Simple Table of Contents from Headings

simpletoc.js

//https://stackoverflow.com/questions/187619/is-there-a-javascript-solution-to-generating-a-table-of-contents-for-a-page

window.onload = function () {
    var toc = "";
    var level = 0;

    document.getElementById("VNAArticle").innerHTML =
        document.getElementById("VNAArticle").innerHTML.replace(
            /<h([\d])>([^<]+)<\/h([\d])>/gi,
            function (str, openLevel, titleText, closeLevel) {
                if (openLevel != closeLevel) {
                    return str;
                }

                if (openLevel > level) {
                    toc += (new Array(openLevel - level + 1)).join("<ul>");
                } else if (openLevel < level) {
                    toc += (new Array(level - openLevel + 1)).join("</ul>");
                }

                level = parseInt(openLevel);

                var anchor = titleText.replace(/ /g, "_");
                toc += "<li><a href=\"#" + anchor + "\">" + titleText
                    + "</a></li>";

                return "<h" + openLevel + "><a name=\"" + anchor + "\">"
                    + titleText + "</a></h" + closeLevel + ">";
            }
        );

    if (level) {
        toc += (new Array(level + 1)).join("</ul>");
    }

    document.getElementById("toc").innerHTML += toc;
}

<section class='page'>
<details class='fr'>
<summary>TOC</summary>
<aside id='toc'></aside>
</details>
<br class='cb'/>
</section>
<article id='VNAArticle'>
<h1></h1>
<h2></h2>
</article>