There are many online calculators. You've probably seen, and perhaps used, several. Some calculate mortgage payments, others calculate costs, and even more calculate goofy things, like the number of days, hours, minutes, and seconds since you were born.

Here's a calculator that calculates ... well ... anything. It's a Windows-like calculator that's similar to the one you have in your wallet that lets you calculate how much money you have left in your checking account after you write a check.

Here's a picture.

When you use this calculator with the ShowModalDialog method (explained in greater detail in my blog post Open a page in a new window of a specific size), you can return a calculated value to the calling page. Here's how it works: you provide a link to the calculator from a Web page that references a JavaScript function that uses the ShowModalDialog method; then when a user closes the calculator, the total is returned to the first page through the JavaScript function that called the ShowModalDialog method.  It's really much more simple than it sounds.

Here's some of the code

In the example page, the "Calculate Numbers" link, code shown below, references a JavaScript function named OpenCalculator.

`<a href="javascript:OpenCalculator();">Calculate Numbers</a>`

The OpenCalculator function, shown in the following code, does a few different things: it displays the calculator, returns a value from the calculator, and places the returned value into a <span> tag.

```function OpenCalculator() {   var ret;  var spanTag;   ret = window.showModalDialog(     "calculator.htm", "",     "dialogHeight:285px;     dialogWidth:215px;     resizable:yes;");   spanTag = document.all.tags     ("span").item("numbers");   spanTag.innerText = ret;}```

The calculator has several functions, too many to show here, and three global variables. The global variables store the numbers and operators that the calculator needs to calculate the values. The variable named `total` stores the current number that is being calculated. When a user clicks the equals button, total resets to zero. The variable named `op` stores the operator so that when the user clicks the plus button, the calculator knows to add the numbers. The last variable, named `isNew`, is a Boolean that indicates whether to reset the calculation to zero.

Each of the buttons in the calculator calls a JavaScript function: the number buttons call the SetNumber function, the operator buttons call the SetOperator function, and the equals button calls the SetTotal function. Each of these functions serves a very specific purpose, but the function that does all the work is the Calculate function, shown in the following code. The Calculate function takes the current operator (the operator button clicked last) and calculates the last number entered (or the value in the number textbox) and places the calculated value into the `total` variable.

``````function Calculate()
{
var num = parseInt(document.Calculator.numbers.value);

switch(op)
{
case "plus":
{
total += num;
break;
}
case "minus":
{
total -= num;
break;
}
case "multiply":
{
total *= num;
break;
}
case "divide":
{
total /= num;
break;
}
default:
{
break;
}
}

document.Calculator.numbers.value = total;
}``````

For an example of the calculator and the full code, see the example page.