<nav>
  <a class="sidebar-chunk" href="page1.html">Page 1</a>
  <a class="sidebar-chunk" href="page2.html">Page 2</a>
  <a class="sidebar-chunk" href="page3.html">Page 3</a>
</nav>  

.sidebar-chunk{
  display:block;
}

const nav = document.querySelector(".sidebar-chunk").parentElement;
const newA = document.createElement("A");
newA.textContent = "New Link";
newA.href = "newPage.html";
nav.appendChild(newA);