Add multiple Li to Ul with javascript

newLi is a reference to the node you wish to append to the formList. It can exist just once.

So, first time it executes formList.appendChild(newLi), it will append it to formList. Second time it executes, it would be removed from the first position and now added to second position. Same for third position.

You cannot append the same node multiple times using appenChild.

The Node.appendChild() method adds a node to the end of the list of children of a specified parent node. If the given child is a reference to an existing node in the document, appendChild() moves it from its current position to the new position (there is no requirement to remove the node from its parent node before appending it to some other node). This means that a node can't be in two points of the document simultaneously. So if the node already has a parent, the node is first removed, then appended at the new position.

Description at MDN

var form = document.getElementById("form");
var newUl = document.createElement('ul');

newUl.id = "formList";
form.appendChild(newUl);

newButton.addEventListener("click", function(){
    var newLi = document.createElement('li');
    newUl.appendChild(newLi);
})

• You need to create the ul once, and assign to it the id = "formList", then append it to the form
• On every click, create new li element
• You don't need to select the ul again, because you already has a reference to it.

Here you can find a working fiddle https://jsfiddle.net/LeoAref/m5d0bzeL/

There's too much to correct in your post all at once. But if you're trying to template your LI, you can clone it using cloneNode

var template_ul = document.createElement('ul');
var template_li = document.createElement('li');
let newButton = document.getElementById('new');
var count = 0;

newButton.addEventListener("click", function() {
  let list = template_ul.cloneNode();
  document.getElementById('form').appendChild(list);

  
  let first_li = template_li.cloneNode();
  var text = document.createTextNode(++count);
  first_li.appendChild(text);
  
  list.appendChild(first_li);
  list.appendChild(template_li.cloneNode());
  list.appendChild(template_li.cloneNode());
});

body { 
  text-align:center;
}
button { margin: 4px auto 1em; }

ul {
  margin: 0 auto 1em;
  padding: 0;
  width: 50%
}

ul:nth-child(2n) li{
  background:#c90;
}


li {
  list-style-type: none;
  background: #09c;
  height: 20px;
  padding: 2px;
}

ul,
li {
  border: 1px solid #444;
}

<button id="new">New</button>
<div id="form">
</div>

function addNode(){
  var x = document.createElement("ul");
  x.setAttribute("id","myList");
  document.body.appendChild(x);

  var myListItem = document.createElement("li");
  var textNode = document.createTextNode("List Item one");
  var t2 = document.createTextNode("second item");

  myListItem.appendChild(textNode);

  document.getElementById("myList").appendChild(newLi("FirstItem"));
  document.getElementById("myList").appendChild(newLi("Second Item"));
  document.getElementById("myList").appendChild(newLi("Third Item"));
  document.getElementById("myList").appendChild(newLi("Fourth Item"));
}

//function which generates a new li tag and accepts a string for TextNode
    function newLi(x)
    {
      var m = document.createElement("li");
      var t = document.createTextNode(x);
      m.appendChild(t);
      return m;
    }