JS Array Rotate (v7)

var rotateArray = function(a, inc) {
    for (var l = a.length, inc = (Math.abs(inc) >= l && (inc %= l), inc < 0 && (inc += l), inc), i, x; inc; inc = (Math.ceil(l / inc) - 1) * inc - l + (l = inc))
    for (i = l; i > inc; x = a[--i], a[i] = a[i - inc], a[i - inc] = x);
    return a;
    };

var arr = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'];
rotateArray(arr, -1);

Array.prototype.rotate = (function() {
  return function(inc) {
    for (var l = this.length, inc = (Math.abs(inc) >= l && (inc %= l), inc < 0 && (inc += l), inc), i, x; inc; inc = (Math.ceil(l / inc) - 1) * inc - l + (l = inc))
    for (i = l; i > inc; x = this[--i], this[i] = this[i - inc], this[i - inc] = x);
    return this;
  };
})();

var arr = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'];
arr.rotate(1);

Array.prototype.rotate = (function() {
  var unshift = Array.prototype.unshift,
      splice = Array.prototype.splice;

  return function(count) {
    var len = this.length >>> 0,
        count = count >> 0;

    unshift.apply(this, splice.call(this, count % len, len));
    return this;
  };
})();

var arr = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'];
arr.rotate(1);

function directionalRotate(direction, counter, arr) {
  counter = direction ? (counter < arr.length - 1 ? counter + 1 : 0) : (counter > 0 ? counter - 1 : arr.length - 1)
  var currentItem = arr[counter]
  var priorItem = arr[counter - 1] ? arr[counter - 1] : arr[arr.length - 1]
  var nextItem = arr[counter + 1] ? arr[counter + 1] : arr[0]
  return {
    "counter": counter,
    "current": currentItem,
    "prior": priorItem,
    "next": nextItem
  }
}
var direction = true // forward
var counter = 0
var arr = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'];

directionalRotate(direction, counter, arr)