JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="reg.php">
   <input type="text" id="username" name="username"></input>        
   <button type="submit" onclick="return val()">Sign Up</button>
</form>

JavaScript

function val(){

    var uname = document.getElementById('username').value;
    if(!uname){
        document.getElementById("error").innerHTML = "Enter a username";
        alert("hello");
        return false;
    }
    else{
        return true;
    }
}