Calculating sales tax with JavaScript

In this form there is some secret math going on behind the scenes.…If I order two of something that costs $15, my line total takes care of itself,…notice the sales tax and the grand total.…So I don't have to figure that out, somehow Acrobat is figuring it out for me.…So there's my sales tax again.…So let's see what I mean when I say there is hidden mask going on behind the scenes.…When I choose my Select Object Tool, you can see that there is a hidden field…here, let's see what job that hidden field has.…

Its calculation is just the sum of the totals, so it's a little way station,…it's just a place for Acrobat to store total so that this tax field can then…incorporate that into some simple JavaScript.…So we'll take a look at this and then we're going to replicate it in a minute.…Essentially what's happening here is we're telling Acrobat, hey, go look in this…field's subtotal, find out what value is in there.…Look in this field's tax and for this field the b field, the value is going to…be what you found in the subtotal times .06, 6% tax.…