Array indexOf() Reference Error?

Hello,

I have these simple code lines and on line 20 I'm getting the error in the comment but I cant find the problem in my code

  var directions      = [0,1,2,3];
  var lockedDirection = 0;
  var lastDirection   = 2; // sample code

  switch (lastDirection) {
  case 0:
       lockedDirection = 2;
       break;
  case 1:
       lockedDirection = 3;
       break;
  case 2:
       lockedDirection = 0;
       break;
  case 3:
       lockedDirection = 1;
       break;
  };


  var a = directions.indexOf(lockedDirection); // ReferenceError: Function directions.indexOf is undefined
  directions.splice(a, 1);