how to prevent sending data if captcha is not resolved successfully

I have an input and a google recaptcha
and need to send input value only if the captcha is solved
after page reload the value of grecaptcha.getResponse() is an empty string
when captcha is resolved by a single click - the value is a long string - differ after each page load
how to know what is the value if the captcha says - you're a robot or something like that
in other words how to cancel sending data if captcha is not solved successfully?

<script src="https://www.google.com/recaptcha/api.js" async defer></script>

<div class="g-recaptcha" data-sitekey="6Lf-wAIaAAAAALa36iP4tIvwCeub0-bAB3QfqWbP"></div>

best I found is here - https://developers.google.com/recaptcha/docs/verify
and tried this:

$('#btn').on('click', function(){
    let rc = grecaptcha.getResponse();
    console.log(rc.success); // undefined
});

any help

Solution

Here you have a working example in Php

 define('SITE_KEY','SDFSDF4UAAAAAM-ISDM2lM5WESDVSDCERGDFGSDFG');
 define('SECRET_KEY','SGSFGFDSAACJ_OxaXzSayvQS_ABCDECSDFSDF');


if ($_POST) {
    function getcaptcha($secretkey){
    $response = file_get_contents("https://www.google.com/recaptcha/api/siteverify? 
    secret=".SECRET_KEY."&response={$secretkey}");
    $respuesta = json_decode($response);
    return $respuesta;
} 

$return = getcaptcha($_POST['g-recaptcha-response']);
    var_dump($return);
        if ($return->success == true && $return->score >0.5) {
         echo "You are a Person...";
         }
         else{
        echo "You are a robot... ";
         }
    }

and the Javascript

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Recaptcha V3 by AleDC</title>


<script src="https://www.google.com/recaptcha/api.js?render=<?php echo SITE_KEY; ?>"></script>

<form action="index.php" method="post">

Nombre: <input type="text"> <br>
TOKEN:  <input type="text" name="g-recaptcha-response" id="g-recaptcha-response"><br>
        <input type="submit" value="submit">



</form>

<script>
grecaptcha.ready(function() {
    grecaptcha.execute('<?php echo SITE_KEY; ?>', {action: 'homepage'})
    .then(function(token) {
       console.log(token);
       document.getElementById("g-recaptcha-response").value=token;
    });
});
</script>

Remember that you must register your website in the google recaptcha portal