iterator.js

// simple iterator
var wm = new WeakMap()
function prev(array) {
  var counter = wm.get(array)
  if (!counter) {
    return undefined
  }
  counter -= 1
  wm.set(array, counter)
  return array[counter]
}
function next(array) {
  var counter = wm.get(array)
  if (counter === undefined) {
    wm.set(array, 0)
    return array[0]
  }
  if (counter > array.length) {
    return undefined
  }
  counter += 1
  wm.set(array, counter)
  return array[counter]
}

iterator_example.js

var array = [..."abcd"]
next(array) // a
next(array) // b
next(array) // c

prev(array) // b
prev(array) // a

next(array) // b
next(array) // c
next(array) // d
next(array) // undefined

prev(array) // d