Customizing Webpack

When you run expo start:web or expo build:web the CLI will check to see if your project has a webpack.config.js in the root directory. If the project doesn't then Expo will use the default @expo/webpack-config (preferred).

This is akin to react-scripts & create-react-app.

If you need to edit the config the best way to do this is by running expo customize:web and selecting the webpack.config.js option. This will install @expo/webpack-config as a devDependency and create a template webpack.config.js into your project. You can now make changes to a config object based on the default config and return it for Expo CLI to use. Deleting the config will cause Expo to fall back to the default again.

If you create a new Webpack config or make any changes to it you'll need to restart your Webpack dev server with expo start:web.

Example

webpack.config.js

const createExpoWebpackConfigAsync = require('@expo/webpack-config');

// Expo CLI will await this method so you can optionally return a promise.
module.exports = async function(env, argv) {
  const config = await createExpoWebpackConfigAsync(env, argv);
  // If you want to add a new alias to the config.
  config.resolve.alias['moduleA'] = 'moduleB';

  // Maybe you want to turn off compression in dev mode.
  if (config.mode === 'development') {
    config.devServer.compress = false;
  }

  // Or prevent minimizing the bundle when you build.
  if (config.mode === 'production') {
    config.optimization.minimize = false;
  }

  // Finally return the new config for the CLI to use.
  return config;
};

Polyfills

React Native for web uses some advanced browser features that might not be available in every browser. Expo web tries to make including these features as simple and efficient as possible with @expo/webpack-config.

ResizeObserver

👉 Browser support

TL;DR: To fully support onLayout install resize-observer-polyfill.

The onLayout prop that's used in all of the core primitives like View, Image, Text, ScrollView, etc. requires an API called ResizeObserver. This API isn't fully supported across all browsers, iOS Safari (in iOS 13) is a good example. If the device doesn't support ResizeObserver then react-native-web will fallback on window.onresize and you'll see a warning in the logs:

onLayout relies on ResizeObserver which is not supported by your browser.
Please include a polyfill, e.g., https://github.com/que-etc/resize-observer-polyfill.
Falling back to window.onresize.

To get everything working properly, you'll want to install and include a global polyfill for ResizeObserver.

Adding ResizeObserver

Install the polyfill: yarn add resize-observer-polyfill
Restart the project and @expo/webpack-config will automatically include the polyfill.

The reason it automatically includes the polyfill is because react-native-web needs it included immediately. Webpack is able to inject the polyfill before any of the application code has been executed. Alternatively you can customize the webpack config and include the polyfill in the entry field yourself.

Testing the ResizeObserver polyfill

Open the running Expo project in iOS Safari
Connect the device to an Apple computer
Open Safari on the computer in go to Develop > [YOUR DEVICE] > [YOUR HOST]
Ensure the logs don't have the onLayout relies on ResizeObserver... warning.

Editing static files

You can also use expo customize:web to generate the static project files: index.html, serve.json, favicon.ico, etc... These can be used to customize your project in a more familiar way.

All of the files you select from the terminal prompt will be copied to a web/ folder in your project's root directory. Think of this folder like public/ in Create React App. We use "web" instead of "public" because Expo projects target more then just web. For mobile platforms, we similarly put platform-specific project files in /ios and /android folders.

Deleting any of these files will cause Expo CLI to fall back to their respective default copies. If at some point you want to reset these files to their initial values simply run expo customize:web --force or -f for short.

Why

Customizing the favicon icon
Adding third-party API code to the <head/> in your index.html
Changing the caching policy in the serve.json file