The better option is to do this calculation in PHP itself but if you want to do this in Javascript the you can do something like this:
Try creating a JS function that takes in all the values and the id of output input field and make sure that the id of output field is distinct.
JS Code
function calculate(unit_price, quantity, discount, tax, output_id) {
var subtotal = "do subtotal calc here";
document.getElementById(output_id).value = subtotal;
}
PHP/HTML Code
<?php
// mySQL Code goes here
while($row = mysql_fetch_array($result, MYSAL_ASSOC))
?>
<div class="centered">
<table border="1">
<tr>
<form id = "invoiceform" action="final.php" method="post">
<td><input type="text" id="Sl_no" value="<?php echo $s; ?>"></td> <!-- DO NOT INCREASE $S HERE YET-->
<td><input type="text" id="partno" value="<?php echo $p[$j]; ?>"></td>
<td><input type="text" id="description" value="<?php $row['Description']; ?>"></td>
<td><input type="text" id="unit_price" value="<?php echo $row['Unit_Price']; ?>"></td>
<td><input type="text" id="quantity" value=""></td>
<td><input type="text" id="discount" value=""></td>
<td><input type="text" id="tax" value=""></td>
<td><input type="text" id="<?php echo 'subtotal_'.$s;?>" value=""></td>
<script>
calculate(<?php echo $row['Unit_Price']; ?>, <?php echo $row['Quantity']; ?>, <?php echo $row['Discount']; ?>, <?php echo $row['Tax']; ?>, <?php echo 'subtotal_'.$s; s++;?>);
</script>
</tr>
</table>
</div>
P.S. Try making all the field ids distinct the way i made subtotal id distinct because a HTML page is supposed to have distinct IDs on elements
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solved JavaScript – How can I run the javascript in all my html rows