How to modify values of a textbox based on a checkbox

Question

Where can I find a guide to creating custom JavaScipt calculation code for Adobe Acrobat fillable forms?
I have a fillable form that will base a textbox value on the following:
value = 0.00;
if checkbox1 is checked, then:
  value = value + 3.60;
end if
if checkbox2 is checked, then:
  value = value + 0.90;
end if
if textbox4 has any number in it, then:
  value = value * the number in textbox4;
end if
if value is NaN, then:
  value = 0.00;
end if
if value is 0.00, then:
  make textbox blank (clear the textbox)
end if
how do I do this in "custom calculation script" JavaScript?
I tried to implement it, but the code gave an error when I tried to add to event.value a second time:
event.value = 0;
event.value = event.value + 3.60; // works
event.value = event.value + 0.90; // doesn't work
I don't understand. Why would it work on one but not the other?

Answer 1

"event.value" is a string. I suggest you work with another variable (a
number) and at the very end apply the value of that variable to event.value.
A good place to start learning about calculations is here:
http://acrobatusers.com/tutorials/how-to-do-not-so-simple-form-calculations