You seem to assume that the passed-in object should also be used as the resulting object. This is false, as they can be decoupled.

In other words, it's still possible to pass in a separate js object instance containing the configuration.

Yet I'm not sure about the whole of your arguments, so I can't tell if it would actually make sense to wrap the default js builder pattern implementation (the javascript object initializer syntax aaaaaaaaaaab refers to).

However, I do method builders often overused in API's where a simple singular builder method combined with builder classes suffice and simplify.