Google reCaptcha

recaptcha.doc

// Site
https://www.google.com/recaptcha/intro/android.html

// Doc
https://developers.google.com/recaptcha/docs/verify

recaptcha.ex1

// Script
<script src='https://www.google.com/recaptcha/api.js?hl=pt-BR' async defer></script>

// HTML
<div class="g-recaptcha" data-sitekey="your-site-key" data-callback="onRecaptcha" data-expired-callback="onRecaptchaExpired"></div>

// JS
var onRecaptcha = function() {
  // code
};

var onRecaptchaExpired = function() {
  // code
  //grecaptcha.reset();
};

recaptcha.ex2

// Script
<script src='https://www.google.com/recaptcha/api.js?hl=pt-BR&onload=onloadCallback&render=explicit' async defer></script>

// HTML
<div id="gCaptchaKey"></div>

// JS
var onRecaptcha = function() {
  // code
};

var onRecaptchaExpired = function() {
  // code
  //grecaptcha.reset();
};

var onloadCallback = function() {
  grecaptcha.render('gCaptchaKey', {
      'sitekey': 'your-site-key',
      'callback': onRecaptcha,
      'expired-callback': onRecaptchaExpired
  });
};