Google recaptcha v3 - JavaScript

form.html

<form method="post">
    <div class="g-recaptcha" data-sitekey="SITE_KEY" data-callback="verifyCaptcha"></div>
    <div id="g-recaptcha-error"></div>
    <input type="button" class="submit" value="Submit" />
</form>

<script src='https://www.google.com/recaptcha/api.js'></script>

<script>
var recaptcha_response = '';

jQuery('.submit').on('click', function(){
    if(recaptcha_response.length == 0) {
        document.getElementById('g-recaptcha-error').innerHTML = '<span style="color:red;">This field is required.</span>';
        return false;
    }
    return true;
})
 
function verifyCaptcha(token) {
    recaptcha_response = token;
    document.getElementById('g-recaptcha-error').innerHTML = '';
}
</script>