Write a counter with Redux - setting state to zero

I’m confused by this function which sets the state to zero. I think of state as an object including properties, so in my mind a state would need a key and value such as state = { count: 0 } .

const counterReducer = (state = 0, action) => { }

const counterReducer = (state = 0, action) => {
  switch (action.type) {
    case INCREMENT:
      return (state += 1);

    case DECREMENT:
      return (state -= 1);

      return state;

I will quote directly from the redux docs:

" The shape of the state is up to you: it can be a primitive, an array, an object, or even an Immutable.js data structure. The only important part is that you should not mutate the state object, but return a new object if the state changes."

Source: https://redux.js.org/introduction/getting-started

Wow, this is confusing. Thank you