Untitled

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="utf-8">
    <title>css</title>

</head>
<body>
<ul id="elem">
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
    <li>text</li>
</ul>

<script>
    let list = document.querySelector("#elem"),
        listItems = list.children,
        listItemsLast = listItems[listItems.length - 1];
    listItemsLast.style.color="red";
</script>
</body>
</html>