Converting Jquery Ajax Call To Vanilla Javascript - Cannot Post /public/ Error

So this is a slight follow up to a question I had yesterday (Update HTML input value in node.js without changing pages), I'm attempting to submit a HTML form, send an ajax request

Solution 1:

all looks good except the two lines:

var num1 = getElementById("numa");
var num2 = getElementById("numb");

it should be like this

var num1  = document.getElementById("numa");
var num2  = document.getElementById("numb");

Note: updated html code (added onsubmit in form and removed from button)

<form id="add" method="post" onsubmit="return add()">
  Number 1:
  <input type="number"id="numa" name="numa" step="any"/><br>
  Number 2:
  <input type="number"id="numb" name="numb" step="any"/><br>
  <input type="number"id="answer" name="answer"readonly/>
  <input type="submit" value="Submit"/>
</form>