function calculate() {
  var transactionAmount = document.getElementById('transactionAmount').value;
  var transactionNumber = document.getElementById('transactionNumber').value;
  var customerYears = document.getElementById('customerYears').value;

  if(transactionAmount && isNumber('transactionAmount') && transactionNumber && isNumber('transactionNumber') && customerYears && isNumber('customerYears')) {
    document.getElementById('lifetimeValue').value = Math.round(transactionAmount * transactionNumber * customerYears * 100)/100;
  }
  else {
    document.getElementById('lifetimeValue').value = "";
  }
}

function isNumber(field) {
  var sText = document.getElementById(field).value;
  var ValidChars = "0123456789.";
  var IsNumber = true;
  var Char;
 
  for (i = 0; i < sText.length && IsNumber == true; i++) { 
    Char = sText.charAt(i); 
    if (ValidChars.indexOf(Char) == -1)
      IsNumber = false;
  }
  return IsNumber;
}