gistfile1.txt

var fields = [document.querySelector('#field-1-id'), document.querySelector('#field-2-id')]

fields.forEach(function (field) {
  field.addEventListener('keydown', function (event) {
    var total = 0
    fields.forEach(function (field) {
      total += field.value
    })
    document.querySelector('#calculated-field-id').value = total
  })
})