Passport.js
Passport.js is an open source library focusing on authentication.
From their own site:
Passport is authentication middleware for Node.js. Extremely flexible and modular, Passport can be unobtrusively dropped in to any Express-based web application. A comprehensive set of strategies support authentication using a username and password, Facebook, Twitter, and more.
Butler Auth uses Passport.js for many (but not all) authentication strategies.
Given that Passport.js uses a consistent interface for all authentication strategies it’s usually quite easy to add additional authentication providers to Butler Auth.