JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<head>
    <script type="text/javascript">    
      var a = new Array();
        a[0] = "";
        a[1] = "kgs";
        a[2] = "gms";
        a[3] = "ltrs";
        a[4] = "ml";
        a[5] = "nos";
        a[6] = "pkt";

      var sup = new Array(),
        sup_map_sup =  new Array(),
        sup_map_item =  new Array();
        
        sup[0] = "";
        sup[1] = "S1";
        sup[2] = "S2";  
        sup_map_sup['0'] = "1";
        sup_map_item['0'] = "2";
        sup_map_sup['1'] = "2";
        sup_map_item['1'] = "3"; 
        sup_map_sup['2'] = "2";
        sup_map_item['2'] = "4";
        sup_map_sup['3'] = "2";
        sup_map_item['3'] = "5";
        sup_map_sup['4'] = "2";
        sup_map_item['4'] = "1";

   function updateunit(thiss){
      var b1 = thiss.value;
      var c = b1.split(',');
      var d = c[1];
      var e = a[d];
      document.getElementById('pane2').innerHTML = e ;
      //document.getElementById('pane3').innerHTML = e ;
      document.getElementById('supplier').length = 1;
        alert(sup_map_item);
      for (var i = 0; i < sup_map_item.length; i++){
        if (sup_map_item[i] === c[0]) {
          addOption(document.getElementById('supplier'), sup_map_sup[i], sup[sup_map_sup[i]]);
        }
      }
    }

    function addOption(selectbox, value, text ){
      var optn = document.createElement("OPTION");
      optn.text = text;
      optn.value = value;

      selectbox.options.add(optn);
    }
</script>
</head>

 <body>
        <h1>Online POS Tool</h1>    
<form action='temp.php' method="POST"> 
Item Name <select name ="itemname" id = "itemname" onchange = "updateunit(this)"><option   value = '1,1' > A </option><option value = '2,1' > B </option><option value = '3,2' > C    </option><option value = '4,3' > D </option><option value = '5,4' > E </option></select>     <br>

Supplier <select name ="supplier" id = "supplier">
<option value = ""> Select an Item </option> 
</select> <br>

Amount <input type="text"...