpevcg sbe gur shgher?')