function calculate() {
      // Get the user's input from the form. Assume it is all valid.
      // Convert interest from a percentage to a decimal, and convert from
      // an annual rate to a monthly rate. Convert payment period in years
      // to the number of monthly payments.
      var principal = document.loandata.principal.value;
      var interest = document.loandata.interest.value / 100 / 12;
      var payments = document.loandata.years.value * 12;
  
      // Now compute the monthly payment figure, using esoteric math.
      var x = Math.pow(1 + interest, payments);
      var monthly = (principal*x*interest)/(x-1);
  
      // Get named <span> elements from the form.
      var payment = document.getElementById("payment");
      var total = document.getElementById("total");
      var totalinterest = document.getElementById("totalinterest");
  
      // Check that the result is a finite number. If so, display the
      // results by setting the HTML content of each <span> element.
      if (isFinite(monthly)) {
          payment.innerHTML = monthly.toFixed(2);
          total.innerHTML = (monthly * payments).toFixed(2);
          totalinterest.innerHTML = ((monthly*payments)-principal).toFixed(2);
      }
      // Otherwise, the user's input was probably invalid, so display nothing.
      else {
          payment.innerHTML = "";
          total.innerHTML = ""
          totalinterest.innerHTML = "";
      }
  }
  </script>