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);