Reducer is a required argument, and must be a function or an object of functions that can be passed to combinereducers

When using the combination combineReducers function from Redux, the reducer argument is required and must be a function or an object of functions. The combineReducers function is responsible for combining multiple reducers into a single reducer function, which can then be used with the Redux store.

Here is an example to illustrate how to use combineReducers with a single reducer function:

    const initialState = {
      counter: 0
    };

    function counterReducer(state = initialState, action) {
      switch(action.type) {
        case 'INCREMENT':
          return {
            ...state,
            counter: state.counter + 1
          };
        case 'DECREMENT':
          return {
            ...state,
            counter: state.counter - 1
          };
        default:
          return state;
      }
    }

    const rootReducer = combineReducers({
      counter: counterReducer
    });

    // Now you can create your Redux store using the rootReducer
    const store = createStore(rootReducer);
  

In the above example, we have a state object with a single property ‘counter’. The counterReducer function handles all the actions related to the ‘counter’ property. We then use the combineReducers function to combine the ‘counterReducer’ into a root reducer called ‘rootReducer’.

Note that combineReducers expects an object of reducers, so the keys in the object correspond to the names of the state properties and the values correspond to the reducer functions that handle those specific properties.

You can add more reducer functions to the object passed to combineReducers if you have more state properties and action types to handle.