Rot13 in Javascript

rot13.js

const LOWER_START = 97;
const UPPER_START = 65;
const LOWER_END = LOWER_START + 26;
const UPPER_END = UPPER_START + 26;

function newAsciiOf(char) {
  var newAscii = char.charCodeAt() + 13;

  if (/[a-z]/.test(char) && newAscii >= LOWER_END) {
    newAscii = (LOWER_START + (newAscii % LOWER_END));
  } else if (/[A-Z]/.test(char) && newAscii >= UPPER_END) {
    newAscii = (UPPER_START + (newAscii % UPPER_END));
  }

  return newAscii;
}

function rot13(string) {
  var newStr = '';
  var char;

  for(var i = 0; i < string.length; i += 1) {
    char = string[i];
    if (/[^a-zA-Z]/.test(char)) {
      newStr += char;
    } else {
      newStr += String.fromCharCode(newAsciiOf(char));
    }
  }

  return newStr;
}

console.log(rot13('Teachers open the door, but you must enter by yourself.'));