const firstLIFO = [];
const secondLIFO = [];

const FIFO = {
  enqueue: (object => {
    firstLIFO.push(object);
  }),

  dequeue: (() => {
    if (secondLIFO.length === 0) {
      if (firstLIFO.length === 0) return null;
      while (firstLIFO.length) {
        secondLIFO.push(firstLIFO.pop());
      }
    }
    return secondLIFO.pop();
  })
}

FIFO.enqueue(1);
FIFO.enqueue(2);
FIFO.enqueue(3);
FIFO.enqueue(4);
console.log(FIFO.dequeue());
console.log(FIFO.dequeue());
console.log(FIFO.dequeue());
FIFO.enqueue(5);
console.log(FIFO.dequeue());
console.log(FIFO.dequeue());