cb username ajax validation

i simply want to understand how CB get username field value? is it use username element's id attribute or use element's class or what? if i understand how cb get username value to check via ajax, i can replicate that code in my Registration Value Layout.

The ajax validation is entirely done through the remote rule in jQuery Validation. It binds to the form and the individual element. More than likely the validation rule is working, but it doesn't know where to output its response. You can check that by monitoring the Network tab of your browsers developer tools after pressing F12. You should see the HTTP request there.

I've no idea what your layout is so there's nothing I can suggest. You should not be replacing the entire HTML of a field. Use [value] to output the HTML CB expects then add additional HTML around it as needed. If you need to see the HTML CB outputs then remove your layout and inspect the page with your browsers developer tools by right clicking and clicking inspect element on the username field.