JSFiddle - React, Tailwind, and code Playground

HTML

<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname"><br><br>
<input type="button" value="確定" onclick="confirm()">

JavaScript

function confirm()
{
  var DisplayString =""
  if (document.getElementById("fname").value=="" )
  {
    DisplayString = DisplayString + "First name 不可空白!" + "\n";
  }
  if (document.getElementById("lname").value=="" )
  {
    DisplayString = DisplayString + "Last name 不可空白!"+ "\n";
  }
  if (DisplayString !="")
  {
    alert(DisplayString);
  }
}