TOC.js

/**
 * TOC.js: create a table of contents for a document.
 *
 * This script runs when the DOMContentLoaded event is fired and
 * automatically generates a table of contents for the document.
 * It does not define any global symbols so it should not conflict
 * with other scripts.
 *
 * When this script runs, it first looks for a document element with
 * an id of "TOC". If there is no such element it creates one at the
 * start of the document. Next, the function finds all <h2> through
 * <h6> tags, treats them as section titles, and creates a table of
 * contents within the TOC element. The function adds section numbers
 * to each section heading and wraps the headings in named anchors so
 * that the TOC can link to them. The generated anchors have names
 * that begin with "TOC", so you should avoid this prefix in your own
 * HTML.
 *
 * The entries in the generated TOC can be styled with CSS. All
 * entries have a class "TOCEntry". Entries also have a class that
 * corresponds to the level of the section heading. <h1> tags generate
 * entries of class "TOCLevel1", <h2> tags generate entries of class
 * "TOCLevel2", and so on. Section numbers inserted into headings have
 * class "TOCSectNum".
 *
 * You might use this script with a stylesheet like this:
 *
 *   #TOC { border: solid black 1px; margin: 10px; padding: 10px; }
 *   .TOCEntry { margin: 5px 0px; }
 *   .TOCEntry a { text-decoration: none; }
 *   .TOCLevel1 { font-size: 16pt; font-weight: bold; }
 *   .TOCLevel2 { font-size: 14pt; margin-left: .25in; }
 *   .TOCLevel3 { font-size: 12pt; margin-left: .5in; }
 *   .TOCSectNum:after { content: ": "; }
 *
 * To hide the section numbers, use this:
 *
 *   .TOCSectNum { display: none }
 **/
document.addEventListener("DOMContentLoaded", () => {
    // Find the TOC container element.
    // If there isn't one, create one at the start of the document.
    let toc = document.querySelector("#TOC");
    if (!toc) {
        toc = document.createElement("div");
        toc.id = "TOC";
        document.body.prepend(toc);
    }

    // Find all section heading elements. We're assuming here that the
    // document title uses <h1> and that sections within the document are
    // marked with <h2> through <h6>.
    let headings = document.querySelectorAll("h2,h3,h4,h5,h6");

    // Initialize an array that keeps track of section numbers.
    let sectionNumbers = [0,0,0,0,0];

    // Now loop through the section header elements we found.
    for(let heading of headings) {
        // Skip the heading if it is inside the TOC container.
        if (heading.parentNode === toc) {
            continue;
        }

        // Figure out what level heading it is.
        // Subtract 1 because <h2> is a level-1 heading.
        let level = parseInt(heading.tagName.charAt(1)) - 1;

        // Increment the section number for this heading level
        // and reset all lower heading level numbers to zero.
        sectionNumbers[level-1]++;
        for(let i = level; i < sectionNumbers.length; i++) {
            sectionNumbers[i] = 0;
        }

        // Now combine section numbers for all heading levels
        // to produce a section number like 2.3.1.
        let sectionNumber = sectionNumbers.slice(0, level).join(".");

        // Add the section number to the section header title.
        // We place the number in a <span> to make it styleable.
        let span = document.createElement("span");
        span.className = "TOCSectNum";
        span.textContent = sectionNumber;
        heading.prepend(span);

        // Wrap the heading in a named anchor so we can link to it.
        let anchor = document.createElement("a");
        let fragmentName = `TOC${sectionNumber}`;
        anchor.name = fragmentName;
        heading.before(anchor);    // Insert anchor before heading
        anchor.append(heading);    // and move heading inside anchor

        // Now create a link to this section.
        let link = document.createElement("a");
        link.href = `#${fragmentName}`;     // Link destination

        // Copy the heading text into the link. This is a safe use of
        // innerHTML because we are not inserting any untrusted strings.
        link.innerHTML = heading.innerHTML;

        // Place the link in a div that is styleable based on the level.
        let entry = document.createElement("div");
        entry.classList.add("TOCEntry", `TOCLevel${level}`);
        entry.append(link);

        // And add the div to the TOC container.
        toc.append(entry);
    }
});