helper.js

  doRecaptchaVerification: function(component, event, helper, token) {
      const action = component.get('c.verifyResponse');

      action.setParams({response: token});

      action.setCallback(this, function(response) {
          const state = response.getState();

          if (state === 'SUCCESS') {
              const valid = response.getReturnValue();
              if (!valid) {
                  component.set('v.formMessage', 'Sorry, we could not verify you.');
              } else if (valid) {
                  component.set('v.formMessage', '');
                  
                // reCaptcha validated! Here is where we'll submit the form.
              }
          } else {
              const errors = response.getError();
              component.set('v.formMessage', 'Sorry, an error occured.'));
          }
      });
      
      $A.enqueueAction(action);
  }