non-recursive array flattening

index.js

var a = [1, 2, [2], [1, 2, [3, 4, [5]]], [[4, 5], 3, 2, 1], [1]];

function flatten(array) {
  var stack = [];
  var out = [];
  var tail;
  stack.push({array: array, idx: 0});
  while(stack.length) {
    tail = stack[stack.length - 1];
    if (tail.idx >= tail.array.length) {
      stack.pop();
    } else if (tail.array[tail.idx] instanceof Array) {
      stack.push({ array: tail.array[tail.idx++], idx: 0 });
    } else {
      out.push(tail.array[tail.idx++]);
    }
  }
  return out;
}

console.log(flatten(a));