calculate two number and diplay next field without reload the page javascript

JavaScript
var text1 = document.getElementById("Text1");
var text2 = document.getElementById("Text2");

function add_number() {
   var first_number = parseFloat(text1.value);
   if (isNaN(first_number)) first_number = 0;
   var second_number = parseFloat(text2.value);
   if (isNaN(second_number)) second_number = 0;
   var result = first_number + second_number;
   document.getElementById("txtresult").value = result;
}<p>Enter First Number :</p>
<br>
<input type="text" id="Text1" name="TextBox1" oninput="add_number()">
<br> <p>Enter Second Number :</p>
<br>
<input type="text" id="Text2" name="TextBox2" oninput="add_number()">
<br>Result :
<br>
<input type="text" id="txtresult" name="TextBox3">
Source

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