JSFiddle - React, Tailwind, and code Playground

HTML

<h1 id="h1-01">Title 1</h1>
<h1 id="h1-02">Titel 2</h1>
<h1 id="h1-03">Titel 3</h1>
<h1 id="h1-04">Titel 4</h1>

JavaScript

var h1 = document.getElementsByTagName('h1'),
    ol = document.createElement('ol');

for (var i=0; i<h1.length; i++) {
    var id  = h1[i].id,
        txt = h1[i].innerHTML,
        li  = document.createElement('li'),
        a   = document.createElement('a');
    
    a.href = id;
    a.innerHTML = txt;
    
    li.appendChild(a);
    ol.appendChild(li);
}

document.body.innerHTML = '';
document.body.appendChild(ol);