Sortable table

Based on work from https://1bestcsharp.blogspot.ch/2017/07/javascript-move-row-up-and-down.html

HTML

<!DOCTYPE html>
<html>
    <head>
        <title>HTML Table Row Up And Down</title>
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <style>
            
            tr{cursor: pointer}
            
            .selected{background-color: red; color: #fff; font-weight: bold }
            
            button{margin-top:10px;background-color: #eee;border: 2px solid #00F;
                  color: #17bb1c;font-weight: bold;font-size: 25px;cursor: pointer}
            
        </style>
    </head>
    <body>
        <table id="myTable" border="1">
            <tr onclick="selectRow(this)">
                <th>First Name</th>
                <th>Last Name</th>
                <th>Age</th>
            </tr>
            <tr onclick="selectRow(this)">
                <td>A1</td>
                <td>B1</td>
                <td>C1</td>
            </tr>
            <tr onclick="selectRow(this)">
                <td>A2</td>
                <td>B2</td>
                <td>C2</td>
            </tr>
            <tr onclick="selectRow(this)">
                <td>A3</td>
                <td>B3</td>
                <td>C3</td>
            </tr>
            <tr onclick="selectRow(this)">
                <td>A4</td>
                <td>B4</td>
                <td>C4</td>
            </tr>
            <tr onclick="selectRow(this)">
                <td>A5</td>
                <td>B5</td>
                <td>C5</td>
            </tr>
        </table>
        <button onclick="upNdown('up');">&ShortUpArrow;</button>
        <button onclick="upNdown('down');">&ShortDownArrow;</button>
        

        <script>
            var index;

            function selectRow(x) {
                var table = document.getElementById("myTable");
                //remove the "selected" class from all rows;
                for (var i=0; i<table.rows.length; i++) {
                    table.rows[i].classList.toggle("selected", false);
 ...