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

Explanation:
In Redux, combineReducers is a helper function that is used to combine multiple reducers into a single reducer function. The reducers are typically defined for different parts of the application state, and combineReducers helps to create a common root reducer that can handle actions for all the reducers.

The combineReducers function takes in a configuration object, where the keys represent different parts of the state, and the values represent the individual reducers for those parts of the state. These reducers can be either functions or objects of functions.

The “reducer” argument mentioned in the query is referring to one of the values in the configuration object passed to combineReducers. It is required to be either a function or an object of functions. A reducer function is responsible for handling specific actions related to a particular part of the state. It takes in the current state and an action, and returns a new state based on the action.

Here is an example to illustrate the usage of combineReducers:

    
      // Import the necessary dependencies
      import { combineReducers } from 'redux';

      // Define individual reducers for different parts of state
      function todosReducer(state = [], action) {
        switch (action.type) {
          case 'ADD_TODO':
            return [...state, action.payload];
          case 'REMOVE_TODO':
            return state.filter(todo => todo.id !== action.payload);
          default:
            return state;
        }
      }

      function userReducer(state = null, action) {
        switch (action.type) {
          case 'SET_USER':
            return action.payload;
          case 'CLEAR_USER':
            return null;
          default:
            return state;
        }
      }

      // Combine the reducers into a root reducer
      const rootReducer = combineReducers({
        todos: todosReducer,
        user: userReducer
      });

      // The rootReducer can now be used as the single reducer in the Redux store
    
  

In the above example, the combineReducers function is used to create a rootReducer that combines the todosReducer and userReducer. The configuration object provided to combineReducers specifies the keys ‘todos’ and ‘user’ as the parts of state, and the associated reducer functions as their values.

The rootReducer can be used when creating the Redux store, allowing actions to be dispatched and state to be managed for both the ‘todos’ and ‘user’ parts of the application state.