如何创建提示弹出窗口，根据提示输入类型显示文本？

<form>
  <input type="text" id="inputField" placeholder="请输入内容">
  <button type="button" onclick="showPopup()">显示弹出窗口</button>
</form>

#popup {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 300px;
  height: 200px;
  background-color: #fff;
  border: 1px solid #ccc;
  z-index: 9999;
}

#overlay {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, 0.5);
  z-index: 9998;
}

function showPopup() {
  var inputField = document.getElementById("inputField");
  var inputValue = inputField.value;
  
  var popupContent = "";
  
  if (inputValue === "") {
    popupContent = "请输入内容";
  } else if (isNaN(inputValue)) {
    popupContent = "输入内容不是数字";
  } else {
    popupContent = "输入内容是数字";
  }
  
  var popup = document.createElement("div");
  popup.id = "popup";
  popup.innerHTML = popupContent;
  
  var overlay = document.createElement("div");
  overlay.id = "overlay";
  
  document.body.appendChild(popup);
  document.body.appendChild(overlay);
}