JavaScript斐波那契计算器:如何将答案输出到文本框中?

JavaScript Fibonacci Calculator: How to output answer into text box?

我目前正在尝试制作一个接受数字输入 (n) 并将斐波那契数列显示为第 n 个序列的计算器。我是 Javascript 的初学者,似乎无法使我的代码正常工作:

HTML:
<head>
  <script src="calculator.js"></script>
</head>
<body>
    <input type="number" name="Input" value="" id="userInput" / >
    <input type="button" value="Go" onclick="calculate();" />
    <input type="number" name="Answer" id="userAnswer" />
</body>

JavaScript:

var input = document.getElementById("userInput");
var output = document.getElementById("userAnswer");
var answer;

function calculate(){
n = input.value;
if (n < 2){
  return 1;
} else {
  answer = calculate(n - 2) + calculate(n - 1);
  return answer;
}
output.value = answer;
}

不要复杂!

function calculate(n){
  if (n < 2){
    return 1;
  } else {
    return calculate(n - 2) + calculate(n - 1);
  }
}
<head>
  <script src="calculator.js"></script>
</head>
<body>
    <input type="number" name="Input" value="" id="userInput" / >
    <input type="button" value="Go" onclick="document.getElementById('userAnswer').value=calculate(document.getElementById('userInput').value);" /><br>
    <input type="number" name="Answer" id="userAnswer" />
</body>