Challenge Solution

const ul = document.querySelector('ul');
const lis = ul.children;
const addItemInput  = document.querySelector('.addItemInput');
const addItemButton = document.querySelector('.addItemButton');
const listItems = document.getElementsByTagName('li');

Array.prototype.forEach.call(lis, li => { attachListItemButtons(li);} );

function addButtonUp(li) {
  let up = document.createElement('button');
  up.className = 'buttonUp';
  up.textContent = 'Up';
  li.appendChild(up);
}

function addButtonDown(li) {
  let down = document.createElement('button');
  down.className = 'buttonDown';
  down.textContent = 'Down';
  li.appendChild(down);
}

function addButtonRemove(li) {
  let remove = document.createElement('button');
  remove.className = 'buttonRemove';
  remove.textContent = 'Remove';
  li.appendChild(remove);
}

function attachListItemButtons(li) {
  if (li.previousElementSibling !== null) {
    addButtonUp(li);
  }
  if (li.nextElementSibling !== null) {
    addButtonDown(li);
  }
  addButtonRemove(li);
}

function swapButtons(first, second){
  // first will be insertBefore()'d second AFTER this function call
  if (second.previousElementSibling === null) {
    let bup = first.getElementsByClassName('buttonUp')[0];
    let bdown = second.getElementsByClassName('buttonDown')[0];
    second.insertBefore(bup, bdown);
  }
  if (first.nextElementSibling === null) {
    let brem = first.getElementsByClassName('buttonRemove')[0];
    let bdown = second.getElementsByClassName('buttonDown')[0];
    first.insertBefore(bdown, brem);
  }
}

ul.addEventListener('click', (event) => {
  if (event.target.tagName ==='BUTTON') {
    let li = event.target.parentNode;
    let prevli = li.previousElementSibling;
    let nextli = li.nextElementSibling;
    let ul = li.parentNode;

    if (event.target.className === 'buttonRemove') {
      if (prevli === null && nextli !== null) {
        let nextUp = nextli.getElementsByClassName('buttonUp')[0];
        nextli.removeChild(nextUp);
      }
      else if (nextli === null && prevli != null) {
        let prevDown = prevli.getElementsByClassName('buttonDown')[0];
        prevli.removeChild(prevDown);
      }
      ul.removeChild(li);
    }

    if (event.target.className === 'buttonUp') {
      if (prevli !== null) {
        swapButtons(li, prevli);
        ul.insertBefore(li, prevli);
      }
    }

    if (event.target.className === 'buttonDown') {
      if (nextli !== null) {
        swapButtons(nextli, li);
        ul.insertBefore(nextli, li); 
      }
    }
  }
});

addItemButton.addEventListener(
  'click', () => {
    let ul = document.getElementsByTagName('ul')[0];

    if (ul.children.length > 0) {
      // if you are adding an item to the end of the list,
      // give the current last item a "Down" button
      let currLast = ul.lastElementChild;
      let downButton = document.createElement('button');
      downButton.className = 'buttonDown';
      downButton.textContent = 'Down';
      let removeButton = currLast.getElementsByClassName('buttonRemove')[0];
      currLast.insertBefore(downButton, removeButton);
    }

    let li = document.createElement('li');
    li.textContent = addItemInput.value;
    ul.appendChild(li);
    attachListItemButtons(li);
    addItemInput.value = '';
})