Textbox not clears when clicks the submit button
<p>I am trying to clear the <code>txtSubTotal</code> text box after clicking the <code>PROCEED</code> button. It's not working though I tried some code examples, even in SO.</p>

<p><strong>btnProceed/HTML</strong></p>

<pre><code>&lt;input type="submit" name="btnProceed" id="btnProceed" value="PROCEED" onclick="clearSubTotal();"/&gt;
</code></pre>

<p><strong>clearSubTotal()/JS</strong></p>

<pre><code>function clearSubTotal() {
$('#txtSubTotal').val('');
}
</code></pre>

<p><strong>txtSubTotal</strong></p>

<pre><code>&lt;input name="txtSubTotal" type="text" id="txtSubTotal" size="15" value="&lt;?php 
$sql=mysqli_query($connection,"select sum(amount) from sales_temp");
$row = mysqli_fetch_array($sql);
echo $row[0];
?&gt;"/&gt;
</code></pre>

<p><strong>form/HTML</strong></p>

<pre><code> &lt;form id="manageSalesForm" name="manageSalesForm" method="post" action="&lt;?php echo BASE_URL?&gt;includes/functions/sales_functions.php"&gt;
</code></pre>

<p><a href="https://i.stack.imgur.com/4Suw3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4Suw3.png" alt="enter image description here"></a></p>

<p>Appreciate your help on this.</p>

<p>NOTE: Found that on the second button press, the text box clears. How to set this correctly for the first button perss?</p>

<p><strong>ADD button/JS</strong></p>

<pre><code>     function submitdata() {
     var listItemName  = document.getElementById("listItemName").value;
     var listStock = document.getElementById("listStock").value;
     var txtUnitPrice = document.getElementById("txtUnitPrice").value;
     var txtQuantity = document.getElementById("txtQuantity").value;
     var listCustomer = document.getElementById("listCustomer").value;
     var txtReceiptNo = document.getElementById("txtReceiptNo").value;
     var TheDate = document.getElementById("TheDate").value;

     // Returns successful data submission message when the entered information is stored in database.
     var dataString = {listItemName:listItemName, listStock: listStock, txtUnitPrice: txtUnitPrice, txtQuantity: txtQuantity, listCustomer: listCustomer, txtReceiptNo: txtReceiptNo};
     if (listItemName == '' || listStock == ''|| txtUnitPrice == ''|| txtQuantity == ''|| listCustomer == ''|| txtReceiptNo == ''|| TheDate == '') {
     salesitemsAddFail();
     } 
     else {
                // AJAX code to submit form.
                $.ajax({
                type: "POST",
                url: "/pms/includes/functions/sales_temp_functions.php",
                data: dataString,
                cache: false,
                success: function(html) {    

     //reload the sales datagrid once add the item details to temporary table (sales_temp)
     $('#list').trigger("reloadGrid",[{page:1}]);
         window.location.reload();
                }
                });
            }
}
$('#btnProceed').click(function(event) {
  event.preventDefault(); // stops form submission
$('#txtSubTotal').val('');
});
</code></pre>

<p><strong>ADD button/HTML</strong></p>

<pre><code>&lt;td width="46"&gt;&lt;button type="button" name="btnSave" id="btnSave" onclick="submitdata(); check_qty(); showSubTotal();"&gt;ADD&lt;/button&gt;&lt;/td&gt;
</code></pre>

<p><strong>sales_functions.php</strong></p>

<pre><code>&lt;?php

//Start the Session
       if(!isset($_SESSION)) 
       { 
           session_start(); 
       } 

include ("/../../pages/sales.php");
include("/../../dbutil.php");

 if(isset($_POST['listCustomer'])){ $customer = $_POST['listCustomer'];}
 if(isset($_POST['staff'])){ $user = $_POST['staff']; }

if(isset($_POST['btnProceed'])){

$result=mysqli_query($connection, 
"INSERT INTO sales(cus_id,item_id,stock_id,receipt_no,qty,unit_price,amount,user_id,purchase_id) 
 SELECT C.cus_id, I.item_id, S.stock_id, $receipt_no, ST.qty, ST.unit_price, ST.amount, U.id, P.purchase_id 
FROM customers C, items I, stock S, sales_temp ST, users U, purchase_items P 
WHERE ST.staff='$user' 
AND C.customer_name='$customer' 
AND I.item_name=ST.item_name 
AND S.stock_code=ST.stock_code 
AND ST.purchase_id=P.purchase_id");


//Update available qty from purchase_items relevant only to the logged in user(sales_temp table may have records from multiple users)
$resultUpdate=mysqli_query($connection, "UPDATE purchase_items P INNER JOIN sales_temp ST ON (P.purchase_id = ST.purchase_id) SET P.avail_qty = (P.avail_qty - ST.qty) WHERE ST.staff='$user'");

//Delete records relevant only to current user. Here 'WHERE' clause use to prevent deleting other user's records.
$resultDelete=mysqli_query($connection, "DELETE FROM sales_temp WHERE staff='$user'");


if (!$result) {
  printf("Errormessage: %s\n", mysqli_error($connection));
}

// use exec() because no results are returned
if ($result) {

}
else
{
  echo '&lt;script type="text/javascript"&gt;',
 'salesAddFail();',
'&lt;/script&gt;';
}}
?&gt;
</code></pre>