JSFiddle - React, Tailwind, and code Playground

HTML

<td>Item 1</td>
<input type="text" name="item1" id="item1"  />

<td>&nbsp;</td>
    <td>&nbsp;</td>

 <td>Quantity</td>
<input type="text" name="quantity" id="quantity"  />

<td>&nbsp;</td>
    <td>&nbsp;</td>

<td>Sub Total</td>
<input name="sub_total1" type="text" id="sub_total1" />

JavaScript

//Calculate Sub Total
    function calculateSubTotal()
{

    var SubTotal =  +item.value *  +quantity.value;
 
    document.getElementById("sub_total1").value = isNaN(SubTotal) ? 0 : SubTotal;
    }
        
    
document.getElementById("item").onchange = calculateSubTotal;
document.getElementById("quantity").onc = calculateSubTotal;