@rnx-kit/metro-config
v2.0.1
Published
Metro config for monorepos
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@rnx-kit/metro-config
@rnx-kit/metro-config
provides helper functions for creating a Metro config
that works in a monorepo.
Installation
yarn add @rnx-kit/metro-config --dev
or if you're using npm
npm add --save-dev @rnx-kit/metro-config
Usage
Import makeMetroConfig
helper function from @rnx-kit/metro-config
:
// metro.config.js
const { makeMetroConfig } = require("@rnx-kit/metro-config");
module.exports = makeMetroConfig({
transformer: {
getTransformOptions: async () => ({
transform: {
experimentalImportSupport: false,
inlineRequires: false,
},
}),
},
});
makeMetroConfig
takes a Metro config as parameter. The config is augmented
with additional fields to make sure it works smoothly in a monorepo.
Expo
@rnx-kit/metro-config
can be used with Expo. Pass its default config to
makeMetroConfig
like below:
// metro.config.js
const { getDefaultConfig } = require("@expo/metro-config");
const { makeMetroConfig } = require("@rnx-kit/metro-config");
const config = getDefaultConfig(__dirname);
module.exports = makeMetroConfig(config);
Known Limitations
While Metro is the de-facto standard, it lacks a few features that would make it optimal; here's our current wishlist, feel free to submit PRs if you want to help with them :)
Bundle size
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.
If you're feeling adventurous, you can try an experimental Metro serializer we've built that adds support for tree shaking: @rnx-kit/metro-serializer-esbuild. Do note that you will need to be on React Native 0.64 or above, and use Metro 0.66.1.
Plugins
Metro doesn't have a plugin system, but it does have hooks that allows you to implement something that functions similarly. @rnx-kit/metro-serializer implements a serializer that allows you to pass plugins that are run just before the JS bundle is serialized and written to disk. Follow the instructions for installing it, then try our plugins:
- @rnx-kit/metro-plugin-cyclic-dependencies-detector is a plugin that detects cyclic imports. These can cause bugs that can be really confusing to debug.
- @rnx-kit/metro-plugin-duplicates-checker detects whether you're bundling multiple copies of the same package in your JS bundle.
- @rnx-kit/metro-plugin-typescript-validation
performs type checking of the TypeScript files being bundled. The Babel plugin
for TypeScript that comes with
metro-react-native-babel-preset
only strips the types. Normally, one would runtsc
separately to ensure that the code is correct. This step can now be replaced with the plugin.
You can of course also provide your own plugins.
Ensuring a single instance of a package
Normally, Metro resolves a module relative to the package it is currently
residing in. For example, with a monorepo such as below, my-awesome-package
would resolve [email protected]
while another-awesome-package
would
resolve [email protected]
. This would lead to duplicate packages in your
bundle and may cause issues.
workspace
├── node_modules
│ └── [email protected] <-- should be ignored
└── packages
├── my-awesome-package
│ └── node_modules
│ └── [email protected] <-- should take precedence
└── another-awesome-package <-- imported by my-awesome-package,
but uses workspace's react-native-msal
If we simply exclude the workspace's copy, Metro will not be able to find
react-native-msal
from another-awesome-package
. It also won't exclude copies
that are installed in other packages. To help Metro resolve to the correct copy,
we need to exclude all other copies, and also add a corresponding entry in
extraNodeModules
. @rnx-kit/metro-config
contains functions to help you set
this up correctly. Given the example above, our metro.config.js
should look
like this:
const {
exclusionList,
makeMetroConfig,
resolveUniqueModule,
} = require("@rnx-kit/metro-config");
const [msalPath, msalExcludePattern] = resolveUniqueModule("react-native-msal");
const additionalExclusions = [msalExcludePattern];
const blockList = exclusionList(additionalExclusions);
module.exports = makeMetroConfig({
resolver: {
extraNodeModules: {
"react-native-msal": msalPath,
},
blacklistRE: blockList, // For Metro < 0.60
blockList, // For Metro >= 0.60
},
});
Error: EMFILE: too many open files, watch
If you're getting an error like below, you need to install Watchman.
events.js:292
throw er; // Unhandled 'error' event
^
Error: EMFILE: too many open files, watch
at FSEvent.FSWatcher._handle.onchange (internal/fs/watchers.js:178:28)
Emitted 'error' event on NodeWatcher instance at:
at NodeWatcher.checkedEmitError (/~/node_modules/sane/src/node_watcher.js:143:12)
at FSWatcher.emit (events.js:315:20)
at FSEvent.FSWatcher._handle.onchange (internal/fs/watchers.js:184:12) {
errno: -24,
syscall: 'watch',
code: 'EMFILE',
filename: null
}
Error: jest-haste-map: Haste module naming collision
Metro will throw an exception if it finds duplicates:
Error: jest-haste-map: Haste module naming collision:
Duplicate module name: react-animated
Paths: /~/node_modules/example/node_modules/react-native/Libraries/Animated/release/package.json collides with /~/node_modules/react-native/Libraries/Animated/release/package.json
This error is caused by `hasteImpl` returning the same name for different files.
at setModule (/~/node_modules/jest-haste-map/build/index.js:569:17)
at workerReply (/~/node_modules/jest-haste-map/build/index.js:641:9)
at processTicksAndRejections (internal/process/task_queues.js:97:5)
at async Promise.all (index 77)
To resolve this, you'll need to exclude one of the paths in your
metro.config.js
, e.g.:
const { exclusionList, makeMetroConfig } = require("@rnx-kit/metro-config");
const blockList = exclusionList([
// Ignore nested copies of react-native
/node_modules\/.*\/node_modules\/react-native\/.*/,
]);
module.exports = makeMetroConfig({
resolver: {
blacklistRE: blockList, // For Metro < 0.60
blockList, // For Metro >= 0.60
},
});
[Flipper] React DevTools is disabled
Flipper only enables React Native plugins if it detects a Metro "device". It
detects one by opening localhost:8081
and checking for some keywords like
"React Native packager is running". However, if one of your packages have an
index.html
in the package root, Metro will serve that file instead. Flipper
will then think that it's not dealing with a React Native app and disable all
related plugins.
The fix is to move index.html
elsewhere, but if you cannot do that, you can
work around this issue by filtering out the offending packages in
metro.config.js
:
const { makeMetroConfig } = require("@rnx-kit/metro-config");
const fs = require("fs");
const path = require("path");
const config = makeMetroConfig();
module.exports = {
...config,
watchFolders: config.watchFolders.filter(
(p) => !fs.existsSync(path.join(p, "index.html"))
),
};