Exempt a field with a button?

Question

So I have a form I am working on, one of the parts is a subtotal, tax, total field. My total field is calculated using sum of subtotal and tax. My tax field is calculated using custom;
var f = this.getField("subtotal");
event.value = Math.round(f.value * 7.00) / 100;
I want a button though that would disable the tax field. This way if I need to apply tax exempt I can press a button and the tax field is no longer automatically calculated and applied.
Any help?

Answer 1

It might make more sense to use a check box. The script could check to see if it's selected and calculate accordingly. Something like:
(function () {
    // Get the subtotal value, as a number
    var v1 = +getField("subtotal").value;
    // Get the value of the "taxable" check box. If not selected, tax = 0
    var v2 = getField("taxable").value;
    // Calculate the tax
    event.value = v2 === "Off" ? 0 : Math.round(v1 * 7.00) / 100;