web用函数求n的阶乘

你们好，我想问下这需要怎么做呢？不太熟悉使用函数和调用函数的步骤，谢谢！

<input type="number" id="num">
<button onclick="show()">Show</button>
<br>
Result: <input type="text" id="result" disabled>

<script>
function fact(num) {
  if (num <= 1) {
    return 1;
  } else {
    return num * fact(num-1);
  }
}

function show() {
  var num = document.getElementById("num").value;
  var result = fact(num);
  document.getElementById("result").value = result;
}
</script>