Array indexOf() Reference Error?

Question

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);

Answer 1

I often add functionality to the built in Javascript objects via their prototype, including indexOf:
* Array.indexOf - Like String.indexOf() but checks for an object in an array
* @method
* @param  {Object} obj object to find in the array
* @returns {Number} location in the array of the object, or -1 if not found
if(!Array.prototype.indexOf) {
    Array.prototype.indexOf = function(obj) {
        for(var i=0; i<this.length; i++) {
            if(obj===this[i]) {
                return i;
        return -1;