Isn't fn.assign useless inside getState?

[deser]

In my application I call getState so frequently that it even affects performance. I started to investigate and and came across fn.assign here:

getState(state) {
      if (Array.isArray(state)) {
        return state.slice()
      } else if (fn.isMutableObject(state)) {
        return fn.assign({}, state)
      }

      return state
}

To my mind it's absolutely useless as it does only shallow copy which means inner mutable objects will still be mutable after assigning. Seems that this code just ... slows down performance?