@rnx-kit/metro-serializer-esbuild
v0.2.1
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esbuild serializer for Metro
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@rnx-kit/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 true
.
drop
Tells esbuild to edit your source code before building to drop certain
constructs. There are currently two possible things that can be dropped:
debugger
and console
.
See the full documentation at https://esbuild.github.io/api/#drop.
By default, this option is not set.
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.
minifyWhitespace
Same as minify
but only removes whitespace.
See the full documentation at https://esbuild.github.io/api/#minify.
By default, this option is not set.
minifyIdentifiers
Same as minify
but only renames local variables to be shorter.
See the full documentation at https://esbuild.github.io/api/#minify.
By default, this option is not set.
minifySyntax
Same as minify
but only rewrites syntax to be more compact.
See the full documentation at https://esbuild.github.io/api/#minify.
By default, this option is not set.
pure
Add /* @__PURE__ */
annotation to the specified new or call expressions. This
tells esbuild they can be removed if the resulting value is unused.
See the full documentation at https://esbuild.github.io/api/#pure.
By default, this option is not set.
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
.
metafile
The path to write metadata to, relative to the package root.
Determines whether esbuild should produce some metadata about the build in JSON format.
See the full documentation at https://esbuild.github.io/api/#metafile.
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 useexport *
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 asno-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.