Function for shifting all letters in a string one letter forward in the alphabet

lettershifter.js

function letterShifter(message) { 
  var lc_alphabet, uc_alphabet, lc_key, uc_key, coded, i, ch, index;

  lc_alphabet = "abcdefghijklmnopqrstuvwxyz";
  uc_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  lc_key = "bcdefghijklmnopqrstuvwxyza";
  uc_key = "BCDEFGHIJKLMNOPQRSTUVWXYZA";
  coded = "";

  for(i = 0; i < message.length; i++) {
    ch = message.charAt(i);
    if(isNaN(ch * 1) === false) {
      coded = coded + ch;
    } else if(ch.match(/^[A-Za-z]+$/) === null) {
      coded = coded + ch;
    } else if(ch === ch.toLowerCase()) {
      index = lc_alphabet.indexOf(ch);
      coded = coded + lc_key.charAt(index);
    } else if(ch === ch.toUpperCase()) {
      index = uc_alphabet.indexOf(ch);
      coded = coded + uc_key.charAt(index);
    }
  }
  return coded;         
}