JS: Convert String to Upper or Lower Based on Even or Odd without using ToUpperCase/ToLowerCase

gistfile1.txt

function output(str, pos) {
  var result = ''
  for (var i = 0; i <= str.length; i++) {
    var mod = i % 2;
    var code = str.charCodeAt(i);
    if (pos === 'even' && mod === 0) {
      result += changeChar(code);
    } else if (pos === 'odd' && mod !== 0) {
      result += changeChar(code);
    } else {
      result += String.fromCharCode(code);
    }
  }
  
  return result
}

function changeChar(code) {
  if (code > 64 && code < 91) {    
    return String.fromCharCode(code + 32)
  } else {
    return String.fromCharCode(code - 32)
  } 
}

document.write('odd (aZbe to azbE): ' + output('aZbe', 'odd'));

document.write('<br />even (aZbe to AZBe): ' + output('aZbe', 'even'));