To run this, put it in a .txt file and then rename it .html and open it.

Source code:

Code:
<script type = "text/javascript">
<!-- TheEliteOne's JavaScript Calculator Example >
var num1 = prompt("Enter number one: ");
var num2 = prompt("Enter number two: ");
document.write(num1 + num2 + "<br>");
document.write(num1 - num2 + "<br>");
document.write(num1 * num2 + "<br>");
document.write(num1 / num2 + "." + num1 % num2);
</script>
Here is the same thing, but this one uses functions to do the math:

Code:
<script type = "text/javascript">
function add(a,b)
{
return a + b;
}
function sub(a,b)
{
return a - b;
}
function mult(a,b)
{
return a * b;
}
function div(a,b)
{
return a / b;
}
function div2(a,b)
{
<!-- This is to get the remainder >
return a % b;
}

var num1 = prompt ("Enter number one: ");
var num2 = prompt ("Enter number two: ");

document.write(add(num1,num2) + "<br>");
document.write(sub(num1,num2) + "<br>");
document.write(mult(num1,num2) + "<br>");
document.write(div(num1,num2) + "." + div2(num1,num2));
</script>
:)