Javascript checkbox onChange

I have a checkbox in a form and I’d like it to work according to following scenario:

• if someone checks it, the value of a textfield (totalCost) should be set to 10.
• then, if I go back and uncheck it, a function calculate() sets the value of totalCost according to other parameters in the form.

So basically, I need the part where, when I check the checkbox I do one thing and when I uncheck it, I do another.