A client side script which executes on submit of an Online Lead Form and sends a POST request to a back-end suitelet which does the reCaptcha verification.

netsuite_recaptcha_css.js

/*
 * This is the external URL of the Suitelet we've created on Part 3 of this tutorial.
 */
var CAPTCHA_VERIFICATION_SUITELET_URL = 'https://forms.na1.netsuite.com/app/site/hosting/scriptlet.nl?script=89&deploy=1&compid=TSTDRV262509&h=cd6d5783f52fba53b0f3';


/**
 * Executes when the user clicks the Submit button.
 * @returns {Boolean} True to continue save, false to abort save
 */
function onSubmit() {
   var captchaChallenge = $('#recaptcha_challenge_field').val(); //Get the value of the recaptcha_challenge_field
   var captchaResponse = $('#recaptcha_response_field').val(); //Get the value of the recaptcha_response_field

   var isToBeSubmitted = true; //Denotes if the form will be submitted or not.

   $.ajax({
      url: CAPTCHA_VERIFICATION_SUITELET_URL + '&challenge=' + captchaChallenge + '&response=' + captchaResponse, //include the challenge and the response parameters
      type: 'POST', //Do a POST request
      accepts: 'application/json',
      dataType: 'json',
      cache: false,

      /*
       * This is important since jQuery.ajax is asynchronous by default.
       * We need to be able to process the response first before we can tell that we can submit the form or not.
       * So we set async to false.
       */
      async: false
   }).done(function (data) {
      if (!data.status.isSuccess) { //If the status returned by the suitelet is not successful, then fail the verification.
         alert('Captcha Verification Failed.');
         Recaptcha.reload();
         isToBeSubmitted = false; //Set this variable to false so that the form will not get submitted to the server.
      }
   });

   return isToBeSubmitted; //If this is true, then the form submits else it prevents the user to submit the form.
}