It’s 2020. Your users shouldn’t have to download your entire app when all they need is a piece of it. If a user is creating a new post, it doesn’t make sense to have them download all the code for the /registration route. If a user is registering, they don’t need the huge rich text editor your app needs on the /settings route. It’s wasteful and some would argue disrespectful to those users who don’t have the privilege of unlimited bandwidth. This idea has not only gained much more popularity in recent years, but it’s also become exponentially easier to pull off - it even has a fancy cool name - code splitting.

The idea is simple, don’t download code until the user needs it. Though in practice, it’s not that simple. The reason for this isn’t because code splitting itself is terribly difficult, but that there are various tools to do it and everyone has an opinion on which is the best. When you’re first starting out, it can be hard to parse what is what.

The two most common approaches are using Webpack and its bundle loader or the ECMAScript dynamic import() proposal which is currently in stage 4 of the ECMAScript process. Any chance I get to not use webpack I take, so we’ll be using dynamic import() in this post.

If you’re familiar with ES modules, you know that they’re completely static. What that means is that you must specify what you’re importing and exporting at compile time, not run time. This also means that you can’t dynamically import a module based on some condition. import s need to be declared at the top of your file or they’ll throw an error.

if ( ! user ) { import * as api from './api' }

Now, what if import didn’t have to be static? Meaning what if the code above worked? What benefits would that give us? First, it would mean we could load certain modules on demand. That would be pretty powerful since it would enable us to get closer to the vision of only downloading code the user needs.

if ( editPost === true ) { import * as edit from './editpost' edit . showEditor ( ) }

Assuming editpost contained a pretty large rich text editor, we’d make sure we didn’t download it until the user was actually ready to use it.

Another cool use case of this would be for legacy support. You could hold off on downloading specific code until you were certain the user’s browser didn’t already have it natively.

Here’s the good news (that I kind of already alluded to earlier). This type of functionality does exist, it’s supported by Create React App, and it’s currently in Stage 4 of the ECMAScript process. The difference is that instead of using import as you typically would, you use it like a function that returns you a promise that resolves with the module once the module is completely loaded.

if ( editPost === true ) { import ( './editpost' ) . then ( ( module ) => module . showEditor ( ) ) . catch ( ( e ) => ) }

Pretty rad, right?

Now that we know how to import modules dynamically, the next step is figuring out how to use it with React and React Router v4.

The first (and probably biggest) question we need to ask ourselves when it comes to code splitting with React is where should we split at? Typically, there are two answers.

Split at the route level. 🙂 Split at the component level. 😃

The more common approach is to split at the route level. You already split your app into different routes, so adding in code splitting on top of that feels pretty natural. How would this actually look?

Let’s start off with a basic React Router example. We’ll have three routes, / , /topics , /settings .

import React from 'react' import { BrowserRouter as Router , Route , Link , } from 'react-router-dom' import Home from './Home' import Topics from './Topics' import Settings from './Settings' class App extends React . Component { render ( ) { return ( < Router > < div > < ul > < li > < Link to = ' / ' > Home </ Link > </ li > < li > < Link to = ' /topics ' > Topics </ Link > </ li > < li > < Link to = ' /settings ' > Settings </ Link > </ li > </ ul > < hr /> < Route exact path = ' / ' component = { Home } /> < Route path = ' /topics ' component = { Topics } /> < Route path = ' /settings ' component = { Settings } /> </ div > </ Router > ) } } export default App

💻 Play with the code.

Now, say our /settings route was super heavy. It contains a rich text editor, an original copy of Super Mario Brothers, and an HD image of Guy Fieri. We don’t want the user to have to download all of that when they’re not on the /settings route. Let’s use our knowledge of dynamic imports and React to code split the /settings route.

Just like we solve any problem with React, let’s make a component. We’ll call it DynamicImport . The goal of DynamicImport is to dynamically load a module, then, once it’s loaded, pass that module to its children .

Once implemented, it’ll look something like this.

const Settings = ( props ) => ( < DynamicImport load = { ( ) => import ( './Settings' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > )

The above code tells us two important details about DynamicImport . First, it will receive a load prop which when invoked, will dynamically import a module using the dynamic import syntax we covered earlier. Second, it will receive a function as its children which will need to be invoked with the imported module.

Before we dive into the implementation of DynamicImport , let’s think about how we might accomplish this. The first thing we need to do is to make sure we call props.load . That will return us a promise that when it resolves, should have the module. Then, once we have the module, we need a way to cause a re-render so we can invoke props.children passing it that module. How do you cause a re-render in React? By setting state. By adding the dynamically imported module to DynamicImport s local state, we follow the exact same process with React as we’re used to - fetch data -> set state -> re-render. Except this time instead of “fetch data”, we have “import module”.

First, let’s add some initial state to DynamicImport . component will eventually be the component that we’re dynamically importing.

class DynamicImport extends React . Component { state = { component : null } }

Now, we need to call props.load . That will return us a promise that when it resolves, should have the module.

class DynamicImport extends React . Component { state = { component : null } componentDidMount ( ) { this . props . load ( ) . then ( ( component ) => { this . setState ( { component } ) } ) } }

There’s one small caveat here. If the module we’re dynamically importing is using ES modules (export default), it’ll have a .default property. If the module is using commonjs (module.exports), it won’t. Let’s change our code to adapt for that.

this . props . load ( ) . then ( ( component ) => { this . setState ( { component : component . default ? component . default : component } ) } )

Now that we’re dynamically importing the module and adding it to our state, the last thing we need to do it figure out what the render method looks like. If you remember, when the DynamicImport component is used, it’ll look like this.

const Settings = ( props ) => ( < DynamicImport load = { ( ) => import ( './Settings' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > )

Notice that we’re passing it a function as its “children” prop. That means we need to invoke children passing is the component state.

class DynamicImport extends React . Component { state = { component : null } componentDidMount ( ) { this . props . load ( ) . then ( ( component ) => { this . setState ( { component : component . default ? component . default : component } ) } ) } render ( ) { return this . props . children ( this . state . component ) } }

Now anytime we want to import a module dynamically, we can wrap it in DynamicImport . If we were to do this to all our routes, our code would look like this.

import React from 'react' import { BrowserRouter as Router , Route , Link , } from 'react-router-dom' class DynamicImport extends React . Component { state = { component : null } componentDidMount ( ) { this . props . load ( ) . then ( ( component ) => { this . setState ( { component : component . default ? component . default : component } ) } ) } render ( ) { return this . props . children ( this . state . component ) } } const Home = ( props ) => ( < DynamicImport load = { ( ) => import ( './Home' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > ) const Topics = ( props ) => ( < DynamicImport load = { ( ) => import ( './Topics' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > ) const Settings = ( props ) => ( < DynamicImport load = { ( ) => import ( './Settings' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > ) class App extends React . Component { render ( ) { return ( < Router > < div > < ul > < li > < Link to = ' / ' > Home </ Link > </ li > < li > < Link to = ' /topics ' > Topics </ Link > </ li > < li > < Link to = ' /settings ' > Settings </ Link > </ li > </ ul > < hr /> < Route exact path = ' / ' component = { Home } /> < Route path = ' /topics ' component = { Topics } /> < Route path = ' /settings ' component = { Settings } /> </ div > </ Router > ) } } export default App

💻 Play with the code.

How do we know this is actually working and code splitting our routes? If you run npm run build with an app created by Create React App, you’ll see our app’s been split.

Each chunk is each dynamic import() in our app.

At this point, you may be wondering why React doesn’t come with a built-in solution for code splitting similar to our DynamicImport component. I have good news, it does, but the implementation is a little different.

Wait, why did we spend all this time building DynamicImport if React has a built-in solution that we can use? - You Cause you’re smarter now, and that’s all I care about - Me

With the DynamicImport component, we passed to it a load prop which was a function that when invoked, would resolve with the module.

const Settings = ( props ) => ( < DynamicImport load = { ( ) => import ( './Settings' ) } > </ DynamicImport > )

With React’s built-in solution, instead of passing the function which wraps a dynamic import as a prop to a component, you pass it as an argument to the React.lazy method.

const Settings = React . lazy ( ( ) => import ( './Settings' ) )

The next difference is how we show the Loading component. With DynamicImport , we used null to check if the component was still loading. If it was, we rendered <Loading /> , if it wasn’t, we rendered the component.

const Settings = ( props ) => ( < DynamicImport load = { ( ) => import ( './Settings' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > )

With React.lazy , it’ll assume that the dynamic import will resolve to a module that exports a React component. What that means is that you don’t have to render the lazily loaded component yourself, instead, you render what React.lazy returns you and it’ll take care of rendering the actual component.

What you do have to worry about though is what you want to show when the module is still loading. To do this, you can use React’s Suspense component giving it a fallback prop which is a React element.

const Settings = React . lazy ( ( ) => import ( './Settings' ) ) function App ( ) { return ( < div > < React.Suspense fallback = { < Loading /> } > < Settings /> </ React.Suspense > </ div > ) }

What’s nice about React.Suspense that we weren’t able to do with our DynamicImport component is that Suspense can take in multiple, lazily loaded components while still only rendering one Loading element.

const AdDashboard = React . lazy ( ( ) => import ( './AdDashboard' ) ) const Analytics = React . lazy ( ( ) => import ( './Analytics' ) ) const Settings = React . lazy ( ( ) => import ( './Settings' ) ) function App ( ) { return ( < div > < React.Suspense fallback = { < Loading /> } > < AdDashboard /> < Analytics /> < Settings /> </ React.Suspense > </ div > ) }

Now let’s update our main app to use both our DynamicImport component as well as React.lazy to see the difference.

import React from 'react' import Loading from './Loading' import DynamicImport from './DynamicImport' import { BrowserRouter as Router , Route , Link , } from 'react-router-dom' const Home = ( props ) => ( < DynamicImport load = { ( ) => import ( './Home' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > ) const Topics = ( props ) => ( < DynamicImport load = { ( ) => import ( './Topics' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > ) const Settings = ( props ) => ( < DynamicImport load = { ( ) => import ( './Settings' ) } > { ( Component ) => Component === null ? < Loading /> : < Component { ... props } /> } </ DynamicImport > ) const LazyHome = React . lazy ( ( ) => import ( './Home' ) ) const LazyTopics = React . lazy ( ( ) => import ( './Topics' ) ) const LazySettings = React . lazy ( ( ) => import ( './Settings' ) ) class App extends React . Component { render ( ) { return ( < Router > < div > < ul > < li > < Link to = ' / ' > Home </ Link > </ li > < li > < Link to = ' /topics ' > Topics </ Link > </ li > < li > < Link to = ' /settings ' > Settings </ Link > </ li > </ ul > < hr /> < Route exact path = ' / ' component = { Home } /> < Route path = ' /topics ' component = { Topics } /> < Route path = ' /settings ' component = { Settings } /> < React.Suspense fallback = { < Loading /> } > < Route exact path = ' / ' component = { LazyHome } /> < Route path = ' /topics ' component = { LazyTopics } /> < Route path = ' /settings ' component = { LazySettings } /> </ React.Suspense > </ div > </ Router > ) } } export default App

💻 Play with the code.

Remember earlier when we talked about how there were two main ways to go about code splitting your app? We had this handy little guide.

Split at the route level. 🙂 Split at the component level. 😃

So far we’ve only covered splitting at the route level. This is where a lot of people stop. Code splitting at the route level only is like brushing your teeth but never flossing. Your teeth will be mostly clean, but you’ll still get cavities.

Instead of thinking about code splitting as splitting your app up by its routes, you should think of it as splitting your app up by its components ( <Route> s are just components, after all). If you have a rich text editor that lives in a modal, splitting by the route only will still load the editor even if the modal is never opened.