JSFiddle - React, Tailwind, and code Playground

HTML

<html>

    <head>
        <title></title>

        <script type="text/javascript">
    
            var a = new Array();
            a[1] = "kgs"; 
            a[2] = "gms"; 
            a[3] = "ltrs"; 
            a[4] = "ml"; 
            a[5] = "nos"; 
            
            function updateunit(){
                var b = document.getElementById("itemname");
                var b1 = b.value;
                var c = b1.split(',');
                var d = c[0];
                var e = a[d];
                //alert(document.write(a[1]));
                document.getElementById('pane2').innerHTML = e ;
            }
            
            window.onload = function() {
                updateunit();
                
                document.getElementById('itemname').onchange = updateunit ;
            
            }    
        </script>
    </head>


<body>

<form action='production.php' method="POST"> 

Item Name <select name ="itemname" id = "itemname" onchange = "updateunit()">
<option value = '1,5' > A </option>
<option value = '2,5' > B </option>
<option value = '3,5' > C </option>
<option value = '4,5' > D </option>
</select> <br>



Amount <input type="text" name="amount"> <span id = "pane2"></span> <br>

</form>        

</body>

</html>