The .js also looks at the captcha, just to make sure that something, anything was written in before the page is allowed to be submitted. The second is where the captcha input is actually validated ...

Researcher uses an old unCAPTCHA trick against latest the audio version of reCAPTCHA, with a 97 percent success rate.

When a web application using reCAPTCHA challenges a visitor, Google provides an image set through JavaScript code. When a visitor solves the image-based puzzle, an HTTP request is triggered.