Sharing the extra work I have done to improve this solution

// Creating a new button that accepts text to name the text and class name
function newButton(li, txt) {
  let button = document.createElement('button');
  button.className = txt;
  button.textContent = txt;
  li.appendChild(button);
}

// declaring the newButton function & using an array of button names.
// Would make it much easier to create new or remove existing buttons
function attachListItemButtons(li, name) {
  name = ['up', 'down', 'remove'];
  for(i = 0; i < name.length; i += 1) {
    newButton(li, name[i]);
  }
}

for(let i = 0; i < lis.length; i += 1) {
  attachListItemButtons(lis[i]);
}