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You don't avoid mutable state by doing "model.count += 1" in a separate update() function rather than a method on the model.

Both are functions that operate on the same data; the OO syntax is just a convenience that was invented exactly for the reason of avoiding endless switch statements in dispatching actions on a particular piece of data.

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Doesn't the OO approach just move the work of the switch statement into the class declaration of the model? In practice, OO is about encapsulation, and FP is not. So it's probably not too surprising that the author chose not mix metaphors.

This is also modeled after an architecture in a statically typed language that expresses possible actions as a single union type, in which every possible type must be handled. There's no similar compile-time guarantee in JS, but a switch statements with constants is a reasonable approximation of the idea.

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