Javascript checkbox onChange

Question

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)

Answer 1

function calc()
{
  if (document.getElementById('xxx').checked) 
  {
      document.getElementById('totalCost').value = 10;
  } else {
      calculate();
  }
}

HTML