Taverse an Array circularly in both direction by adding prev and next methods to the Array prototype

array_traverse.js

function ExtendedArray() {
  let arr = [];
  arr.push.apply(arr, arguments);
  arr.pos = 0;
  arr.prev = function () {
    return this[this.pos = (this.pos || this.length)-1];
  };
  arr.next = function () {
    return this[this.pos = (++this.pos % this.length)];
  };
  return arr;
}

var a = ExtendedArray(1, 2, 3, 4, 5);

console.log(a.prev()); // 5
console.log(a.prev()); // 4
console.log(a.prev()); // 3
console.log(a.prev()); // 2
console.log(a.prev()); // 1
console.log(a.prev()); // 5
console.log(a.prev()); // 4
console.log('----------------');
console.log(a.next()); // 5
console.log(a.next()); // 1
console.log(a.next()); // 2
console.log(a.next()); // 3
console.log(a.next()); // 4