adding to innerHTML after JavaScript for loop

Question

So im just testing a basic functionality on JavaScript. Im trying to set the <li> within the <ul> but for some simple reason my li gets placed outside the ul.

var test = document.getElementById('testdiv');
var data = [1,2,3,4,5];
    
test.innerHTML += '<ul>'; 
for (var i = 0; i < data[i]; i++)
{
test.innerHTML += '<li>' + i + '=' + data[i] + '</li>';
} 
test.innerHTML += '</ul>';

.start{
    border: 1px solid #000;
}  

  
     <div class="start" id="testdiv"></div>
  

Current html outcome looks like this:

<div class="start" id="testdiv">
 <ul></ul>
 <li>0=1</li>
 <li>1=2</li>
 <li>2=3</li>
 <li>3=4</li>
 <li>4=5</li>
</div>

Expected outcome with an simple explanation would be much appreciated.

<div class="start" id="testdiv">
  <ul>
   <li>0=1</li>
   <li>1=2</li>
   <li>2=3</li>
   <li>3=4</li>
   <li>4=5</li>
  </ul>
</div>

Answer 1

@MikeChristensen explains why it is bad practice to use innerHtml within for loop.

Every time innerHTML is set, the HTML has to be parsed, a DOM constructed, and inserted into the document.

Try to use temporary string variable instead:

var myList = '<ul>';
for (var i = 0; i < data[i]; i++) { 
  myList += '<li>' + i + '=' + data[i] + '</li>'; 
} 
myList += '</ul>';
test.innerHTML = myList;

Answer 2

Instead of using above approach you can use below one.

Possible reason is browser might adding the closing tag automatically to the opened tag.

var test = document.getElementById('testdiv');
var data = [1,2,3,4,5];
var ulelement = document.createElement("ul");    
for (var i = 0; i < data[i]; i++)
{
ulelement.innerHTML += '<li>' + i + '=' + data[i] + '</li>';
} 
test.appendChild(ulelement);

.start{
    border: 1px solid #000;
}

<div class="start" id="testdiv"></div>