Array flatter without recursion.

app.js

function flatter (arr) {
  var clonedList = arr.slice(0)
  var flatList = []
  while (clonedList.length) {
    var item = clonedList.shift()
    if (item instanceof Array === true) {
      clonedList = item.concat(clonedList)
    } else {
      flatList.push(item)
    }
  }
  return flatList
}

var res = flatter([1,[2,3,[4,5,6,[7,8]],9],10]);
console.log(res);