A simple circular javascript data structure (backed by an array)

Circular.js

/**
 * A simple circular data structure
 */
function Circular(arr, startIntex){
  this.arr = arr;
  this.currentIndex = startIntex || 0;
}

Circular.prototype.next = function(){
  var i = this.currentIndex, arr = this.arr;
  this.currentIndex = i < arr.length-1 ? i+1 : 0;
  return this.current();
}

Circular.prototype.prev = function(){
  var i = this.currentIndex, arr = this.arr;
  this.currentIndex = i > 0 ? i-1 : arr.length-1;
  return this.current();
}

Circular.prototype.current = function(){
  return this.arr[this.currentIndex];
}



/**
 *     USAGE
 */
var c = new Circular([1,2,3,4]);
c.current(); // 1
c.next();    // 2
c.current(); // 2
c.next();    // 3
c.next();    // 4
c.next();    // 1
c.next();    // 2
c.prev();    // 1
c.current(); // 1
c.prev();    // 4
c.current(); // 4

You can use the modular (%) operator: https://stackoverflow.com/questions/18725428/going-back-to-the-first-index-after-reaching-the-last-one-in-an-array