JSFiddle - React, Tailwind, and code Playground

HTML

<form name="SimpleForm" onsubmit="return validateForm()">
    <input type="text" name="firstname" id="firstname" />
    <input type="text" name="lastname" id="lastname">
     <input type="submit" value="Submit">
</form>

JavaScript

function validateForm()
{
 // get value using Id
   var firtsname =document.getElementById("firstname").value;
   var lastname =document.getElementById("lastname").value;
   
   if(!firtsname )
   {
     alert("Enter First name");
     // return false will not allow to submit form to server
      return false;
   }
   else if (!lastname)
   {
       alert("Enter last name");
       return false;
   }
}