JSFiddle - React, Tailwind, and code Playground

HTML

<form>First Name :
    <input type='text' id='fname' placeholder='Enter First Name'>
    <br>
    <br>Last Name :
    <input type='text' id='lname' placeholder='Enter Last Name'>
    <br>
    <br>Age :
    <input type='text' id='age' placeholder='Age'>
    <br>
    <br>Sex :
    <input type='radio' class='sex'>Male
    <input type='radio' class='sex'>Female</form>
<button id='submit'>Submit</button>

JavaScript

$(document).ready(function () {
    $("#submit").click(function () {
        var fname = document.getElementById('fname').value;
        var lname = document.getElementById('lname').value;
        var age = document.getElementById('age').value;
        
        if (fname.length == 0) {
            alert("Please input a first name");
        } else if (lname.length == 0) {
            alert("Please input a last name");
        } else if (age.length == 0) {
            alert("Please input an age");
        }
    });
});