XOR String Encryption with non static key

var alphabet = 'abcdefghijklmnopqrstuvwxyz',
shipher = [],
key,
output = [];

function generateKey(input, string){
  for(var j = 0; j < input.length; j++){
    var numRand = Math.floor( Math.random() * (string.length-1) ); //gets a random integer between [0 - alphabet.length]
    shipher.push( string.charAt(numRand).toUpperCase() ); //appends next char to shipher.
  }
  return shipher; //this is the final char array(which is gonna be our key).
}

function encryptDecrypt(input) {

  if( !(input === output.join("")) ) //prevents generateKey() to run when decrypting, to avoid repetitions.
    key = generateKey(input, alphabet); //get key.

  output = []; //makes output empty, before appending to it.

  for (var i = 0; i < input.length; i++) {
    var charCode = input.charCodeAt(i) ^ key[i % key.length].charCodeAt(0); //XOR
    output.push( String.fromCharCode(charCode) ); //appends encrypted/decrypted value
  }
  return output.join("");
}


var encrypted = encryptDecrypt( "string to encrypt" );
console.log("Encrypted: " + encrypted);

var decrypted = encryptDecrypt(encrypted);
console.log("Decrypted: " + decrypted);