cognito-express | Authenticates API requests on a Node application | AWS library

In your own code you would never do that: once you use promises, there is no gain in using the old-style callback system.

Apparently the author of this API wanted to offer both callback-based and promise-based support via the init method, so that the client of that API could still choose, or maintain compatibility with a previous version of this API which might have only supported the callback-style.

Now the init method takes a required callback argument: if it would not be passed, there would be an exception when callback() is executed. This is a weak spot in the API, because now the constructor code (which belongs to the API itself) needs to pass a callback argument to its own call of init. Although that constructor does not need to be "called back", it must provide a valid callback, and so it passes a dummy one: