combineSelectors(selectors)

This module follows the patterns set out by redux with combineReducers. It allows you to colocate your selectors with the reducer state they work on and simplifies their usage. It follows a pattern outlined by Dan Abramov in a free egghead.io lesson here

The basic idea is to use combineSelectors everywhere you use combineReducer as you progressively build up your store state. This allows a convenient way to access more complicated derived state (provided through selector functions) following your store state.

Example:

export const selectors = combineSelectors({
  potato: potatoSelectors,
  tomato: tomatoSelectors
});
// Like combineReducers, this will produce an object where the selectors are called with the section of the redux state it's keyed by
{
  potato: {
    // ... potato selectors
  },
  tomato: {
    // ... tomato selectors
  },
}

The access of your selectors now mirrors your redux state.

Example:

reducers/todos.js

export default function todos(state = {}, action) {
  switch (action.type) {
    case "ADD_TODO":
      return {
        ...state,
        [action.payload.id]: action.payload.text,
      };
    default:
      return state;
  }
}

// Selectors
export const getTodo = (state, id) => state[id];
export const getAllIds = (state) => Object.keys(state);

reducers/counter.js

export default function counter(state = 0, action) {
  switch (action.type) {
    case "INCREMENT":
      return state + 1;
    case "DECREMENT":
      return state - 1;
    default:
      return state;
  }
}

// Selectors
export const getCount = (state) => state;

reducers/index.js

import { combineReducers } from "redux";
import { combineSelectors } from "combine-selectors-redux";
import todos, * as todoSelectors from "./todos";
import counter, * as counterSelectors from "./counter";

export default combineReducers({
  todos,
  counter,
});

export const selectors = combineSelectors({
  todos: todoSelectors,
  counter: counterSelectors,
});

App.js

import { createStore } from "redux";
import reducer, { selectors } from "./reducers/index";

let store = createStore(reducer);
console.log(store.getState());
// {
//   todos: [],
//   counter: 0
// }

console.log(selectors);
// {
//   todos: function() {...},
//   counter: function() {...}
// }

console.log(selectors.todos.getAllIds());
// []

console.log(selectors.counter.getCount());
// 0