Question

This is a general question. Using an example below, suppose that I have a Javascript that performs an AJAX call on a PHP script to pull data from a database. I can choose to:

a) manipulate the data on the PHP script itself before encoding an sending it back to the Javascript, OR;

<?php
 while ($row = mysqli_fetch_assoc($result))
 {
  extract($row);
  array_push($data, array('product' => $product, 'discount' => $price*0.15));
 }
 echo json_encode($data);
?>

b) get raw data from the PHP script and do the manipulation on the Javascript.

<SCRIPT type='text/javascript'>
 $.each(json, function(index, element) {
  element.discount = 0.15*element.price
 }
</SCRIPT>

Though I already know that I can get better performance (server-side) using option b), I would like to hear from the community whether it is a wiser choice and whether there are any logical arguments against it which I may have overlooked. Thanks!

NOTE:

• This is just a simple scenario, the calculations are more complicated
  than *0.15.
• Javascript is a must-have to pull data and for application to
  work.

Answer 1

Some things you do not want to be stingy on when it comes to performance or time taken to execute.

• Cryptography is one of them.
• Anything with a financial aspect to it, is another.

The fact that we can do XYZ on the client layer, through JavaScript doesn’t mean that we should be compelled to do it.
What happens

A) If JavaScript is turned off.
B) You rely on the value POSTed from JavaScript. (Imagine a shopping cart which has $_POST[‘total_amount’] which is used by the server. I am going to edit that POST or hidden input to get free things.

If what you are doing is purely informative, then this does not matter as much, but if you rely on any information, then you should be checking the prices multiple times.

Additionally, what happens when you want to start providing an API or something to your service. Your transport is now broken because it doesn’t contain the calculated prices.

DO it server side, at whichever layer better. As Duncan says, SQL could be one option, or PHP. I don’t have a preference on this one since any gains for a smallset of items will be largely negligible.