JSFiddle - React, Tailwind, and code Playground

HTML

<div id="myoldform">
    Guess: <input type="text" id="guess"/>
    <button id="b1" type="button">Submit</button>
</div>

JavaScript

document.getElementById('b1').addEventListener('click', checkForm);

x = Math.floor(Math.random()*99)

function randomNumber(){
    return x;
}


function checkForm() {

    var number = document.getElementById('guess').value; 

    if (number == null || number == "" || isNaN(number) || number < 0 || number > 99){

        alert("Invalid input");
        return false;
    }
    else if (number == x)
    {
        alert("You're correct!");
        document.getElementById('b1').disabled = true;
        return true;
    }
    else {
        alert("Wrong! Try again.")
    }
}

randomNumber();
console.log(x); //checks value for testing