Minifying JavaScript

Learn about customizing the JavaScript minification process in Expo CLI with Metro bundler.

Minification is an optimization build step. It removes unnecessary characters such as collapses whitespace, removes comments, and shortens static operations, from the source code. This process reduces the final size and improves load times.

Minification in Expo CLI

In Expo CLI, minification is performed on JavaScript files during the production export (when npx expo export, npx expo export:embed, eas build, and so on, commands run).

For example, consider following code snippet in a project:

// This comment will be stripped
console.log('a' + ' ' + 'long' + ' string' + ' to ' + 'collapse');

This will be minified by the Expo CLI:

console.log('a long string to collapse');

Tip: Comments can be preserved by using the /** @preserve */ directive.

The default minification of Expo CLI is sufficient for most projects. However, you can customize the minifier to optimize for speed or remove additional features like logs.

Remove console logs

You can remove console logs from your production build. Use the drop_console option in the Terser minifier config.

const { getDefaultConfig } = require('expo/metro-config');

const config = getDefaultConfig(__dirname);

config.transformer.minifierConfig = {
  compress: {
    // The option below removes all console logs statements in production.
    drop_console: true,
  },
};

module.exports = config;

You can also pass an array of console types to drop if you want to preserve certain logs. For example: drop_console: ['log', 'info'] will remove console.log and console.info but preserve console.warn and console.error.

Customizing the minifier

Different minifiers have tradeoffs between speed and compression. You can customize the minifier used by Expo CLI by modifying the metro.config.js file in your project.

Terser

terser is the default minifier in SDK 48 and above: (Metro@0.73.0 changelog).

const { getDefaultConfig } = require('expo/metro-config');

const config = getDefaultConfig(__dirname);

config.transformer.minifierPath = 'metro-minify-terser';
config.transformer.minifierConfig = {
  // Terser options...
};

module.exports = config;

Unsafe Terser options

For additional compression that may not work in all JavaScript engines, enable the unsafe compress options:

const { getDefaultConfig } = require('expo/metro-config');

const config = getDefaultConfig(__dirname);

config.transformer.minifierPath = 'metro-minify-terser';

config.transformer.minifierConfig = {
  compress: {
    // Enable all unsafe optimizations.
    unsafe: true,
    unsafe_arrows: true,
    unsafe_comps: true,
    unsafe_Function: true,
    unsafe_math: true,
    unsafe_symbols: true,
    unsafe_methods: true,
    unsafe_proto: true,
    unsafe_regexp: true,
    unsafe_undefined: true,
    unused: true,
  },
};

module.exports = config;

esbuild

esbuild is used to minify exponentially faster than uglify-es and terser. For more information, see metro-minify-esbuild usage.

Uglify

For projects SDK 48 and above, you can use uglify-es by following the steps below:

const { getDefaultConfig } = require('expo/metro-config');

const config = getDefaultConfig(__dirname);

config.transformer.minifierPath = 'metro-minify-uglify';
config.transformer.minifierConfig = {
  // Options: https://github.com/mishoo/UglifyJS#compress-options
};

module.exports = config;