examples/intro/calculator.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>Calculator</title>
</head>
<body>

x: <input id="x">
operator: <input id="op">
y: <input id="y">

<button id="go">Calculate</button>

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

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

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

function calculate() {
    var x = Number(document.getElementById('x').value);
    var y = Number(document.getElementById('y').value);
    var op = document.getElementById('op').value;
    var result;

    switch (op) {
        case '+' : {
            result = x+y;
            break;
        };
        case '*' : {
            result = x*y;
            break;
        };
        case '-' : {
            result = x-y;
            break;
        };
        case '/' : {
            result = x/y;
            break;
        };
    };

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

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