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metro-serializer-esbuild

Allow Metro to use esbuild for bundling and serialization.

This tool is in Beta, and has been yielding good results so far. See the list of known issues below for more information.

Motivation​

Metro currently does not implement tree shaking, i.e. it does not attempt to remove unused code from the JS bundle. For instance, given this code snippet:

import { partition } from "lodash";

Metro will bundle all of lodash in your bundle even though you're only using a small part of it. In lodash's case, you can add babel-plugin-lodash to your Babel config to help Metro strip away some modules, but not all libraries will come with such helpers. For more details, see issues #227 and #632.

metro-serializer-esbuild addresses this by letting esbuild take over bundling.

Requirements​

This plugin currently depends on some unstable features introduced in Metro 0.66.1. Breaking changes were introduced in Metro 0.60, so this plugin will not work with React Native below 0.64.

Usage​

esbuild works best when we pass it ES6 modules. The first thing we must do is to disable import/export transformation by enabling disableImportExportTransform in babel.config.js:

+const env = process.env.BABEL_ENV || process.env.NODE_ENV;
 module.exports = {
   presets: [
     [
       "module:metro-react-native-babel-preset",
+      {
+        disableImportExportTransform:
+          env === "production" && process.env["RNX_METRO_SERIALIZER_ESBUILD"],
+      },
     ],
   ],
 };

To avoid issues with dev server, we only want to enable disableImportExportTransform when bundling for production.

If you're using @rnx-kit/babel-preset-metro-react-native, you don't need to make any changes.

Note that Hermes currently does not fully implement the ES6 spec. esbuild, on the other hand, does not fully support lowering to ES5. This essentially means that you may have to add additional plugins if you're seeing esbuild outputting "target environment is not supported yet" errors during bundle. For an example, see the error and its solution in #1743.

Next, configure Metro to use the esbuild serializer by making the following changes to metro.config.js:

 const { makeMetroConfig } = require("@rnx-kit/metro-config");
+const {
+  MetroSerializer,
+  esbuildTransformerConfig,
+} = require("@rnx-kit/metro-serializer-esbuild");

 module.exports = makeMetroConfig({
   serializer: {
+    customSerializer: MetroSerializer(),
   },
+  transformer: esbuildTransformerConfig,
 });

Note that esbuildTransformerConfig is incompatible with dev server and debug builds. It should only be set when bundling for production.

We can now create a bundle as usual, e.g.:

react-native bundle --entry-file index.js --platform ios --dev false ...

Options​

Similar to metro-serializer, metro-serializer-esbuild also supports plugins. Additionally, you can configure the output of the plugin by passing an options object as the second parameter. For instance, to output ES6 compliant code, set the target option:

 const myPlugins = [...];
 module.exports = makeMetroConfig({
   serializer: {
     customSerializer: MetroSerializer(myPlugins, {
+      target: "es6"
     }),
   },
   transformer: esbuildTransformerConfig,
 });

Below are all the currently supported options.

target​

Sets the target environment for the transpiled JavaScript code.

See the full documentation at https://esbuild.github.io/api/#target.

Values: Any JS language version string such as es6 or esnext. You can also use environment names. See the full documentation for a list of supported names.

Defaults to hermes0.7.0.

fabric​

When enabled, includes Fabric-enabled version of React. You can save some bytes by disabling this if you haven't migrated to Fabric yet.

Defaults to false.

minify​

When enabled, the generated code will be minified instead of pretty-printed.

See the full documentation at https://esbuild.github.io/api/#minify.

Defaults to true in production environment; false otherwise.

sourceMapPaths​

Determines whether paths in the output source map are absolute or relative to the directory containing the source map.

Values: absolute | relative

Defaults to relative.

strictMode​

By default, the "use strict"; directive is added by Babel and esbuild when lowering to ES5. You can save some bytes by telling this serializer to strip them from the bundle.

Note that disabling strictMode here will definitely break source maps. It is recommended to try disabling strict mode in Babel or TypeScript first before considering this option. If you can target ES6, that is a better alternative.

This flag only affects production environment.

Defaults to true.

analyze​

Sets whether esbuild should output a report at the end of bundling.

See the full documentation at https://esbuild.github.io/api/#analyze.

Values: false | true | verbose

Defaults to false.

logLevel​

The log level passed to esbuild.

See the full documentation at https://esbuild.github.io/api/#log-level

Values: verbose | debug | info | warning | error | silent

Defaults to warning.

Metro + ESM Support​

Metro currently does not support ESM. However, if you're looking to save even more bytes, and are comfortable with solving CJS vs ESM resolution issues, you can try adding module to resolver.resolverMainFields in metro.config.js. This will tell Metro to always pick ESM over CJS when possible. Note that this can lead to unexpected errors since you cannot import ESM from CJS. Until https://github.com/facebook/metro/issues/670 lands, you are basically on your own to fix any issues that might come up.

Known Limitations​

  • Dev server does not play well with esbuildTransformerConfig. To work around this limitation, you can save the esbuild specific Metro config to a separate file and only specify it when needed, e.g.:
    react-native bundle ... --config metro+esbuild.config.js
  • esbuild does not properly tree-shake export *. This is a known limitation (see https://github.com/evanw/esbuild/issues/1420). It is also not recommended to use export * in your code as they may lead to duplicate exports. For more details, read https://hackmd.io/Z021hgSGStKlYLwsqNMOcg. This can be mitigated with an ESLint rule, such as no-export-all from @rnx-kit/eslint-plugin.
  • esbuild is incompatible with RAM bundle. If you require RAM bundles, you cannot use this serializer. In fact, Metro will simply ignore it.