A default is provided for the onFailedAttempt option, but only if onFailedAttempt is not in the options object Relevant code:

This means that if options.onFailedAttempt is null or undefined, you get a TypeError when attempting to invoke it. But in passing null or undefined, the caller probably wanted to use the default onFailedAttempt handler (I know that's what I want).

This is obviously easy to workaround once you know about it, but does seem like a gotcha.