srv/auth/DataSourceAuthenticationProvider.js
function (registrationRequest, callback) {
            this.fetchUser(registrationRequest, function (err, user) {
                if (!err && user) {
                    err = RegistrationError.USER_ALREADY_EXISTS
                }
                callback(err);
            });
        }
function (registrationRequest, callback) {
            this.fetchUser(registrationRequest, function (err, user) {
                if (!err && user) {
                    err = RegistrationError.USER_ALREADY_EXISTS
                }
                callback(err);
            });
        }
function ( registrationRequest , callback ) { this . fetchUser ( registrationRequest , function ( err , user ) { if ( ! err && user ) { err = RegistrationError . USER_ALREADY_EXISTS } callback ( err ) ; } ) ; }
Checks if the user already exists

@param registrationRequest
@param callback
Checks if the user already exists