examples/intro/add_numbers.html
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">

  <title>Add numbers</title>
</head>
<body>

<input id="x">
<input id="y">
<button id="go">+</button>

<div id="result"></div>

<script src="add_numbers.js"></script>

</body>
</html>
examples/intro/add_numbers.js
"use strict";

function add() {
    var x = Number(document.getElementById('x').value);
    var y = Number(document.getElementById('y').value);

    document.getElementById('result').innerHTML = x+y;
    return false;
}

document.getElementById('go').addEventListener('click', add);