Tutorial: How to share code between iOS, Android & Web using React Native, react-native-web and monorepo

Let's make our react-native app work in the browser, the right way.

This tutorial was made for react-native <= 0.61. If you are using a newer version, I recommend that you fork this repository instead: brunolemos/react-native-web-monorepo, I am keeping it updated 🙌

Why am I writing this?

Hi 👋 I'm Bruno Lemos. I recently launched a project called DevHub - TweetDeck for GitHub and one of the things that caught people's attention was the fact that it is an app made by a single developer and available on 6 platforms: Web (react-native-web), iOS (react native), Android (react native), macOS, Windows and Linux (electron, for now), with almost 100% code sharing between them. It even shares some code with the server! This is something that would require a team of 3+ until a couple years ago.

Since then, I've received dozens of tweets and private messages asking how to achieve the same and in this tutorial I'll walk you through it.

What's react-native-web?

If you are not familiar with react-native-web, it's a lib by Necolas (ex Twitter engineer) to make your React Native code render in the browser. Roughly speaking, you will write <View /> and it will render <div />, making sure all styles render the exact same thing. It does more than that, but let's keep it simple.

The new Twitter was created using this technology and it's awesome.

If you already know react-native, you don't need to learn any new syntax. It's the same API.

Summary

• Starting a new React Native project
• Turning our folder structure into a monorepo
• Making react-native work in a monorepo
• Sharing code between our monorepo packages
• Creating a new web project using create-react-app and react-native-web
• Making CRA work inside our monorepo with code sharing
• ???
• Profit

Step-by-step tutorial

Starting a new React Native project

• $ react-native init myprojectname
• $ cd myprojectname
• $ git init && git add . -A && git commit -m "Initial commit"

Note: It's much easier to create a cross platform app from scratch than trying to port an existing mobile-only (or even harder: web-only) project, since they may be using lot's of platform specific dependencies.

EDIT: If you use expo, it seems they will soon have built in support for web!

Turning our folder structure into a monorepo

Monorepo means having multiple packages in a single repository so you can easily share code between them.  It's a bit less trivial than it sounds because both react-native and create-react-app require some work to support monorepo projects. But hey, at least it's possible!

We'll use a feature called Yarn Workspaces for that.
Requirements: Node.js, Yarn and React Native. 

• Make sure you are at the project root folder
• $ rm yarn.lock && rm -rf node_modules
• $ mkdir -p packages/components/src packages/mobile packages/web
• Move all the files (except .git) to the packages/mobile folder
• Edit the name field on packages/mobile/package.json from packagename to mobile
• Create this package.json at the root directory to enable Yarn Workspaces:

{
  "name": "myprojectname",
  "private": true,
  "workspaces": {
    "packages": [
      "packages/*"
    ],
    "nohoist": []
  }
  "dependencies": {
    "react-native": "0.61.3"
  }
}

• Create a .gitignore at the root directory:

.DS_Store
.vscode
node_modules/
yarn-error.log

• $ yarn

Making react-native work in a monorepo

• Check where react-native got installed. If it was at /node_modules/react-native, all right. If it was at /packages/mobile/node_modules/react-native, something is wrong. Make sure you have the latest versions of node and yarn. Also make sure to use the exact same version of dependencies between the monorepo packages, e.g. "react": "16.11.0" on both mobile and components, not a different version between them.

• Open your favorite editor and use the Search & Replace feature to replace all occurrences of node_modules/react-native/ with ../../node_modules/react-native/.

• For react-native <= 0.59, open packages/mobile/package.json. Your start script currently ends in /cli.js start. Append this to the end: --projectRoot ../../.

• Open packages./mobile/metro.config.js and set the projectRoot field on it as well so it looks like this:

const path = require('path')

module.exports = {
  projectRoot: path.resolve(__dirname, '../../'),
  transformer: {
    getTransformOptions: async () => ({
      transform: {
        experimentalImportSupport: false,
        inlineRequires: false,
      },
    }),
  },
}

• [Workaround] You currently need to add the react-native dependency to the root package.json to be able to bundle the JS:

  "dependencies": {
    "react-native": "0.61.3"
  },

iOS changes

• $ open packages/mobile/ios/myprojectname.xcodeproj/
• Open AppDelegate.m, find jsBundleURLForBundleRoot:@"index" and replace index with packages/mobile/index
• Still inside Xcode, click on your project name on the left, and then go to Build Phases > Bundle React Native code and Images. Replace its content with this:

export NODE_BINARY=node
export EXTRA_PACKAGER_ARGS="--entry-file packages/mobile/index.js"
../../../node_modules/react-native/scripts/react-native-xcode.sh

• $ yarn workspace mobile start

You can now run the iOS app! 💙 Choose one iPhone emulator and press the "Run" triangle button inside Xcode.

Android changes

• $ studio packages/mobile/android/
• Open packages/mobile/android/app/build.gradle. Search for the text project.ext.react = [...]. Edit it so it looks like this:

project.ext.react = [
    entryFile: "packages/mobile/index.js",
    root: "../../../../"
]

• Android Studio will show a Sync Now popup. Click on it.
• Open packages/mobile/android/app/src/main/java/com/myprojectname/MainApplication.java. Search for the getJSMainModuleName method. Replace index with packages/mobile/index, so it looks like this:

@Override
protected String getJSMainModuleName() {
  return "packages/mobile/index";
}

If you get the Cannot get property 'packageName' on null object error, try disabling auto linking

You can now run the Android app! 💙 Press the "Run" green triangle button inside Android Studio and choose the emulator or device.

Sharing code between our monorepo packages

We've created lots of folders in our monorepo, but only used mobile so far. Let's prepare our codebase for code sharing and then move some files to the components package, so it can be reused by mobile, web and any other platform we decide to support in the future (e.g.: desktop, server, etc.).

• Create the file packages/components/package.json with the following contents:

{
  "name": "components",
  "version": "0.0.1",
  "private": true
}

• [optional] If you decide to support more platforms in the future, you'll do the same thing for them: Create a packages/core/package.json, packages/desktop/package.json, packages/server/package.json, etc. The name field must be unique for each one.

• Open packages/mobile/package.json. Add all the monorepo packages that you are using as dependencies. In this tutorial, mobile is only using the components package:

"dependencies": {
  "components": "0.0.1",
  ...
}

• Stop the react-native packager if it's running
• $ yarn
• $ mv packages/mobile/App.js packages/components/src/
• Open packages/mobile/index.js. Replace import App from './App' with import App from 'components/src/App'. This is the magic working right here. One package now have access to the others!
• Edit packages/components/src/App.js, replace Welcome to React Native! with Welcome to React Native monorepo! so we know we are rendering the correct file.
• $ yarn workspace mobile start

Yay! You can now refresh the running iOS/Android apps and see our screen that's coming from our shared components package. 🎉

• $ git add . -A && git commit -m "Monorepo"

Web project

Note: You can reuse up to 100% of the code, but that doesn't mean you should. It's recommended to have some differences between platforms to make them feel more natural to the user. To do that, you can create platform-specific files ending with .web.js, .ios.js, .android.js or .native.js. See example.

Creating a new web project using CRA and react-native-web

• $ cd packages/
• $ npx create-react-app web
• $ cd ./web (stay inside this folder for the next steps)
• $ rm src/* (or manually delete all files inside packages/web/src)
• Make sure the dependencies inside package.json are the exact same between all monorepo packages. For example, update the "react" version to "16.9.0" (or any other version) on both web and mobile packages.
• $ yarn add react-native-web react-art
• $ yarn add --dev babel-plugin-react-native-web
• Create the file packages/web/src/index.js with the following contents:

import { AppRegistry } from 'react-native'

import App from 'components/src/App'

AppRegistry.registerComponent('myprojectname', () => App)
AppRegistry.runApplication('myprojectname', {
  rootTag: document.getElementById('root'),
})

Note: when we import from react-native inside a create-react-app project, its webpack config automatically alias it to react-native-web for us.

• Create the file packages/web/public/index.css with the following contents:

html,
body,
#root,
#root > div {
  width: 100%;
  height: 100%;
}

body {
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

• Edit packages/web/public/index.html to include our CSS before closing the head tag:

...
<title>React App</title>
<link rel="stylesheet" href="%PUBLIC_URL%/index.css" />
</head>

Making CRA work inside our monorepo with code sharing

CRA doesn't build files outside the src folder by default. We need to make it do it, so it can understand the code from our monorepo packages, which contains JSX and other non-pure-JS code.

• Stay inside packages/web/ for the next steps
• Create a .env file (packages/web/.env) with the following content:

SKIP_PREFLIGHT_CHECK=true

• $ yarn add --dev react-app-rewired
• Replace the scripts inside packages/web/package.json with this:

"scripts": {
  "start": "react-app-rewired start",
  "build": "react-app-rewired build",
  "test": "react-app-rewired test",
  "eject": "react-app-rewired eject"
},

• Create the packages/web/config-overrides.js file with the following contents: 

const fs = require('fs')
const path = require('path')
const webpack = require('webpack')

const appDirectory = fs.realpathSync(process.cwd())
const resolveApp = relativePath => path.resolve(appDirectory, relativePath)

// our packages that will now be included in the CRA build step
const appIncludes = [
  resolveApp('src'),
  resolveApp('../components/src'),
]

module.exports = function override(config, env) {
  // allow importing from outside of src folder
  config.resolve.plugins = config.resolve.plugins.filter(
    plugin => plugin.constructor.name !== 'ModuleScopePlugin'
  )
  config.module.rules[0].include = appIncludes
  config.module.rules[1] = null
  config.module.rules[2].oneOf[1].include = appIncludes
  config.module.rules[2].oneOf[1].options.plugins = [
    require.resolve('babel-plugin-react-native-web'),
  ].concat(config.module.rules[2].oneOf[1].options.plugins)
  config.module.rules = config.module.rules.filter(Boolean)
  config.plugins.push(
    new webpack.DefinePlugin({ __DEV__: env !== 'production' })
  )

  return config
}

The code above overrides some create-react-app's webpack config so it includes our monorepo packages in CRA's build step

• $ git add . -A && git commit -m "Web project"

That's it! You can now run yarn start inside packages/web (or yarn workspace web start at the root directory) to start the web project, sharing code with our react-native mobile project! 🎉

Some gotchas

• react-native-web supports most of the react-native API, but a few pieces are missing like Alert, Modal, RefreshControl and WebView;
• If you come across a dependency that doesn't work well with the monorepo structure, you may add it to the nohoist list; But avoid that if possible, because it may cause other issues, specially with the metro bundler.

Some tips

• Navigation may be a bit of a challenge; you can use something like react-navigation which recently added web support or you can try using two different navigators between and mobile, in case you want the best of both worlds by compromising some code sharing;
• If you plan sharing code with the server, I recommend creating a core package that only contain logic and helper functions (no UI-related code);
• For Next.js, you can check their official example with react-native-web
• For native windows, you can try react-native-windows;
• For native macOS, you can the new Apple Project Catalyst, but support for it is not 100% there yet (see my tweet);
• To install new dependencies, use the command yarn workspace components add xxx from the root directory. To run a script from a package, run yarn workspace web start, for example; To run a script from all packages, run yarn workspaces run scriptname;

Thanks for reading! 💙

If you like react, consider following me here on Dev.to and on Twitter.

Links

• Source code: react-native-web-monorepo
• DevHub: devhubapp/devhub (production app using this structure + Desktop + TypeScript)
• Twitter: @brunolemos