The serialization/deserialization is just JSON for now, though I plan on adding some configurability and perhaps changing the implementation at some point. There is some runtime checking to make sure the arguments are correctly serializable.

In local mode, a process is created up to the concurrency limit you specify, and each process is reused for subsequent calls (mimicking how Lambda reuses containers, allowing you to use the same caching behavior you'd use on Lambda). I'm not currently using webworkers, but that's something I could see a new mode for easily. For larger data, I would recommend storing arguments and return values directly in cloud storage like S3, or on local disk in local mode.

I would be interested to learn how your experiment with faast.js goes!