Task completion for a checklist in JavaScript

Attach an event to the button 'done' in the same way:

javascriptlet mark = document.createElement('span');
mark.classList.add('mark');
mark.textContent = 'done';
mark.addEventListener('click', function() {
  // here will be the code for marking the task as done
});
li.appendChild(mark);

In order to mark the task as done, you need to attach some CSS class to the corresponding li tag. Let's name, for example, this class - done.

Then for this class you can write the corresponding CSS code:

css#list .done {
  text-decoration: line-through;
}