<script src="https://www.google.com/recaptcha/api.js"></script>
grecaptcha.ready(function () {
if (grecaptcha.getResponse() === "") {
alert("Please validate the Google reCaptcha.");
} else {
alert("Successful validation! Now you can submit this form to your server side processing.");
}
});
<form action="?" method="POST">
<div class="g-recaptcha" data-sitekey="your_site_key"></div>
<br/>
<input type="submit" value="Submit">
<script src="https://www.google.com/recaptcha/api.js"></script>
</form>