Why am I getting \u0000 instead of characters when using String.fromCharCode?

Question

I could not find this particular problem addressed.

The object is to decode a Caesar cipher. My code works to the point of putting the correct letter codes into an array. I can manually turn that array into the correct string like this:

String.fromCharCode(89, 79, 85, 32, 68, 73, 68, 32, 73, 84, 33);

But when I try to turn the array into a string like this:

  return String.fromCharCode(arr.join(", "));

it returns \u0000, which I gather is the Unicode null character.

Can anybody explain what's going on?

Here's my complete code:

function rot13(str) {
  var arr = [];
  for (var i = 0; i < str.length; i++){
    if (str.charCodeAt(i) > 77 && str.charCodeAt(i) < 91){
      arr.push(str.charCodeAt(i) - 13);
    } else if (str.charCodeAt(i) <=77 && str.charCodeAt(i) > 64) {
      arr.push(str.charCodeAt(i) + 13);
    } else {
      arr.push(str.charCodeAt(i));
    }
  }
  console.log(arr);
  return String.fromCharCode(arr.join(", "));
}

rot13("LBH QVQ VG!");
String.fromCharCode(89, 79, 85, 32, 68, 73, 68, 32, 73, 84, 33);

Answer 1

arr.join(',') will not expand to a list of arguments to a function. You either need to use Function.apply (.apply(null, arr)) or if you have ES6 available, use the spread operator:

return String.fromCharCode(...arr);

or

return String.fromCharCode.apply(null, arr);

function rot13(str) {
  var arr = [];
  for (var i = 0; i < str.length; i++){
    if (str.charCodeAt(i) > 77 && str.charCodeAt(i) < 91){
      arr.push(str.charCodeAt(i) - 13);
    } else if (str.charCodeAt(i) <=77 && str.charCodeAt(i) > 64) {
      arr.push(str.charCodeAt(i) + 13);
    } else {
      arr.push(str.charCodeAt(i));
    }
  }

  return String.fromCharCode.apply(null, arr);
}

console.log(rot13("LBH QVQ VG!"));