20. Refactoring the Reducers

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Earlier, we removed the visibilityFilter reducer, and so the root reducer in the app now combines only a single todos reducer. Since index.js acts effectively as a proxy to the todos reducer, we will remove index.js completely. Then we will rename todos.js to index.js, thereby making todos the new root reducer.

The root reducer file now contains byId, allIds, activeIds, and completedIDs. We're going to extract some of them into separate files.

Creating a file called byid.js, where we paste the code for the byId reducer.

Now we'll add a named export for a selector called getTodo that takes the state and id, where the state corresponds to the state of the byId reducer. Now going back to index.js, we can import the reducer as a default import.

We can also import any associated selectors in a single object with a namespace import:

import byId, * as fromById from './byid'

Now if we take a look at the reducers managing the IDs, we will notice that their code is almost exactly the same except for the filter value which they compare action.filter to.

Creating createList

Let's create a new function called createList that takes filter as an argument.

createList will return another function– a reducer that handles the ids for the specified filter– , so its state shape is an array. To save time, we can copy & paste the implementation from allIds, and then just change the 'all`` literal tocreateList'sfilter` argument, so that we can create it for any filter.

const createList = (filter) => {
  return (state = [], action) => {
    if (action.filter !== filter) {
      return state;
    }
    switch (action.type) {
      case 'RECEIVE_TODOS':
        return action.response.map(todo => todo.id);
      default:
        return state;
    }
  };
};

Now we can remove the allIds, activeIds, and completedIds reducer code completely. Instead, we will generate the reducers using the new createList function, and pass the filter as an argument to it.

Next, extract the createList function into a separate file called createList.js.

Now that it's in a separate file, we will add a public API for accessing the state in form of a selector. For now, we will call it getIds, and will just returns the state of the list (we may change this in the future).

Finishing Up

Back in index.js, we will import createList and any named selectors from this file.

import createList, * as fromList from './createList';

We will also rename idsByFilter to listByFilter because now that the list implementation is in a separate file, we will use the getIds selector that it exports.

export const getVisibleTodos = (state, filter) => {
  const ids = fromList.getIds(state.listByFilter[filter]);
  return ids.map(id => fromById.getTodo(state.byId, id));
};

Since we also moved the byId reducer into a separate file, we want to make sure we don't make an assumption that it's just a lookup table. With this in mind, we will use the fromById.getTodo selector that it exports and pass its state and the corresponding ID.

With this refactor, we can change the state shape of any reducer in the future without rippling changes across the codebase.

Recap at 3:41 in video