This code is not working giving error 'currentOperand' is not defined

function reducer(state, action) {
  switch(action.type) {
    case 'choose_operation':
      currentOperand = 0;
      return {...state, {/*operation*/}};
  }
}
const [{ currentOperand, previousOperand, operation }, dispatch] = useReducer(reducer, {});
dispatch({type: 'choose_operation'})

Neither this one showing same error 'currentOperand' is not defined

function reducer(state, action) {
  switch(action.type) {
    case 'choose_operation':
      state[0] = 0; // state.currentOperand = 0;
      return {...state, {/*operation*/}};
  }
}
const [[ currentOperand, previousOperand, operation ], dispatch] = useReducer(reducer, []);
dispatch({type: 'choose_operation'})

This one is working but I have to specify the array in every return

function reducer([ currentOperand, previousOperand, operation ], action) {
  switch(action.type) {
    case 'choose_operation':
      currentOperand = 0;
      return [ currentOperand, previousOperand, operation ];
  }
}
const [[ currentOperand, previousOperand, operation ], dispatch] = useReducer(reducer, [",","]);
dispatch({type: 'choose_operation'})