What I want my program to do
When I enter the correct discount code ("discount"), 10% will be taken off the total price and the new 'grand total' will be displayed. If no discount code is entered/correct one entered, then the grand total will be the same as the 'total price'.

Problem
Nothing is showing at all in grand total. This is my output when I click submit with the correct discount code entered: [Image of output][1] [1]: https://i.stack.imgur.com/WUwnK.png

What I've tried
In my JS script I have tried the following:

<script>
function getGrandTotal(){
const total = document.getElementbyId("total").value;
const dis = document.getElementbyId("discountCode").value;

 if(dis === "discount"){
  document.getElementById("grandTotal").value=
 total * 0.9;
 }else{
grandTotal = total;
}
 }
document.getElementById('btnSubmit').addEventListener('click', getGrandTotal);
</script>

My total amount was assigned in this function:

<script>
function getTotalAmount() {
const choice = document.getElementById("burgerSize").value;
const amount = document.getElementById("quantity").value;

if (choice === "4") {
  document.getElementById("total").value =
    amount * 4;
}
if (choice === "6") {
  document.getElementById("total").value =
    amount * 6;
}
if (choice === "10") {
  alert("Health Warning");
  document.getElementById("total").value =
    amount * 10;
}
}
document.getElementById('burgerSize').addEventListener('change', getTotalAmount);
document.getElementById('quantity').addEventListener('change', getTotalAmount);
</script>

I was thinking the same structure of method would apply to both of these tasks. I have also tried using if(dis !== "discount"){grandTotal = total;} instead of my else statement, but this is also not working. Any help would be great!

Here's my relevant html code:

<label> Total cost: </label>
<input type="text" id="total" /><br>

<label for = "discountCode"> Discount code: </label>
<input type="text" id="discountCode" /><br>
<input type="button" id="btnSubmit" value="Submit " onclick="return getGrandTotal()" /><br>

<label> Grand total: </label>
<input type="text" id="grandTotal" /><br>