JSFiddle - React, Tailwind, and code Playground

HTML

<h2>Glossary:</h2>


<h3>Links:</h3> 
<div id="quicklinks"></div>

<h3>Entries:</h3> 
<dl class="glossary"> <a name="Bobcat">
    <dt>Bobcat</dt>
        <dd>A medium-sized, furry mammal that can eat children.</dd>
    <a name="Cat">
    <dt>Cat</dt>
        <dd>A small, furry mammal that meows.</dd>
    <a name="Dog">
    <dt>Dog</dt>
        <dd>A small, furry mammal that barks.</dd>
    <a name="Fish">
    <dt>Fish</dt>
        <dd>A scaly aquatic animal that swims in water.</dd>
    <a name="Flamingo">
    <dt>Flamingo</dt>
        <dd>A pink bird.</dd>
    <a name="Gnat">
    <dt>Gnat</dt>
        <dd>A small flying insect.</dd>
    <a name="Llama">
    <dt>Llama</dt>
        <dd>A shaggy-haired beast of burden with a long neck.</dd>
    <a name="Mouse">
    <dt>Mouse</dt>
        <dd>A tiny mammal.</dd>
    <a name="Quail">
    <dt>Quail</dt>
        <dd>A small bird.</dd>
    <a name="Rhino">
    <dt>Rhino</dt>
        <dd>A scaly mammal.</dd>
    <a name="Swan">
    <dt>Swan</dt>
        <dd>A white bird.</dd>
    <a name="Toucan">
    <dt>Toucan</dt>
        <dd>A colorful bird.</dd>
    <a name="Whale">
    <dt>Whale</dt>
        <dd>A huge aquatic mammal.</dd>
    <a name="Yak">
    <dt>Yak</dt>
        <dd>A shaggy-haired beast of burden.</dd>
    <a name="Zebra">
    <dt>Zebra</dt>
        <dd>Sort of a horse with white/black stripes.</dd>

</dl>

CSS

span.letterLinkSpan {
    display: inline-block;
}
span.letterLinkSpan>a.letterLink {
    display: inline-block;
    width: 1.1em;
    text-align: center;
    padding: 0.2em;
    font-size: medium;
    color: #000000;
    background-color: #BBCCFF;
}

span.letterLinkSpan>a.letterLink.entriesPresent {
    font-weight: bold;
}
span.letterLinkSpan>a.letterLink.entriesAbsent {
    font-weight: normal;
    color: #FFEEEE;
}
span.letterLinkSpan>a.letterLink:hover {
    background-color: #DDDDFF;
}

JavaScript

var terms = $("dl.glossary dt").sort();

var letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
for (var i = 0; i < letters.length; i++) {
    var nextChar = letters.charAt(i);

    // Need to find first of each letter
    var foundOne = -999;
    for (var j = 0; j < terms.length; j++) {
        var termj = terms[j].innerText;
        var termJJ = termj.toUpperCase();
        if (termJJ.charAt(0) == nextChar) {
            foundOne = j;
            break;
        }
    }

    // Create links and anchors based on whether something was found
    if (foundOne == -999) {

        // Create a new letterLink (with class "entriesAbsent")
        newLetterLink = "<span class=\"letterLinkSpan\"><a class=\"letterLink entriesAbsent\">" + nextChar + "</a></span>";

    } else {

        // Create a new letterLink (with class "entriesPresent")
        newLetterLink = "<span class=\"letterLinkSpan\"><a class=\"letterLink entriesPresent\" href=\"#" + nextChar + "\">" + nextChar + "</a></span>";

        // Create an anchor for the letterLink BEFORE the first glossary term starting with that letter
        $("<a class=\"letterAnchor\" name=\"" + nextChar + "\">").insertBefore(terms[foundOne]);

    }

    // Either way, add the letterLink to the list of letterLinks
    $("div#quicklinks").append(newLetterLink);
}