FAQ: Strategies for Complex State - combineReducers

Frequently Asked Questions Uncategorized
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This community-built FAQ covers the “combineReducers” exercise from the lesson “Strategies for Complex State”.

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This exercise can be found in the following Codecademy content:

Learn Redux

FAQs on the exercise combineReducers

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The last 3-4 lessons have given me errors even though i had done everything correctly, identical to the solution. They usually error on the first task and wont let me move on even though it is correct. The only way to pass the lesson is to use the ‘replace with solution’ button. This is extremely annoying, as it defeats the whole purpose of doing those tasks one at a time before moving on to the next. Instead, i have to blindly do all the tasks and then compare them to the solution, and to move on: replace my code…

2 Likes
3 
const reducers = {
    todos: todosReducer,
    filter: filterReducer
};
const rootReducer = combineReducers(reducers);

How does rootReducer know the “appropriate slice of state” for each slice reducer?

4

I’ve run into this problem too, mostly because of spacing and/or trailing semi-colons. Instead of using ‘Replace with Solution’ button, I just make the applicable changes to the task that I’m working on. This way subsequent tasks aren’t completed in my code.

5

Unless I’m mistaken, the root reducer sends the action to each slice reducer. If the action type is not applicable in that reducer, it simply returns the default state for that slice.

6

I’m trying to understand this concept of root reducers:

import { createStore } from 'redux';

// todosReducer and filterReducer omitted

const rootReducer = (state = {}, action) => {
  const nextState = {
    todos: todosReducer(state.todos, action),
    filter: filterReducer(state.filter, action)
  };
  return nextState;
};

const store = createStore(rootReducer);

how would a dispatch of store.dispatch({ type: 'addTodo', payload:'Understand Redux'}) know to get sent to the todosReducer? Does the action get sent to BOTH reducers, and filterReducer returns the default, and todosReducer matches the case, and then updates todos?

7

All of the slice reducers get called by the root reducer when an action is dispatched. Because of the default cases in the switch statements, the other slice reducers are able to handle unknown action types by just returning the existing state for that slice (i.e. so the state remains unchanged for the other slices where the action is not relevant to them).