MC-XOBAHCK,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<ul>
<li>Пункт 1</li>
<li>Пункт 2</li>
<li>Пункт 3</li>
</ul>
<script>
function navigation(e) {
let el = e.target;
let listItem = [...this.children];
let index = listItem.indexOf(el);
console.log(index);
}
document.querySelector('ul').addEventListener('click', navigation);
</script>
</body>
</html>