i think that the code is the same but i don't know why it make different results

Why does the code make difference output and it is almost the same input?
Input:

var Add = document.getElementById('AddElement'); //the button
var Element = document.getElementById('element'); // the input text
var ListParent = document.querySelector('ul');
    
Add.addEventListener('click', function(){
    var AddText = document.createElement('li').appendChild(document.createTextNode("hello"));
    ListParent.appendChild(AddText);
})

Output:

first click:
hello

second click:
hellohello

Input:

var Add = document.getElementById('AddElement');
var Element = document.getElementById('element');
var ListParent = document.querySelector('ul');
    
Add.addEventListener('click', function(){
    var AddText = document.createElement('li');
    AddText.appendChild(document.createTextNode('hello'));   
    ListParent.appendChild(AddText);
})

Output:
first click:

• hello

second click:

• hello
• hello

>Solution :

appendChild returns the appended node:

const div = document.createElement('div');
const parent = document.createElement('div');
const result = parent.appendChild(div);
console.log(result === div);

So when you do

var AddText=document.createElement('li').appendChild(document.createTextNode("hello"));
                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The value put into addText is the created text node. In contrast, with

var  AddText = document.createElement('li');

it’s the <li>, not the text node.

So, with the first approach, the created <li> doesn’t get used (and probably isn’t what you want).

If you wanted to use the <li>, and slim your code down, assign to the .textContent of the <li> instead.

Add.addEventListener('click', function(){
    ListParent.appendChild(document.createElement('li'))
      .textContent = element.value;
})