クロージャ版iterator

iterator.js

var iterator = function(arr) {
  if (!arr.length) throw new TypeError(typeof arr + ' is not permitted.');
  var copy = (arr instanceof Array) ? arr.slice() : Array.prototype.slice.call(arr);
  var len = copy.length;
  var _move = function(method_name) {
    var current = copy[method_name]();
    len--;
    if (len === -1) throw new Error('Stop Iteration');
    return current;
  };
  return {
    next: function() {
      return _move('shift');
    },
    prev: function() {
      return _move('pop');
    }
  };
}