JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<table width="300" border="0" cellspacing="0" cellpadding="0">
      <tbody>
           <tr>

            <td>Item Name</td>
            <td>Qty</td>
            <td>Price(Per/Kg)</td>
            <td>Amount</td>
          </tr>

          <tr class="amount-row">
    <td>Test Item 1</td>
            <td><input class="quantity"  name="input" type="text" /></td>
            <td><input class="price" name="input" type="text" /></td>
            <td><input class="amount" name="input" type="text" disabled="disabled"/></td>
          </tr>
                    <tr class="amount-row">
    <td>Test Item 2</td>
            <td><input class="quantity"  name="input" type="text" /></td>
            <td><input class="price" name="input" type="text" /></td>
            <td><input class="amount" name="input" type="text " disabled="disabled"/></td>
          </tr>
      </tbody>
    </table>
    <p>&nbsp;</p>
    <table width="300" border="0">
      <tr>
        <td width="100"><strong>Total Amount</strong></td>
        <td id="total-amount">&nbsp;</td>
      </tr>
      <tr>
        <td><strong>Discount</strong></td>
        <td><input type="text" id="discount"></td>
      </tr>
      <tr>
        <td><strong>Net Amount</strong></td>
        <td id="net-amount">&nbsp;</td>
      </tr>
    </table>

JavaScript

var totalAmount = 0;
    $('.quantity, .price').keyup(function() {
      totalAmount = 0;
      var netAmount = 0;
      var discount = 0;

      $(".amount-row").each(function(index, row) { 
    	var quantity = $(row).find('.quantity').val();
      	var price = $(row).find('.price').val();
        
        var amount = quantity * price;
        $(row).find('.amount').val(amount.toFixed(2));
     
      	totalAmount += amount;
      });

      netAmount = totalAmount;
      discount = $("#discount").val();
      if (discount) {
      	netAmount = totalAmount - discount;
      }
      
      $("#total-amount").html(totalAmount.toFixed(2))
      $("#net-amount").html(netAmount.toFixed(2))
    });
    
    
    $("#discount").keyup(function() {
    	var discount = +$("#discount").val();
 
      
    	$("#net-amount").html((totalAmount - discount).toFixed(2));
    });