Change the text of submit button when clicked OKAY on alert pop-up

Question

I have a shopping cart system where a customer can take orders and check out if they have anything on the cart but can't continue if it has nothing.

An alert will pop-up if they click on Submit button. I have this javascript for that one.

function validateForm()
{
var x=document.forms["form1"]["total"].value;
if (x==null || x=="")
  {
  alert("Take your order first.");
  return false;
  }
var con = confirm("Are you sure you want to order this item/s?");
if (con ==false)
{
return false;
}
}

How can I change the text of the submit button when the Okay is clicked on the pop-up message? Instead of "Add Order" I want it to be changed to "Update order" when the customer added his/her order once.

Answer 1

You mean this?

document.getElementById("myForm").addEventListener("submit",function(e) {
  var x = this.total.value.trim(), sure = false;
  if (this.subBut.value=="Update order" && x !=="") return true;
  e.preventDefault();
  if (x == "") {
    alert("Please take your order first.");
  }
  else sure =  confirm("Are you sure you want to order this item/s?");
  if (sure) {
    this.subBut.value="Update order";
  }
  return sure;
});

<form id="myForm">
<input type="text" name="total" value="" />
<input type="submit" name="subBut" value="Add order" />
</form>