JavaScript client-side login-checks?
===

**[RFC 2617][1]** specifies the *HTTP Basic and Digest Auth* standard and works as summarised in [Wikipedia's "Example with explanation" subsection][2]:

>- The client asks for a page that requires authentication but does not provide a username and password. Typically this is because the user simply entered the address or followed a link to the page.
>- The server responds with the 401 "client-error" response code, providing the authentication realm and a randomly-generated, single-use value called a nonce.
>- At this point, the browser will present the authentication realm (typically a description of the computer or system being accessed) to the user and prompt for a username and password. The user may decide to cancel at this point.
>- Once a username and password have been supplied, the client re-sends the same request but adds an authentication header that includes the response code.
>- In this example, the server accepts the authentication and the page is returned. If the username is invalid and/or the password is incorrect, the server might return the "401" response code and the client would prompt the user again.

>Note: A client may already have the required username and password without needing to prompt the user, e.g. if they have previously been stored by a web browser.

I am performing login using [OAuth2 draft-standard][3] as implemented by [Facebook][4].

**Given a server with an exposed public API (e.g.: JSONRPC or REST), how could the client-side JavaScript frontend be written to include the sort of functionality as the previously mentioned RFC 2617 example, but for Facebook auth?**


  [1]: http://tools.ietf.org/html/rfc2617
  [2]: http://en.wikipedia.org/wiki/Digest_access_authentication#Example_with_explanation
  [3]: http://tools.ietf.org/html/draft-ietf-oauth-v2-29
  [4]: https://developers.facebook.com/docs/authentication/server-side/