Toggle a attribute with id 'user'+i with a checkbox with id 'checkbox'+i

first thanks for your help in advance. I'm pretty newbie in jquery and javascript and I don't speak so much english so i cound't found nothing liked in my research.

This is my html

<li></li>
<li></li>
<li id="user1"></li>
<input type="checkbox" id="checkbox1">

<li></li>
<li></li>
<li id="user2"></li>
<input type="checkbox" id="checkbox2">

I want to everytime that a checkbox with id ("checkbox"+i) is checked I toggle a class in the ("user"+i) to push in a list and then send with post request.

I tryed this

for(i = 1; i < 3; i++) {    
   document.getElementById('checkbox' + i).addEventListener('change', function() {
        document.getElementById('user' + i).setAttribute("class", "passInterestIds");
  });
} 

but it only gives me user3 independent of the checkbox

How can I Make this work?