Web Performance

Expo uses react-native-web which is the highly optimized framework used to power major websites and PWAs like Twitter, Major League Soccer, Flipkart, Uber, The Times, DataCamp.

Presets

There are a number of performance tools at your disposal that will not only optimize your web app, but also improve the performance of your native app! Here are the officially recommended and most optimal set of tools to use when creating universal projects:

Babel
babel-preset-expo extends the default react-native preset and adds support for all other Expo platforms. In the browser this has massive performance benefits by enabling tree-shaking of the unused react-native-web modules.
Jest
jest-expo A universal solution for testing your code against all of the platforms it runs on. Learn more about Universal Testing.
Webpack Config
@expo/webpack-config A default Webpack config that's optimized for running react-native-web apps and creating progressive web apps.

Optimize Your Assets

The easiest and most highly recommended way to improve you project is to optimize your assets. You can reduce the size of your assets with the Expo Optimize CLI.

Terminal

# Make sure you can successfully install the native image editing library Sharp

→ npm install -g sharp-cli

# Then in your project run:

→ npx expo-optimize

📦 What Makes My App Large?

To inspect bundle sizes, you can use a Webpack plugin called Webpack Bundle Analyzer. This plugin will help you visualize the size of your static bundles. You can use this to identify unwanted large packages that you may not have bundled intentionally.

Using Bundle Analyzer

Install the bundle analyzer:
Terminal
→ yarn add -D webpack-bundle-analyzer
Reveal the Webpack Config:
- Run expo customize:web and select webpack.config.js.
Customize the config to generate a web report:

const createExpoWebpackConfigAsync = require('@expo/webpack-config');
const { BundleAnalyzerPlugin } = require('webpack-bundle-analyzer');

module.exports = async (env, argv) => {
  const config = await createExpoWebpackConfigAsync(env, argv);

  // Optionally you can enable the bundle size report.
  // It's best to do this only with production builds because it will add noticeably more time to your builds and reloads.
  if (env.mode === 'production') {
    config.plugins.push(
      new BundleAnalyzerPlugin({
        path: 'web-report',
      })
    );
  }

  return config;
};

Finding Tree-Shaking Errors

If you want to track down why a package was included, you can build your project in debug mode.

Terminal

→ EXPO_WEB_DEBUG=true expo build:web

This will make your bundle much larger, and you shouldn't publish your project in this state.

You can now search for unwanted packages by name and see which files or methods are preventing them from being tree-shaken.

⚡️ Lighthouse

Lighthouse is a great way to see how fast, accessible, and performant your website is. You can test your project with the Audit tab in Chrome, or with the Lighthouse CLI.

After creating a production build with expo build:web and serving it somewhere, run Lighthouse with the URL your site is hosted at.

lighthouse <url> --view