fblogin.js

FB.getLoginStatus(function(response) {
    if (response.status === "connected") {
        // use the response.authResponse
    }
    else if (response.status === "not_authorized") {
        FB.login(function(response) {
            ...
        }, { scope: "email, read_stream" });
    }
    else {
        // user not logged in to facebook
    }
});