Redux Lego Pieces - Above and Beyond

November 12, 2017

In the first post on Redux Lego Pieces we discussed the essential building blocks of Redux and how they fit together. If you already have the basics of Redux down then continue on, otherwise you may want to checkout the first post in this series. By the end of this post you should have some familiarity with what libraries you will need to pull together to kick Redux into overdrive.

Lego Piece #5: combineReducers

It is unrealistic for any complex application to have a single reducer that manages the entire application’s state. The more idiomatic approach is to have multiple reducers that each manage a specific part of the applications state. Although, createStore requires a single reducer as input so if we are intentionally writing multiple reducers we need to be able to still pass in a single reducer to createStore. The combineReducers function is used to (you guessed it) combine multiple reducers into a single reducer. The input requires a object which has each reducer mapped by the property name you would like that piece of state to be called in your state tree to the reducer that will manage that piece of state.

const posts = (state = [], action) => {
switch(action.type) {
// Implementation...

const comments = (state = [], action) => {
switch(action.type) {
// Implementation...

const entitiesReducer = combineReducers({

The structure of your reducers will match the structure of your state since each reducer maps directly to a piece of state. In the example above I used combineReducers to create a new reducer that is responsible for managing all of my post and comment entities. The entitiesReducer when called will produce a state object with a posts property and comments property. If I wanted, I could change the name of the state a reducer is managing by changing the property name in the object you pass into the combineReducers function.

Reducers that are made as a result of combineReducers are no different than the reducers you write yourself. The reducers you create with combineReducers are going to expect a larger piece of state as input, in fact they will expect the combined pieces of state as input as demonstrated below.

const entitiesReducer = (state = {posts: [], comments=[]}, action) => {
return {
posts: posts(state.posts, action),
comments: comments(state.comments, action)

Above is an example of what a ‘combined’ reducer looks like. Rather than hard coding this behavior combineReducers does it dynamically for you so you don’t have to statically type out combined reducers. Your combined reducers will ultimately combine into a single reducer that represents your entire application state.

Lego Piece #6: Middleware

Middleware allows for Redux to be extended to support different kinds of functionality that do not ship with Redux by default. What exactly does middleware do in the context of Redux? It can really do anything you would like. Although, a more definitive answer would be that it allows you to take some sort of action in between the moment you dispatch an action and the moment it reaches the stores reducer.

The createStore function supports an enhancer argument. Which is used to add functionality to the store such as logging, persistence and async actions (more on that later). Redux ships with a single store enhancer called applyMiddleware which takes in a list of middleware to chain together.

const store = createStore(

Writing your own middleware is not incredibly complicated but I will not be covering that here for the sake of brevity. The important part to note is that you can utilize middleware to enhance your store and that all middleware are chained together from first to last in the order that they are passed into applyMiddleware. The only caveat being that one middleware could do something with the action that results in middleware down the chain not seeing it.

Lego Piece #7: Async Actions

There are a variety of different ways that asynchronous actions can be handled with Redux. One way that I think is simple is to expand our definition of what an action is defined as. Up until now an action was simply an object. Which is great for updating state synchronously but how would you utilize that to do something that is asynchronous like an HTTP request to obtain data from a server? In short, you can’t. Although, if we expand the definition of what an action is to include objects and functions we will be able to do asynchronous actions.

What if we added a middleware to our application that was responsible for checking if an action was either an object or a function. In the event that it is a function we would invoke it and pass in the dispatch and getState functions allowing us to dispatch other actions and look at values in our state. This is exactly what redux-thunk does.

const store = createStore(

In the above code snippet we add the thunk middleware to our store using the applyMiddleware enhancer that we discussed earlier. The thunk middleware is going to look at every single action and invoke them if they are functions and ignore them if they are not functions, providing the opportunity for you to do any asynchronous work you would like to do.

Asynchronous actions look different than synchronous actions since they are functions. To define an asynchronous action it is best to define an action creator function that will return the function to be called by redux-thunk. This allows you to pass in any additional arguments you may have at dispatch time which is similar to regular action creators. Below is an example of an async action to fetch a list of posts from an API.

const fetchPosts => () {
return async (dispatch, getState) => {
try {
const response = await fetch('/posts')
if (response.ok) {
const json = await response.json()
} else {
throw new Error('Failed to fetch.')
} catch(error) {

On a 2XX response we will dispatch a loadPostsSuccess action to hydrate our store with our newly loaded posts. In the event of a failure we will dispatch a loadPostsFailure action that will allow us to inform our users that we were unable to fetch the posts.

To invoke the async action you simply need to call dispatch with the result of fetchPosts which would be the function for redux-thunk to invoke.


As you can see, by dispatching the fetchPosts async action we were able to attempt to fetch the posts for the application and inform the users of the asynchronous success or failure. All while still allowing the user to perform other actions in the application.

Lego Piece #8: Selectors

If redux is like a database for your relational state and reducers are like your tables the only missing piece we have not discussed are queries. Selectors are used to query for state from your redux state tree and compute derived values so you can store the minimal possible state. You can use selectors the same way you would use queries with an actual database. They can be used to do filtering, searching, and aggregation operations. In its most simple form a selector takes in the redux state and then does a query against that state.

const getPosts = (state) => state.entities.posts

Selectors can decouple the structure of your data from your view. Your view no longer needs to know about the structure of your state object, but simply that there is a function that takes in the application state and returns the posts. You are free to now change the structure to whatever you would like.

Since all selectors are functions that use state as their input you can compose selectors within one another.

const getNumberOfPosts = (state) => getPosts(state).length

Selectors really begin to shine when you want to do more completed operations that require multiple pieces of data. Using a library like reselect you can join queries the same way you would with a typical database. Although, the syntax is a little bit different since we are working with JavaScript functions rather than SQL queries but the concept is still the same.

const getSelectedPostId = (state) => state.posts.selectedId

const getSelectedPost = createSelector(
(posts, selectedPostId) => {
return posts.find(post => === selectedPostId)

The createSelector function accepts a variable number of selectors as input and the final argument is known as the result function. The result function is similar to your joined database query. The result function will be passed in all of the results from the input selectors you passed as arguments to createSelector allowing you use all of the results within a single function, essentially joining all of the results together.

One thing that makes reselect so awesome is that selectors made with createSelector are memoized, which means they do not recompute unless their arguments change. This is great, anytime we call getSelectedPost it will only have to do the find operation if it has not done it already for the specific posts and selectedPostId arguments. If either one of those values change then the selector will recompute otherwise it will return the cached values.

Closing Thoughts

In this post we learned how to utilize combineReducers to combine reducers so that we could decompose reducers into more legible functions that are only responsible for managing a single part of the state tree rather than multiple parts. In conjunction with that we learned how we can leverage Redux’s middleware to allow us to do asynchronous actions using redux-thunk. Lastly, we discussed the notion of Redux being akin to a database that you can query using selectors you wrote yourself or ones that were built with reselect. In the next post in this series we will discuss how you can leverage all the different lego pieces we have discussed so far to create a model layer that is agnostic of your view layer. Stay tuned!