if (document.anchors.length >= 5) {
  console.log("found too many anchors");
}

<h1>Title</h1>
<h2><a name="contents">Contents</a></h2>
<ul id="toc"></ul>

<h2><a name="plants">Plants</a></h2>
<ol>
  <li>Apples</li>
  <li>Oranges</li>
  <li>Pears</li>
</ol>

<h2><a name="veggies">Veggies</a></h2>
<ol>
  <li>Carrots</li>
  <li>Celery</li>
  <li>Beats</li>
</ol>

const toc = document.getElementById("toc");
for (const anchor of document.anchors) {
  const li = document.createElement("li");
  const newAnchor = document.createElement("a");
  newAnchor.href = `#${anchor.name}`;
  newAnchor.textContent = anchor.text;
  li.appendChild(newAnchor);
  toc.appendChild(li);
}