Using data in React with the Fetch API and axios

If you are new to React, and perhaps have only played with building to-do and counter apps, you may not yet have run across a need to pull in data for your app. There will likely come a time when you’ll need to do this, as React apps are most well suited for situations where you’re handling both data and state.

The first set of data you may need to handle might be hard-coded into your React application, like we did for this demo from our Error Boundary tutorial:

See the Pen error boundary 0 by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

What if you want to handle data from an API? That’s the purpose of this tutorial. Specifically, we’ll make use of the Fetch API and axios as examples for how to request and use data.

The Fetch API

The Fetch API provides an interface for fetching resources. We’ll use it to fetch data from a third-party API and see how to use it when fetching data from an API built in-house.

Using Fetch with a third-party API

See the Pen React Fetch API Pen 1 by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

We will be fetching random users from JSONPlaceholder, a fake online REST API for testing. Let’s start by creating our component and declaring some default state.

class App extends React.Component { state = { isLoading: true, users: [], error: null } render() { <React.Fragment> </React.Fragment> }

There is bound to be a delay when data is being requested by the network. It could be a few seconds or maybe a few milliseconds. Either way, during this delay, it’s good practice to let users know that something is happening while the request is processing.

To do that we’ll make use of isLoading to either display the loading message or the requested data. The data will be displayed when isLoading is false, else a loading message will be shown on the screen. So the render() method will look like this:

render() { const { isLoading, users, error } = this.state; return ( <React.Fragment> <h1>Random User</h1> // Display a message if we encounter an error {error ? <p>{error.message}</p> : null} // Here's our data check {!isLoading ? ( => { const { username, name, email } = user; return ( <div key={username}> <p>Name: {name}</p> <p>Email Address: {email}</p> <hr /> </div> ); }) // If there is a delay in data, let's let the user know it's loading ) : ( <h3>Loading...</h3> )} </React.Fragment> );

The code is basically doing this:

  1. De-structures isLoading, users and error from the application state so we don’t have to keep typing this.state.
  2. Prints a message if the application encounters an error establishing a connection
  3. Checks to see if data is loading
  4. If loading is not happening, then we must have the data, so we display it
  5. If loading is happening, then we must still be working on it and display “Loading…” while the app is working

For Steps 3-5 to work, we need to make the request to fetch data from an API. This is where the JSONplaceholder API will come in handy for our example.

fetchUsers() { // Where we're fetching data from fetch(``) // We get the API response and receive data in JSON format... .then(response => response.json()) // ...then we update the users state .then(data => this.setState({ users: data, isLoading: false, }) ) // Catch any errors we hit and update the app .catch(error => this.setState({ error, isLoading: false }));

We create a method called fetchUser() and use it to do exactly what you might think: request user data from the API endpoint and fetch it for our app. Fetch is a promise-based API which returns a response object. So, we make use of the json() method to get the response object which is stored in data and used to update the state of users in our application. We also need to change the state of isLoading to false so that our application knows that loading has completed and all is clear to render the data.

The fact that Fetch is promise-based means we can also catch errors using the .catch() method. Any error encountered is used a value to update our error’s state. Handy!

The first time the application renders, the data won’t have been received — it can take seconds. We want to trigger the method to fetch the users when the application state can be accessed for an update and the application re-rendered. React’s componentDidMount() is the best place for this, so we’ll place the fetchUsers() method in it.

componentDidMount() { this.fetchUsers();

Using Fetch With Self-Owned API

So far, we’ve looked at how to put someone else’s data to use in an application. But what if we’re working with our own data in our own API? That’s what we’re going to cover right now.

See the Pen React Fetch API Pen 2 by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

I built an API which is available on GitHub. The JSON response you get has been placed on AWS — that’s what we will use for this tutorial.

As we did before, let’s create our component and set up some default state.

class App extends React.Component { state = { isLoading: true, posts: [], error: null } render() { <React.Fragment> </React.Fragment> }

Our method for looping through the data will be different from the one we used before but only because of the data’s structure, which is going to be different. You can see the difference between our data structure here and the one we obtained from JSONPlaceholder.

Here is how the render() method will look like for our API:

render() { const { isLoading, posts, error } = this.state; return ( <React.Fragment> <h1>React Fetch - Blog</h1> <hr /> {!isLoading ? Object.keys(posts).map(key => <Post key={key} body={posts[key]} />) : <h3>Loading...</h3>} </React.Fragment> );

Let’s break down the logic

{ !isLoading ? Object.keys(posts).map(key => <Post key={key} body={posts[key]} />) : <h3>Loading...</h3>

When isLoading is not true, we return an array, map through it and pass the information to the Post component as props. Otherwise, we display a “Loading…” message while the application is at work. Very similar to before.

The method to fetch posts will look like the one used in the first part.

fetchPosts() { // The API where we're fetching data from fetch(``) // We get a response and receive the data in JSON format... .then(response => response.json()) // ...then we update the state of our application .then( data => this.setState({ posts: data, isLoading: false, }) ) // If we catch errors instead of a response, let's update the app .catch(error => this.setState({ error, isLoading: false }));

Now we can call the fetchPosts method inside a componentDidMount() method

componentDidMount() { this.fetchPosts();

In the Post component, we map through the props we received and render the title and content for each post:

const Post = ({ body }) => { return ( <div> { => { const { _id, title, content } = post; return ( <div key={_id}> <h2>{title}</h2> <p>{content}</p> <hr /> </div> ); })} </div> );

There we have it! Now we know how to use the Fetch API to request data from different sources and put it to use in an application. High fives. ✋


OK, so we’ve spent a good amount of time looking at the Fetch API and now we’re going to turn our attention to axios.

Like the Fetch API, axios is a way we can make a request for data to use in our application. Where axios shines is how it allows you to send an asynchronous request to REST endpoints. This comes in handy when working with the REST API in a React project, say a headless WordPress CMS.

There’s ongoing debate about whether Fetch is better than axios and vice versa. We’re not going to dive into that here because, well, you can pick the right tool for the right job. If you’re curious about the points from each side, you can read here and here.

Using axios with a third-party API

See the Pen React Axios 1 Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

Like we did with the Fetch API, let’s start by requesting data from an API. For this one, we’ll fetch random users from the Random User API.

First, we create the App component like we’ve done it each time before:

class App extends React.Component { state = { users: [], isLoading: true, errors: null }; render() { return ( <React.Fragment> </React.Fragment> ); }

The idea is still the same: check to see if loading is in process and either render the data we get back or let the user know things are still loading.

To make the request to the API, we’ll need to create a function. We’ll call the function getUsers(). Inside it, we’ll make the request to the API using axios. Let’s see how that looks like before explaining further.

getUsers() { // We're using axios instead of Fetch axios // The API we're requesting data from .get("") // Once we get a response, we'll map the API endpoints to our props .then(response => => ({ name: `${} ${}`, username: `${user.login.username}`, email: `${}`, image: `${user.picture.thumbnail}` })) ) // Let's make sure to change the loading state to display the data .then(users => { this.setState({ users, isLoading: false }); }) // We can still use the `.catch()` method since axios is promise-based .catch(error => this.setState({ error, isLoading: false }));

Quite different from the Fetch examples, right? The basic structure is actually pretty similar, but now we’re in the business of mapping data between endpoints.

The GET request is passed from the API URL as a parameter. The response we get from the API contains an object called data and that contains other objects. The information we want is available in data.results, which is an array of objects containing the data of individual users.

Here we go again with calling our method inside of the componentDidMount() method:

componentDidMount() { this.getUsers();

Alternatively, you can do this instead and basically combine these first two steps:

componentDidMount() { axios .get("") .then(response => => ({ name: `${} ${}`, username: `${user.login.username}`, email: `${}`, image: `${user.picture.thumbnail}` })) ) .then(users => { this.setState({ users, isLoading: false }); }) .catch(error => this.setState({ error, isLoading: false }));

If you are coding locally from your machine, you can temporarily edit the getUsers() function to look like this:

getUsers() { axios .get("") .then(response => console.log(response)) .catch(error => this.setState({ error, isLoading: false }));

Your console should get something similar to this:

We map through the results array to obtain the information we need for each user. The array of users is then used to set a new value for our users state. With that done, we can then change the value of isLoading.

By default, isLoading is set to true. When the state of users is updated, we want to change the value of isLoading to false since this is the cue our app is looking for to make the switch from “Loading…” to rendered data.

render() { const { isLoading, users } = this.state; return ( <React.Fragment> <h2>Random User</h2> <div> {!isLoading ? ( => { const { username, name, email, image } = user; return ( <div key={username}> <p>{name}</p> <div> <img src={image} alt={name} /> </div> <p>{email}</p> <hr /> </div> ); }) ) : ( <p>Loading...</p> )} </div> </React.Fragment> );

If you log the users state to the console, you will see that it is an array of objects:

The empty array shows the value before the data was obtained. The returned data contains only the name, username, email address and image of individual users because those are the endpoints we mapped out. There is a lot more data available from the API, of course, but we’d have to add those to our getUsers method.

Using axios with your own API

See the Pen React Axios 2 Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

You have seen how to use axios with a third-party API but we can look at what it’s like to request data from our own API, just like we did with the Fetch API. In fact, let’s use same JSON file we used for Fetch so we can see the difference between the two approaches.

Here is everything put together:

class App extends React.Component { // State will apply to the posts object which is set to loading by default state = { posts: [], isLoading: true, errors: null }; // Now we're going to make a request for data using axios getPosts() { axios // This is where the data is hosted .get("") // Once we get a response and store data, let's change the loading state .then(response => { this.setState({ posts:, isLoading: false }); }) // If we catch any errors connecting, let's update accordingly .catch(error => this.setState({ error, isLoading: false })); } // Let's our app know we're ready to render the data componentDidMount() { this.getPosts(); } // Putting that data to use render() { const { isLoading, posts } = this.state; return ( <React.Fragment> <h2>Random Post</h2> <div> {!isLoading ? ( => { const { _id, title, content } = post; return ( <div key={_id}> <h2>{title}</h2> <p>{content}</p> <hr /> </div> ); }) ) : ( <p>Loading...</p> )} </div> </React.Fragment> ); }

The main difference between this method and using axios to fetch from a third-party is how the data is formatted. We’re getting straight-up JSON this way rather than mapping endpoints.

The posts data we get from the API is used to update the value of the component’s posts state. With this, we can map through the array of posts in render(). We then obtain the id, title and content of each post using ES6 de-structuring, which is then rendered to the user.

Like we did before, what is displayed depends on the value of isLoading. When we set a new state for posts using the data obtained from the API, we had to set a new state for isLoading, too. Then we can finally let the user know data is loading or render the data we’ve received.

async and await

Another thing the promise-based nate of axios allows us to do is take advantage of is async and await . Using this, the getPosts() function will look like this.

async getPosts() { const response = await axios.get(""); try { this.setState({ posts:, isLoading: false }); } catch (error) { this.setState({ error, isLoading: false }); }

Base instance

With axios, it’s possible to create a base instance where we drop in the URL for our API like so:

const api = axios.create({ baseURL: ""

…then make use of it like this:

async getPosts() { const response = await api.get(); try { this.setState({ posts:, isLoading: false }); } catch (error) { this.setState({ error, isLoading: false }); }

Simply a nice way of abstracting the API URL.

Now, data all the things!

As you build React applications, you will run into lots of scenarios where you want to handle data from an API. Hopefully you know feel armed and ready to roll with data from a variety of sources with options for how to request it.

Want to play with more data? Sarah recently wrote up the steps for creating your own serverless API from a list of public APIs.

The post Using data in React with the Fetch API and axios appeared first on CSS-Tricks.

Finite State Machines with React

As JavaScript applications on the web have grown more complex, so too has the complexity of dealing with state in those applications — state being the aggregate of all the data that an application needs to perform its function. Over the last several years, there has been a ton of great innovation in the realm of state management through tools like Redux, MobX, and Vuex. Something that hasn’t gotten quite as much attention, though, is state design.

What in the heck do I mean by state design?

Let’s set the scene a little bit. In the past, when building an application that needs to fetch some data from a backend service and display it to the user, I’ve designed my state to use boolean flags for various things like isLoading, isSuccess, isError, and so on down the line. As this number of boolean flags grows, though, the number of possible states that my application can have grows exponentially with it — significantly increasing the likelihood of a user encountering an unintentional or error state.

To solve this issue, I’ve spent the last several months exploring the use of finite state machines as a way to better design the state of my applications.

Finite state machines are a mathematical model of computation, initially developed in the early 1940s, that have been used for decades to build both hardware and software for a wide array of technologies.

A finite state machine can be defined as any abstract machine that exists in exactly one of a finite number of states at a given time. In more practical terms, though, a state machine is characterized by a list of states with each state defining a finite, deterministic set of states that can be transitioned to by a given action.

Due to this finite and deterministic nature, we can use state diagrams to visualize our application — before or after it’s been built.

For example, if we wanted to visualize an authentication workflow, we could have three overarching states that our application could be in for a user: logged in, logged out, or loading.

A state diagram showing an application going from logged out, to loading, to logged in.

State machines, owing to their predictability, are especially popular in applications where reliability is critical — such as aviation software, manufacturing, and even the NASA Space Launch System. They’ve been a mainstay in the game development community for decades, as well.

In this article, we’ll tackle building something that most applications on the web use: authentication. We’ll use the state diagram above to guide us.

Before we get started, though, let’s familiarize ourselves with some of the libraries and APIs we’ll be using to build this application.

React Context API

React 16.3 introduced a new, stable version of the Context API. If you’ve worked much with React in the past, you may be familiar with how data is passed from parent to child through props. When you have certain data that is needed by a variety of components, you can end up doing what’s known as prop drilling — passing data through multiple levels of the component tree to get the data to a component that needs it.

Context helps alleviate the pain of prop drilling by providing a way to share data between components without having to explicitly pass that data through the component tree, making it perfect for storing authentication data.

When we create context, we get a Provider and Consumer pair. The provider will act as the “smart,” stateful component that contains our state machine definition, and maintains a record of the current state of our application.


xstate is a JavaScript library for functional, stateless finite state machines and statecharts — it will provide us a nice, clean API for managing definitions and transitions through our states.

A stateless finite state machine library might sound a bit strange, but essentially what it means is that xstate only cares about the state and transition that you pass it — meaning it’s up to your application to keep track of its own current state.

xstate has a lot of features worth mentioning that we won’t cover much in this article (since we’ll only begin to scratch the surface on statecharts): hierarchical machines, parallel machines, history states, and guards, just to name a few.

The approach

So, now that we’ve had a little bit of an introduction to both Context and xstate, let’s talk about the approach we’ll be taking.

We’ll start by defining the context for our application, then creating a stateful <App /> component (our provider) that will contain our authentication state machine, along with information about the current user and a method for the user to logout.

To set the stage a bit, let’s take a quick look at a CodePen demo of what we’ll be building.

See the Pen Authentication state machine example by Jon Bellah (@jonbellah) on CodePen.

So, without further ado, let’s dig into some code!

Defining our context

The first thing we need to do is define our application context and set it up with some default values. Default values in context are helpful for allowing us to test components in isolation, since the default values are only used if there is no matching provider.

For our application, we’re going to set up a few defaults: authState which will be the authentication state of the current user, an object called user which will contain data about our user if they’re authenticated, then a logout() method that can be called anywhere in the app if the user is authenticated.

const Auth = React.createContext({ authState: 'login', logout: () => {}, user: {},

Defining our machine

When we think about how authentication behaves in an application, in its simplest form, there are three primary states: logged out, logged in, and loading. These are the three states we diagramed earlier.

Looking back at that state diagram, our machine consists of those same three states: logged out, logged in, and loading. We also have four different action types that can be fired: SUBMIT, SUCCESS, FAIL, and LOGOUT.

We can model that behavior in code like so:

const appMachine = Machine({ initial: 'loggedOut', states: { loggedOut: { onEntry: ['error'], on: { SUBMIT: 'loading', }, }, loading: { on: { SUCCESS: 'loggedIn', FAIL: 'loggedOut', }, }, loggedIn: { onEntry: ['setUser'], onExit: ['unsetUser'], on: { LOGOUT: 'loggedOut', }, }, },

So, we just expressed the diagram from earlier in code, but are you ready for me to let you in on a little secret? That diagram was generated from this code using David Khourshid’s xviz library — which can be used to visually explore the actual code that powers your state machines.

If you’re interested in diving deeper into complex user interfaces using finite state machines, David Khourshid has a related article here on CSS-Tricks worth checking out.

This can be an incredibly powerful tool when trying to debug problematic states in your application.

Referring back to the code above now, we define our initial application state — which we’re calling loggedOut since we’ll want to show the login screen on an initial visit.

Note that in a typical application, you’d probably want to start from the loading state and determine if the user was previously authenticated… but since we’re faking the login process, we’re starting from the logged out state.

In the states object, we define each of our states along with the corresponding actions and transitions for each of those states. Then we pass all that as an object to the Machine() function, which is imported from xstate.

Along with our loggedOut and loggedIn states, we’ve defined some actions that we want to fire when our application enters or exits those states. We’ll look at what those actions do in a bit.

This is our state machine.

To break things down one more time, let’s look at the loggedOut: { on: { SUBMIT: 'loading'} } line . This means that if our application is in the loggedOut state and we call our transition function with an action of SUBMIT, our application will always transition from the loggedOut state to the loading state. We can make that transition by calling appMachine.transition('loggedOut', 'SUBMIT').

From there, the loading state will either move the user along as an authenticated user or send them back to the login screen and display an error message.

Creating our context provider

The context provider will be the component that sits at the top level of our application and houses all the data related to an authenticated — or unauthenticated — user.

Working in the same file as our state machine definition, let’s create an <App /> component and set it up with everything we’ll need. Don’t worry, we’ll cover what each method does in just a moment.

class App extends React.Component { constructor(props) { super(props); this.state = { authState: appMachine.initialState.value, error: '', logout: e => this.logout(e), user: {}, }; } transition(event) { const nextAuthState = appMachine.transition(this.state.authState, event.type); const nextState = nextAuthState.actions.reduce( (state, action) => this.command(action, event) || state, undefined, ); this.setState({ authState: nextAuthState.value, ...nextState, }); } command(action, event) { switch (action) { case 'setUser': if (event.username) { return { user: { name: event.username } }; } break; case 'unsetUser': return { user: {}, }; case 'error': if (event.error) { return { error: event.error, }; } break; default: break; } } logout(e) { e.preventDefault(); this.transition({ type: 'LOGOUT' }); } render() { return ( <Auth.Provider value={this.state}> <div className="w5"> <div className="mb2">{this.state.error}</div> {this.state.authState === 'loggedIn' ? ( <Dashboard /> ) : ( <Login transition={event => this.transition(event)} /> )} </div> </Auth.Provider> ); }

Whew, that was a lot of code! Let’s break it down into manageable chunks by taking a look at each method of this class individually.

In the constructor(), we’re setting our component state to the initial state of our appMachine, as well as setting our logout function in state, so that it can be passed through our application context to any consumer that needs it.

In the transition() method, we’re doing a few important things. First, we’re passing our current application state and the event type or action to xstate, so we can determine our next state. Then, in nextState, we take any actions associated with that next state (which will be one of our onEntry or onExit actions) and run them through the command() method — then we take all of the results and set our new application state.

In the command() method, we have a switch statement that returns an object — depending on the action type — which we use to pass data into our application state. This way, once a user has authenticated, we can set relevant details about that user — username, email, id, etc. — into our context, making it available to any of our consumer components.

Finally, in our render() method, we’re actually defining our provider component and then passing all of our current state through the value props, which makes the state available to all of the components beneath it in the component tree. Then, depending on the state of our application, we’re rendering either the dashboard or the login form for the user.

In this case, we have a pretty flat component tree beneath our provider (Auth.Provider), but remember that context allows that value to be available to any component beneath our provider in the component tree, regardless of depth. So, for example, if we have a component nested three or four levels down and we want to display the current user name, we can just grab that out of context, rather than drilling it all the way down to that one component.

Creating context consumers

Now, let’s create some components that consume our application context. From these components, we can do all sorts of things.

We can start by building a login component for our application.

class Login extends Component { constructor(props) { super(props); this.state = { yourName: '', } this.handleInput = this.handleInput.bind(this); } handleInput(e) { this.setState({ yourName:, }); } login(e) { e.preventDefault(); this.props.transition({ type: 'SUBMIT' }); setTimeout(() => { if (this.state.yourName) { return this.props.transition({ type: 'SUCCESS', username: this.state.yourName, }, () => { this.setState({ username: '' }); }); } return this.props.transition({ type: 'FAIL', error: 'Uh oh, you must enter your name!', }); }, 2000); } render() { return ( <Auth.Consumer> {({ authState }) => ( <form onSubmit={e => this.login(e)}> <label htmlFor="yourName"> <span>Your name</span> <input id="yourName" name="yourName" type="text" value={this.state.yourName} onChange={this.handleInput} /> </label> <input type="submit" value={authState === 'loading' ? 'Logging in...' : 'Login' } disabled={authState === 'loading' ? true : false} /> </form> )} </Auth.Consumer> ); }

Oh my! That was another big chunk of code, so let’s walk through each method again.

In the constructor(), we’re declaring our default state and binding the handleInput() method so that it references the proper this internally.

In handleInput(), we’re taking the value of our form field from our render() method and setting that value in state — this is referred to as a controlled form.

The login() method is where you would normally place your authentication logic. In the case of this app, we’re just faking a delay with setTimeout() and either authenticating the user — if they’ve provided a name — or returning an error if the field was left empty. Note that the transition() function that it calls is actually the one we defined in our <App /> component, which has been passed down via props.

Finally, our render() method displays our login form, but notice that the <Login /> component is also a context consumer. We’re using the authState context to determine whether or not to show our login button in a disabled, loading state.

Using context from deep in the component tree

Now that we’ve handled the creation of our state machine and a way for users to login to our application, we can now rely on having information about that user within any component nested under our <Dashboard /> component — since it will only ever be rendered if the user is logged in.

So let’s create a stateless component that grabs the username of the current authenticated user and displays a welcome message. Since we’re passing the logout() method to all of our consumers, we can also give the user the option of logging out from anywhere in the component tree.

const Dashboard = () => ( <Auth.Consumer> {({ user, logout }) => ( <div> <div>Hello {}</div> <button onClick={e => logout(e)}> Logout </button> </div> )} </Auth.Consumer>

Building larger applications with statecharts

Using finite state machines with React doesn’t have to be limited to authentication, nor does it have to be limited to the context API.

Using statecharts, you can have hierarchical machines and/or parallel machines — meaning individual React components can have their own internal state machine, but still be connected to the overall state of your application.

In this article, we’ve primarily focused on using xstate directly with the native Context API; in larger applications, I highly recommend looking at react-automata, which provides a thin layer of abstraction over the top of xstate. react-automata has the added benefit of being able to automagically generate Jest tests for your components.

State machines and state management tools are not mutually exclusive

It’s easy to get confused by thinking you must either use, say, xstate or Redux; but it’s important to note that state machines are more of an implementation concept, concerned with how you design your state — not necessarily how you manage it.

In fact, state machines can be used with just about any un-opinionated state management tool. I encourage you to explore various approaches to determine what works best for you, your team, and your application(s).

In conclusion

These concepts can be extended to the real world without having to refactor your entire application. State machines are an excellent refactor target — meaning, next time you’re working on a component that is littered with boolean flags like isFetching and isError, consider refactoring that component to use a state machine.

As a front-end developer, I’ve found that I’m often fixing one of two categories of bugs: display-related issues or unexpected application states.

State machines make the second category virtually disappear.

If you’re interested in diving deeper into state machines, I’ve spent the last several months working on a course on finite state machines — if you sign up for the email list, you’ll receive a discount code when the course launches in August.

The post Finite State Machines with React appeared first on CSS-Tricks.

Render Children in React Using Fragment or Array Components

What comes to your mind when React 16 comes up? Context? Error Boundary? Those are on point. React 16 came with those goodies and much more, but In this post, we’ll be looking at the rendering power it also introduced — namely, the ability to render children using Fragments and Array Components.

These are new and really exciting concepts that came out of the React 16 release, so let’s look at them closer and get to know them.


It used to be that React components could only return a single element. If you have ever tried to return more than one element, you know that you’ll will be greeted with this error: Syntax error: Adjacent JSX elements must be wrapped in an enclosing tag. The way out of that is to make use of a wrapper div or span element that acts as the enclosing tag.

So instead of doing this:

class Countries extends React.Component { render() { return ( <li>Canada</li> <li>Australia</li> <li>Norway</li> <li>Mexico</li> ) }

…you have to add either an ol or ul tag as a wrapper on those list items:

class Countries extends React.Component { render() { return ( <ul> <li>Canada</li> <li>Australia</li> <li>Norway</li> <li>Mexico</li> </ul> ) }

Most times, this may not be the initial design you had for the application, but you are left with no choice but to compromise on this ground.

React 16 solves this with Fragments. This new features allows you to wrap a list of children without adding an extra node. So, instead of adding an additional element as a wrapper like we did in the last example, we can throw <React.Fragment> in there to do the job:

class Countries extends React.Component { render() { return ( <React.Fragment> <li>Canada</li> <li>Australia</li> <li>Norway</li> <li>Mexico</li> </React.Fragment> ) }

You may think that doesn’t make much difference. But, imagine a situation where you have a component that lists different items such as fruits and other things. These items are all components, and if you are making use of old React versions, the items in these individual components will have to be wrapped in an enclosing tag. Now, however, you can make use of fragments and do away with that unnecessary markup.

Here’s a sample of what I mean:

class Items extends React.Component { render() { return ( <React.Fragment> <Fruit /> <Beverages /> <Drinks /> </React.Fragment> ) }

We have three child components inside of the fragment and can now create a component for the container that wraps it. This is much more in line with being able to create components out of everything and being able to compile code with less cruft.

Fragment Shorthand

There is a shorthand syntax when working with Fragments, which you can use. Staying true to its fragment nature, the syntax is like a fragment itself, leaving only only empty brackets behind.

Going back to our last example:

class Fruit extends React.Component { render() { return ( <> <li>Apple</li> <li>Orange</li> <li>Blueberry</li> <li>Cherry</li> </> ) }

Question: Is a fragment better than a container div?

You may be looking for a good reason to use Fragments instead of the container div you have always been using. Dan Abramov answered the question on StackOverflow. To summarize:

  1. It’s a tiny bit faster and has less memory usage (no need to create an extra DOM node). This only has a real benefit on very large and/or deep trees, but application performance often suffers from death by a thousand cuts. This is one less cut.
  2. Some CSS mechanisms like flexbox and grid have a special parent-child relationship, and adding divs in the middle makes it harder to maintain the design while extracting logical components.
  3. The DOM inspector is less cluttered.

Keys in Fragments

When mapping a list of items, you still need to make use of keys the same way as before. For example, let’s say we want to pass a list of items as props from a parent component to a child component. In the child component, we want to map through the list of items we have and output each item as a separate entity. Here’s how that looks:

const preload = { "data" : [ { "name": "Reactjs", "url": "", "description": "A JavaScript library for building user interfaces", }, { "name": "Vuejs", "url": "", "description": "The Progressive JavaScript Framework", }, { "name": "Emberjs", "url": "", "description": "Ember.js is an open-source JavaScript web framework, based on the Model–view–viewmodel pattern" } ]
} const Frameworks = (props) => { return ( <React.Fragment> { => ( <React.Fragment key={}> <h2>{}</h2> <p>{item.url}</p> <p>{item.description}</p> </React.Fragment> ))} </React.Fragment> )
} const App = () => { return ( <Frameworks items={preload} /> )

See the Pen React Fragment Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

You can see that now, in this case, we are not making use of any divs in the Frameworks component. That’s the key difference!

Render Children Using an Array of Components

The second specific thing that came out of React 16 we want to look at is the ability to render multiple children using an array of components. This is a clear timesaver because it allows us to cram as many into a render instead of having to do it one-by-one.

Here is an example:

class Frameworks extends React.Component { render () { return ( [ <p>JavaScript:</p> <li>React</li>, <li>Vuejs</li>, <li>Angular</li> ] ) }

You can also do the same with a stateless functional component:

const Frontend = () => { return [ <h3>Front-End:</h3>, <li>Reactjs</li>, <li>Vuejs</li> ]
} const Backend = () => { return [ <h3>Back-End:</h3>, <li>Express</li>, <li>Restify</li> ]
} const App = () => { return [ <h2>JavaScript Tools</h2>, <Frontend />, <Backend /> ]

See the Pen React Fragment 2 Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.


Like the Context API and Error Boundary feature that were introduced in React 16, rendering children components with Fragment and multiples of them with Array Components are two more awesome features you can start making use of as you build your application.

Have you started using these in a project? Let me know how in the comments so we can compare notes. 🙂

The post Render Children in React Using Fragment or Array Components appeared first on CSS-Tricks.

Handling Errors with Error Boundary

Thinking and building in React involves approaching application design in chunks, or components. Each part of your application that performs an action can and should be treated as a component. In fact, React is component-based and, as Tomas Eglinkas recently wrote, we should leverage that concept and err on the side of splitting any large chunking into smaller components.

Splitting inevitably introduces component hierarchies, which are good because they bloated components and architecture. However, things can begin to get complicated when an error occurs in a child component. What happens when the whole application crashes?! Seriously, React, why do the parent and sibling components have to pay for the sins of another component? Why?

Error Boundaries

React 16 came with a lot of goodies, one of which is error boundaries. Let’s consult the documentation and break down what it says about this gem because we can use it to spot errors where they occur and resolve them faster and with less headache!

Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed. Error boundaries catch errors during rendering, in lifecycle methods, and in constructors of the whole tree below them.

That’s a lot of jargon but, like components, we can break it down into less complex chunks.

Error boundaries are React Components

This makes a lot of sense and useful because it’s a concept we have using all along. The difference is that juice was sprayed on it to make it different from a normal component. Still, don’t forget the basic idea that error boundaries are themselves React Components!

Error boundaries catch JavaScript errors anywhere in their child component tree

In case you have forgotten how children component tree work, here is an example:

<ParentComponent> <FirstChild> <FirstChildDaughter> </FirstChildDaughter> </FirstChild> <SecondChild> </SecondChild>

We have two parent and three child components. According to what we have learned so far about error boundaries, we can replicate the above tree to:

<ErrorBoundaryComponent> <ParentComponent> <FirstChild> <FirstChildDaughter> </FirstChildDaughter> </FirstChild> <SecondChild> </SecondChild> </ParentComponent>

By wrapping the whole tree up in an ErrorBoundaryComponent, we can catch any JavaScript errors that occur in its child components. Cool, right?

Error boundaries log those errors

When errors are caught, we want boundaries errors to do something with them, preferably something to tell us about the. Developers often make use of error logging platforms to monitor errors that occur on their software. With error boundaries, we can do the same.

Error boundaries display a fallback UI

Instead of displaying the whole annoying combo of reds in different shades, you can choose a customized user interface to display when an error occurs. That can come in super handy because it allows you to tailor errors in a style that makes it easier for you to read and scan. Super cool, right?

Like me, you’ll think that this means error boundaries will catch all JavaScript errors. Sadly, that’s not true. Here are errors that they will gracefully ignore:

  • Event handlers
  • Asynchronous code (e.g. setTimeout or requestAnimationFrame callbacks)
  • Server-side rendering
  • Errors thrown in the error boundary itself (rather than its children)


The extra juice that makes a component an error boundary is componentDidCatch() — this is a lifecycle method that works like the JavaScript catch{} block, but for components. When an error is found in a child component, the error is handled by the closest error boundary. Only class components can be error boundaries.

componentDidCatch() accepts two parameters:

  • error: This is the error that was thrown
  • info: An object which contains a trace of where the error occurred

Error Boundary In Action

Say we are working on a feature that lists locations where conferences can be held. Something like this:

See the Pen error boundary 0 by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

The application lists locations from the Location component and the individual locations are output as Location Cards. We take a little extra care to ensure the name of each location is rendered in uppercase for consistency. For this tutorial purpose, we will add an empty object to the list of locations.

class Location extends React.Component { state = { locations: [ { "name": "Ojo", "zone": "Lagos State", "region": "South West" }, { "name": "Ahiazu Mbaise", "zone": "Imo State", "region": "South East" }, { "name": "Akoko-Edo", "zone": "Edo State", "region": "South South" }, { "name": "Anka", "zone": "Zamfara State", "region": "North West" }, { "name": "Akwanga", "zone": "Nasarawa State", "region": "North Central" }, { } ] } render() { return ( <div> <div> <div> <h2>Locations</h2> </div> </div> <div> {this.state.locations .map(location => <LocationCard key={} {...location} /> )} </div> </div> ) }
} const LocationCard = (props) => { return ( <div> <hr /> <p><b>Name:</b> {}</p> <p><b>Zone:</b> {}</p> <p><b>Region:</b> {props.region}</p> <hr /> </div> )
} const App = () => ( <div> <Location /> </div>
) ReactDOM.render(<App />, document.getElementById("root"));

If you run this in the browser, you will see an error similar to this screenshot:

A screenshot of the Type Error providing the error message Cannot read property toUpperCase of undefined. Background color is tan and there is a block of code with a light red background indicating where the error is in the code base.

That’s not totally helpful, so let’s apply an error boundary to handle help us out. First, we’ll create an ErrorBoundary component:

class ErrorBoundary extends React.Component { constructor(props) { super(props); this.state = { hasError: false, error: null, info: null }; } componentDidCatch(error, info) { this.setState({ hasError: true, error: error, info: info }); } render() { if (this.state.hasError) { return ( <div> <h1>Oops, something went wrong :(</h1> <p>The error: {this.state.error.toString()}</p> <p>Where it occured: {}</p> </div> ); } return this.props.children; }

An initial state for hasError, error, and info is created. Then, the componentDidCatch() lifecycle method is added. If an error occurs in the constructor, render or lifecycle method of any of its children components, the hasError state will be changed to true. When this happens, the ErrorBoundary component renders and displays the error. But if there are no errors, the children of the ErrorBoundary component are rendered instead as we’d expect.

Next, we need to add both the ErrorBoundary and Location components to our main App component:

const App = () => ( <div> <ErrorBoundary> <Location /> </ErrorBoundary> </div>

See the Pen error boundary 2 by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

We don’t see that annoying TypeError UI anymore! Things are working!

There’s one little thing we can do to improve the app. If you check the code in the demo, you’ll see an empty object we added at the end. Is it possible to have the other credible locations render? Most definitely! Inside the Location component, we can wrap the LocationCard component with the ErrorBoundary component to scope error logging directly to the cards:

class Location extends React.Component { state = { locations: [ { "name": "Ojo", "zone": "Lagos State", "region": "South West" }, { "name": "Ahiazu Mbaise", "zone": "Imo State", "region": "South East" }, { "name": "Akoko-Edo", "zone": "Edo State", "region": "South South" }, { "name": "Anka", "zone": "Zamfara State", "region": "North West" }, { "name": "Akwanga", "zone": "Nasarawa State", "region": "North Central" }, { // Empty! } ] } render() { return ( <div> <div> <div> <h2>Locations</h2> </div> </div> <div> {this.state.locations .map(location => <ErrorBoundary> // Should render all locations, but the empty instance <LocationCard key={} {...location} /> </ErrorBoundary> )} </div> </div> ) }

This time, the credible locations show, except the one that is empty. You can choose to wrap the whole component tree with an error boundary component once, or you can wrap different components at strategic places. The decision is up to you.

Wrapping Up

I encourage you to start making use of error boundaries in your applications. Similarly, it’s worth digging in a little deeper. For that, here are some issues in the React repo on Error Boundaries and event handles, go through them so you can see the current state of where things are at:

  • componentDidCatch is not getting called when there is an error in promise
  • Why are Error Boundaries not triggered for event handlers?

The post Handling Errors with Error Boundary appeared first on CSS-Tricks.

Managing State in React With Unstated

As your application becomes more complex, the management of state can become tedious. A component’s state is meant to be self-contained, which makes sharing state across multiple components a headache. Redux is usually the go-to library to manage state in React, however, depending on how complex your application is, you might not need Redux.

Unstated is an alternative that provides you with the functionality to manage state across multiple components with a Container class and Provider and Subscribe components. Let’s see Unstated in action by creating a simple counter and then look at a more advanced to-do application.

Using Unstated to Create a Counter

The code for the counter we’re making is available on GitHub:

View Repo

You can add Unstated to your application with Yarn:

yarn add unstated


The container extends Unstated’s Container class. It is to be used only for state management. This is where the initial state will be initialized and the call to setState() will happen.

import { Container } from 'unstated' class CounterContainer extends Container { state = { count: 0 } increment = () => { this.setState({ count: this.state.count + 1 }) } decrement = () => { this.setState({ count: this.state.count - 1 }) }
} export default CounterContainer

So far, we’ve defined the Container (CounterContainer), set its starting state for count at the number zero and defined methods for adding and subtracting to the component’s state in increments and decrements of one.

You might be wondering why we haven’t imported React at this point. There is no need to import it into the Container since we will not be rendering JSX at all.

Events emitters will be used in order to call setState() and cause the components to re-render. The components that will make use of this container will have to subscribe to it.


The Subscribe component is used to plug the state into the components that need it. From here, we will be able to call the increment and decrement methods, which will update the state of the application and cause the subscribed component to re-render with the correct count. These methods will be triggered by a couple of buttons that contain events listeners to add or subtract to the count, respectively.

import React from 'react'
import { Subscribe } from 'unstated' import CounterContainer from './containers/counter' const Counter = () => { return ( <Subscribe to={[CounterContainer]}> {counterContainer => ( <div> <div> // The current count value Count: { counterContainer.state.count } </div> // This button will add to the count <button onClick={counterContainer.increment}>Increment</button> // This button will subtract from the count <button onClick={counterContainer.decrement}>Decrement</button> </div> )} </Subscribe> )
} export default Counter

The Subscribe component is given the CounterContainer in the form of an array to its to prop. This means that the Subscribe component can subscribe to more than one container, and all of the containers are passed to the to prop of the Subscribe component in an array.

The counterContainer is a function that receives an instance of each container the Subscribe component subscribes to.

With that, we can now access the state and the methods made available in the container.


We’ll make use of the Provider component to store the container instances and allow the children to subscribe to it.

import React, { Component } from 'react';
import { Provider } from 'unstated' import Counter from './Counter' class App extends Component { render() { return ( <Provider> <Counter /> </Provider> ); }
} export default App;

With this, the Counter component can make use of our counterContainer.

Unstated allows you to make use of all the functionality that React’s setState() provides. For example, if we want to increment the previous state by one three times with one click, we can pass a function to setState() like this:

incrementBy3 = () => { this.setState((prevState) => ({ count: prevState.count + 1 })) this.setState((prevState) => ({ count: prevState.count + 1 })) this.setState((prevState) => ({ count: prevState.count + 1 }))

The idea is that the setState() still works like it does, but this time with the ability to keep the state contained in a Container class. It becomes easy to spread the state to only the components that need it.

Let’s Make a To-Do Application!

This is a slightly more advanced use of Unstated. Two components will subscribe to the container, which will manage all of the state, and the methods for updating the state. Again, the code is available on Github:

View Repo

The container will look like this:

import { Container } from 'unstated' class TodoContainer extends Container { state = { todos: [ 'Mess around with unstated', 'Start dance class' ], todo: '' }; handleDeleteTodo = (todo) => { this.setState({ todos: this.state.todos.filter(c => c !== todo) }) } handleInputChange = (event) => { const todo = this.setState({ todo }); }; handleAddTodo = (event) => { event.preventDefault() this.setState(({todos}) => ({ todos: todos.concat(this.state.todo) })) this.setState({ todo: '' }); } } export default TodoContainer

The container has an initial todos state which is an array with two items in it. To add to-do items, we have a todo state set to an empty string.

We’re going to need a CreateTodo component that will subscribe to the container. Each time a value is entered, the onChange event will trigger then fire the handleInputChange() method we have in the container. Clicking the submit button will trigger handleAddTodo(). The handleDeleteTodo() method receives a to-do and filters out the to-do that matches the one passed to it.

import React from 'react'
import { Subscribe } from 'unstated' import TodoContainer from './containers/todoContainer' const CreateTodo = () => { return ( <div> <Subscribe to={[TodoContainer]}> {todos => <div> <form onSubmit={todos.handleAddTodo}> <input type="text" value={todos.state.todo} onChange={todos.handleInputChange} /> <button>Submit</button> </form> </div> } </Subscribe> </div> );
} export default CreateTodo

When a new to-do is added, the todos state made available in the container is updated. The list of todos is pulled from the container to the Todos component, by subscribing the component to the container.

import React from 'react';
import { Subscribe } from 'unstated'; import TodoContainer from './containers/todoContainer' const Todos = () => ( <ul> <Subscribe to={[TodoContainer]}> {todos => => ( <li key={todo}> {todo} <button onClick={() => todos.handleDeleteTodo(todo)}>X</button> </li> )) } </Subscribe> </ul>
); export default Todos

This component loops through the array of to-dos available in the container and renders them in a list.

Finally, we need to wrap the components that subscribe to the container in a provider like we did in the case of the counter. We do this in our App.js file exactly like we did in the counter example:

import React, { Component } from 'react';
import { Provider } from 'unstated' import CreateTodo from './CreateTodo'
import Todos from './Todos' class App extends Component { render() { return ( <Provider> <CreateTodo /> <Todos /> </Provider> ); }
} export default App;

Wrapping Up

There are different ways of managing state in React depending on the complexity of your application and Unstated is a handy library that can make it easier. It’s worth reiterating the point that Redux, while awesome, is not always the best tool for the job, even though we often grab for it in these types of cases. Hopefully you now feel like you have a new tool in your belt.

The post Managing State in React With Unstated appeared first on CSS-Tricks.

How React Reconciliation Works

React is fast! Some of that speed comes from updating only the parts of the DOM that need it. Less for you to worry about and a speed gain to boot. As long as you understand the workings of setState(), you should be good to go. However, it’s also important to familiarize yourself with how this amazing library updates the DOM of your application. Knowing this will be instrumental in your work as a React developer.

The DOM?

The browser builds the DOM by parsing the code you write, it does this before it renders the page. The DOM represents documents in the page as nodes and objects, providing an interface so that programming languages can plug in and manipulate the DOM. The problem with the DOM is that it is not optimized for dynamic state changes, React has to calculate if it is necessary to update the DOM. It does this by creating a virtual DOM and comparing it with the current DOM. In this context, the virtual DOM will contain the new state of the component.

Let’s build a simple component that adds two numbers. The numbers will be entered in an input field.

See the Pen reconciliation Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

First, we’ll need to set up the initial state for the fields, then update the state when a number is entered. The component will look like this:

class App extends React.Component { state = { result: '', entry1: '', entry2: '' } handleEntry1 = (event) => { this.setState({entry1:}) } handleEntry2 = (event) => { this.setState({entry2:}) } handleAddition = (event) => { const firstInt = parseInt(this.state.entry1) const secondInt = parseInt(this.state.entry2) this.setState({result: firstInt + secondInt }) } render() { const { entry1, entry2, result } = this.state return( <div> <div> <p>Entry 1: { entry1 }</p> <p>Entry 2: { entry2 }</p> <p>Result: { result }</p> </div> <br /> <div> Entry 1:  <input type='text' onChange={this.handleEntry1} /> </div> <br /> <div> Entry 2:  <input type='text' onChange={this.handleEntry2} /> </div> <div> <button onClick={this.handleAddition} type='submit'>Add</button> </div> </div> ) }

On initial render, the DOM tree will look like this;

A screenshot from DevTools that shows the HTML rendering of the app.

When an entry is made in the first input field, React creates a new tree. The new tree which is the virtual DOM will contain the new state for entry1. Then, React compares the virtual DOM with the old DOM and, from the comparison, it figures out the difference between both DOMs and makes an update to only the part that is different. A new tree is created each time the state of App component changes — when a value is entered in either of the inputs field, or when the button is clicked.

Animated gif showing how the markup in DevTools changes when numbers are added to the input field.

Diffing Different Elements

When the state of a component changes so that an element needs to be changed from one type to another, React unmounts the whole tree and builds a new one from scratch. This causes every node in that tree to be destroyed.

Let’s see an example:

class App extends React.Component { state = { change: true } handleChange = (event) => { this.setState({change: !this.state.change}) } render() { const { change } = this.state return( <div> <div> <button onClick={this.handleChange}>Change</button> </div> { change ? <div> This is div cause it's true <h2>This is a h2 element in the div</h2> </div> : <p> This is a p element cause it's false <br /> This is another paragraph in the false paragraph </p> } </div> ) }

On initial render, you will see the div and its contents and how clicking the button causes React to destroy the div’s tree with its content and build a tree for the <p> element instead. Same happens if we have the same component in both cases. The component will be destroyed alongside the previous tree it belonged to, and a new instance will be built. See the demo below;

See the Pen reconciliation-2 Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

Diffing Lists

React uses keys to keep track of items in a list. The keys help it figure out the position of the item on a list. What happens when a list does not have keys? React will mutate every child of the list even if there are no new changes.

In other words, React changes every item in a list that does not have keys.

Here’s an example:

const firstArr = ['codepen', 'codesandbox']
const secondArr = ['github', 'codepen', 'bitbucket', 'codesanbox'] class App extends React.Component { state = { change: true } handleChange = (event) => { this.setState({change: !this.state.change}) } render() { const { change } = this.state return( <div> <div> <button onClick={this.handleChange}>Change</button> </div> <ul> { change ? => <li>{e}</li>) : => <li>{e}</li>) } </ul> </div> ) }

Here, we have two arrays that get rendered depending on the state of the component. React has no way of keep track of the items on the list, so it is bound to change the whole list each time there is a need to re-render. This results in performance issues.

In your console, you will see a warning like this:

Warning: Each child in an array or iterator should have a unique "key" prop.

To fix this, you add a unique key for each item on the list.

const firstArr = ['codepen', 'codesandbox']
const secondArr = ['github', 'codepen', 'bitbucket', 'codesanbox'] class App extends React.Component { state = { change: true } handleChange = (event) => { this.setState({change: !this.state.change}) } render() { const { change } = this.state return( <div> <div> <button onClick={this.handleChange}>Change</button> </div> <ul> { change ?, index) => <li key={e.index}>{e}</li>) :, index) => <li key={e.index}>{e}</li>) } </ul> </div> ) }

See the Pen reconciliation-3 Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

Wrapping Up

In summary, here are the two big takeaways for understanding how the concept of reconciliation works in React:

  • React can make your reconciliation process.
  • React doesn’t do a full rerender of your DOM nodes. It only changes what it needs to. The diffing process is so fast that you might not notice it.

The post How React Reconciliation Works appeared first on CSS-Tricks.

Understanding React `setState`

React components can, and often do, have state. State can be anything, but think of things like whether a user is logged in or not and displaying the correct username based on which account is active. Or an array of blog posts. Or if a modal is open or not and which tab within it is active.

React components with state render reconciliation. The reconciliation process is the way React updates the DOM, by making changes to the component based on the change in state. When the request to setState() is triggered, React creates a new tree containing the reactive elements in the component (along with the updated state). This tree is used to figure out how the Search component’s setState Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

Let’s register the component and define the markup for the Understanding React `setState` appeared first on CSS-Tricks.

What are Higher-Order Components in React?

If you have been in the React ecosystem for a while, there is a possibility that you have heard about Higher Order Components. Let’s look at a simple implementation while also trying to explain the core idea. From here you should get a good idea of how they work and even put them to use.

Why Higher-Order Components?

As you build React applications, you will run into situations where you want to share the same functionality across multiple components.

For example: you need to manage the state of currently logged in users in your application. Instead of managing that state across all of the components that need that state, you could create a higher-order component to separate the logged in user state into a container component, then pass that state to the components that will make use of it.

The components that receive state from the higher-order component will function as presentational components. State gets passed to them and they conditionally render UI based on it. They do not bother with the management of state.

Let’s see another example. Say you have three JSON files in your application. These files contain different data that will be loaded in your application in three different components. You want to give your users the ability to search the data loaded from these files. You could implement a search feature in all three of the components. This duplication may not be an issue at first, but as your application grows and more components need this functionality, the constant duplication will be cumbersome and prone to problems.

A better way forward is to create a higher-order component to handle the search functionality. With it, you can wrap the other components individually in your higher-order component.

How do Higher-Order Components Work?

The hoc Pen by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.


Higher-Order Components do not have to be scary. After understanding the basics, you can put the concept to use by abstracting away functionalities that can be shared among different components.

More Resources

  • Higher-Order Components – React Documentation
  • React Higher Order Components in depth – by Fran Guijarro
  • Higher Order Components in React – by Chris Nwamba
  • More Examples – CodePen collection

The post What are Higher-Order Components in React? appeared first on CSS-Tricks.

React’s New Context API Explained

In this video tutorial, Wes Bos looks into the new context API and the problem that it’s trying to solve:

React 16.3 has a new Context API which makes accessing data and functions anywhere in your application a snap. If you ever find yourself passing data down via props 4-5 levels deep, context might be what you are looking for.

Don’t forget about Neal Fennimore’s recent post on putting things into context. It covers the concept in great detail and provides some workarounds for the quirks you might encounter when using it.

Direct Link to Article — Permalink

The post React’s New Context API Explained appeared first on CSS-Tricks.

Putting Things in Context With React

Context is currently an experimental API for React – but soon to be a first class citizen! There are a lot of reasons it is interesting but perhaps the most is that it allows for parent components to pass data implicitly to their children, no matter how deep the component tree is. In other words, data can be added to a parent component and then any child can tap into it.

See the Pen React Context Lights by Neal Fennimore (@nealfennimore) on CodePen.

While this is often the use case for using something like Redux, it’s nice to use if you do not need complex data management. Think about that! We create a custom downstream of data, deciding which props are passed and at which levels. Pretty cool.

Context is great in areas of where you have a lot of components that depend on a single piece of data, but are deep within the component tree. Explicitly passing each prop to each individual component can often be overwhelming and it is a lot easier just to use context here.

For example, let’s consider how we would normally pass props down the tree. In this case, we’re passing the color red using props on each component in order to move it on down the stream.

class Parent extends React.Component { render(){ return <Child color="red" />; }
} class Child extends React.Component { render(){ return <GrandChild color={this.props.color} /> }
} class GrandChild extends React.Component { render(){ return ( <div style={{color: this.props.color}}> Yep, I'm the GrandChild </div> ); }

What if we never wanted the Child component to have the prop in the first place? Context saves us having to go through the Child component with color and pass it directly from the Parent to the GrandChild:

class Parent extends React.Component { // Allow children to use context getChildContext() { return { color: 'red' }; } render(){ return <Child />; }
} Parent.childContextTypes = { color: PropTypes.string
}; class Child extends React.Component { render() { // Props is removed and context flows through to GrandChild return <GrandChild /> }
} class GrandChild extends React.Component { render() { return ( <div style={{color: this.context.color}}> Yep, I'm still the GrandChild </div> ); }
} // Expose color to the GrandChild
GrandChild.contextTypes = { color: PropTypes.string

While slightly more verbose, the upside is exposing the color anywhere down in the component tree. Well, sometimes…

There’s Some Gotchas

You can’t always have your cake and eat it too, and context in it’s current form is no exception. There are a few underlying issues that you’ll more than likely come into contact with if you end up using context for all but the simplest cases.

Context is great for being used on an initial render. Updating context on the fly? Not so much. A common issue with context is that context changes are not always reflected in a component.

Let’s dissect these gotchas in more detail.

Gotcha 1: Using Pure Components

Context is hard when using PureComponent since by default it does not perform any shallow diffing by default with context. Shallow diffing with PureComponent is testing for whether the values of the object are strictly equal. If they’re not, then (and only then) will the component update. But since context is not checked, well… nothing happens.

See the Pen React Context Lights with PureComponents by Neal Fennimore (@nealfennimore) on CodePen.

Gotcha 2: Should Component Update? Maybe.

Context also does not update if a component’s shouldComponentUpdate returns false. If you have a custom shouldComponentUpdate method, then you’ll also need to take context into consideration. To enable updates with context, we could update each individual component with a custom shouldComponentUpdate that looks something like this.

import shallowEqual from 'fbjs/lib/shallowEqual'; class ComponentThatNeedsColorContext extends React.PureComponent { // nextContext will show color as soon as we apply ComponentThatNeedsColorContext.contextTypes // NOTE: Doing the below will show a console error come react v16.1.1 shouldComponentUpdate(nextProps, nextState, nextContext){ return !shallowEqual(this.props, nextProps) || !shallowEqual(this.state, nextState) || !shallowEqual(this.context, nextContext); }
} ComponentThatNeedsColorContext.contextTypes = { color: PropTypes.string

However, this does not solve the issue of an intermediary PureComponent between the parent and the child blocking context updates. This means that every PureComponent between the parent and child would need to have contextTypes defined on it, and they would also need to have an updated shouldComponentUpdate method. And at this point, that’s a lot of work for very little gain.

Better Approaches to the Gotchas

Fortunately, we have some ways to work around the gotchas.

Approach 1: Use a Higher Order Component

A Higher Order Component can read from context and pass the needed values on to the next component as a prop.

import React from 'react'; const withColor = (WrappedComponent) => { class ColorHOC extends React.Component { render() { const { color } = this.context; return <WrappedComponent style={{color: color}} {...this.props} /> } } ColorHOC.contextTypes = { color: React.PropTypes.string }; return ColorHOC;
}; export const Button = (props)=> <button {...props}>Button</button> // ColoredButton will render with whatever color is currently in context with a style prop
export const ColoredButton = withColor( Button );

See the Pen React Context Lights with HOC by Neal Fennimore (@nealfennimore) on CodePen.

Approach 2: Use Render Props

Render Props allow us to use props to share code between two components.

class App extends React.Component { getChildContext() { return { color: 'red' } } render() { return <Button /> }
} App.childContextTypes = { color: React.PropTypes.string
} // Hook 'Color' into 'App' context
class Color extends React.Component { render() { return this.props.render(this.context.color); }
} Color.contextTypes = { color: React.PropTypes.string
} class Button extends React.Component { render() { return ( <button type="button"> {/* Return colored text within Button */} <Color render={ color => ( <Text color={color} text="Button Text" /> ) } /> </button> ) }
} class Text extends React.Component { render(){ return (  {this.props.text}  ) }
} Text.propTypes = { text: React.PropTypes.string, color: React.PropTypes.string,

Approach 3: Dependency Injection

A third way we can work around these gotchas is to use Dependency Injection to limit the context API and allow components to subscribe as needed.

The New Context

The new way of using context, which is currently slated for the next minor release of React (16.3), has the benefits of being more readable and easier to write without the “gotchas” from previous versions. We now have a new method called createContext, which defines a new context and returns both a Provider and Consumer.

The Provider establishes a context that all sub-components can hook into. It’s hooked in via Consumer which uses a render prop. The first argument of that render prop function, is the value which we have given to the Provider. By updating the value within the Provider, all consumers will update to reflect the new value.

As a side benefit with using the new context, we no longer have to use childContextTypes, getChildContext, and contextTypes.

const ColorContext = React.createContext('color');
class ColorProvider extends React.Component { render(){ return ( <ColorContext.Provider value={'red'}> { this.props.children } </ColorContext.Provider> ) }
} class Parent extends React.Component { render(){ // Wrap 'Child' with our color provider return ( <ColorProvider> <Child /> </ColorProvider> ); }
} class Child extends React.Component { render(){ return <GrandChild /> }
} class GrandChild extends React.Component { render(){ // Consume our context and pass the color into the style attribute return ( <ColorContext.Consumer> {/* 'color' is the value from our Provider */} { color => ( <div style={{color: color}}> Yep, I'm still the GrandChild </div> ) } </ColorContext.Consumer> ); }

Separate Contexts

Since we have more granular control in how we expose context and to what components are allowed to use it, we can individually wrap components with different contexts, even if they live within the same component. We can see this in the next example, whereby using the LightProvider twice, we can give two components a separate context.

See the Pen React Context Lights with new Context by Neal Fennimore (@nealfennimore) on CodePen.


Context is a powerful API, but it’s also very easy to use incorrectly. There are also a few caveats to using it, and it can be very hard to figure out issues when components go awry. While Higher-Order Components and dependency injection offer alternatives for most cases, context can be used beneficially in isolated portions of your code base.

With the next context though, we no longer have to worry about the gotchas we had with the previous version. It removes having to define contextTypes on individual components and opens up the potential for defining new contexts in a reusable manner.

The post Putting Things in Context With React appeared first on CSS-Tricks.