function calculate(event) {
  // stop form from reloading the page on submit
  event.preventDefault();

  // declare variables to hold the form data we need
  let firstNumber = event.target.elements.num1.value;
  let secondNumber = event.target.elements.num2.value;
  let op = event.target.elements.operator.value;
  let answerEl = document.getElementById('answer-value');

  if (firstNumber == "" || secondNumber == ""){
    answerEl.innerText = "First number and second number must not be empty";
    answerEl.style.color = "red";
    return;
  }

  answerEl.innerText = eval(firstNumber+op+secondNumber);
  answerEl.style.color = "green";
}