Warm-up JS exercise to start off the day - Vigenère Cipher

Vigenère cipher.js

function encryptMessage(message, keyword) {
  var cipher = [];
  var alpha = 'abcdefghigklmnopqrstuvwxyz'.split('');
  var alphaLength = alpha.length;
  //split input
  var message = message.split('');
  var keyword = keyword.split('');
  //config for key position
  var keyIndex = 0;
  var keyLength = keyword.length;
  //encode loop
  for(var i=0; i < message.length; i++) {
      //get message alpha index
      var messageValue = alpha.indexOf(message[i]);
      // get keyword alpha index
      var keyValue = alpha.indexOf(keyword[keyIndex]);
      //add the two
      var charValue = (messageValue + keyValue);
      //readjust if charValue is greater than alpha
      //then assign char
      if (charValue >= alphaLength){
        var char = alpha[(charValue - alphaLength)];
      }
      // assign char that is less then alphalength
      else {
        var char = alpha[charValue];
      }
      //reset the keyindex
      if (++keyIndex >= keyLength){
        keyIndex = 0;
      }
      else {
        keyIndex;
      }
      console.log(keyIndex);
      //push character into cypher
      cipher.push(char)
    }
  return cipher.join('');
}