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A metro bundler configuration customizer to more easily detect and workaround symlinked dependencies which metro does not support.
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README.md

@carimus/metro-symlinked-deps

Utilities to customize the metro bundler configuration in order to workaround its lack of support for symlinks.

The primary use case for this package is to support development on react native dependencies using yarn link or npm link.

Motivation

Facebook's metro bundler used by React Native doesn't support symlinks which is a huge hindrance in the ability to share code locally.

It's related and dependent on this issue with jest since metro uses jest-haste-map internally to track and watch file changes.

The general process for developing on a dependency that is sharing components with the main app would be to use yarn link / npm link to symlink the dependency into the app's node_modules. Since Metro ignores symlinks though, it simply doesn't work out of the box with metro. There's mountains of workarounds to this that work to varying degrees. This is the one that worked for us that we're going to re-use until it's unnecessary.

Usage

Install as a dev dependency using npm or yarn:

yarn add --dev @carimus/metro-symlinked-deps

Option 1: Automatic

If you don't need greater control of the resolver.blacklistRE outside of adding additional paths or expressions to the list, you can safely use the single applyConfigForLinkedDependencies function which will use metro-config's mergeConfig to merge in the configuration updates required for the resolver.blacklistRE and watchFolders.

  1. Modify your metro.config.js (creating it if it doesn't exist, or converting your metro.config.json to metro.config.js if its present) to require and call applyConfigForLinkedDependencies on your existing configuration:

    const {
        applyConfigForLinkedDependencies,
    } = require('@carimus/metro-symlinked-deps');
    
    module.exports = applyConfigForLinkedDependencies(
        {
            /* Your existing configuration, optional */
        },
        {
            /* Options to pass to applyConfigForLinkedDependencies, optional */
        },
    );

applyConfigForLinkedDependencies takes the following options:

  • projectRoot (string, optional but recommended): The root of the metro bundled project. If not provided, it will be detected assuming the current process.cwd() is the project root. It's recommended to explicitly provide this to avoid detection issues.
  • blacklistLinkedModules (string[], defaults to []): a list of modules to blacklist/ignore if they show up in any linked dependencies' node_modules. If you get naming collisions for certain modules, add those modules by name here and restart the bundler using --reset-cache. A common one is react-native which will typically show up as a dev dependency in react native packages since it's used in tests.
  • blacklistDirectories (string[], defaults to []): a list of absolute or relative (to projectRoot) directories that should be blacklisted in addition to the directories determined via blacklistLinkedModules.
  • resolveBlacklistDirectoriesSymlinks (boolean, defaults to true): whether or not to resolve symlinks when processing blacklistDirectories.
  • additionalWatchFolders (string[], defaults to []): a list of additional absolute paths to watch, merged directly into the watchFolders option.
  • resolveAdditionalWatchFoldersSymlinks (boolean, defaults to true): whether or not to resolve symlinks when processing additionalWatchFolders.
  • resolveNodeModulesAtRoot (boolean, defaults to false): Set this to true to set up a Proxy for resolver.extraNodeModules in order to ensure that all modules (even the ones required by linked dependencies or any other out-of-root watch folders) will resolve to the project root's node_modules directory. This is primarily useful if the linked dependencies rely on the presence of peerDependencies installed in the project root.
  • silent (boolean, defaults to false): Set this to true to suppress warning output in the bundler that shows up when linked dependencies are detected.
  • debug (boolean, defaults to false): Set this to true to log out valuable debug information like the final merged metro configuration.

Example

This setup should work for an out of the box react-native 0.60+ project:

const {
    applyConfigForLinkedDependencies,
} = require('@carimus/metro-symlinked-deps');

module.exports = applyConfigForLinkedDependencies(
    {
        transformer: {
            getTransformOptions: async () => ({
                transform: {
                    experimentalImportSupport: false,
                    inlineRequires: false,
                },
            }),
        },
    },
    {
        projectRoot: __dirname,
        blacklistLinkedModules: ['react-native'],
    },
);

Option 2: Manual

TODO

Caveats

  • At the time of writing the blacklist approach appears to fix the naming collision error however it requires that the developer knows which packages are in-common and that they provide that list to this package in order to generate the regular expression
  • The naming collision doesn't appear to occur for ALL in-common packages. It's not clear if it also considers versions too, though that would make sense.

How it works

This is a workaround and as such it was built by incrementally addressing errors that show up.

Error #1: Module not found

Out of the box, if you try to use a symlinked dependency, you get the following error from the bundler when it first builds the bundle (not on during the transform step):

error: bundling failed: Error: Unable to resolve module `your-symlinked-module` from `/path/in/project/that/requires/the/module.js`: Module `your-symlinked-module` does not exist in the Haste module map

This might be related to https://github.com/facebook/react-native/issues/4968
To resolve try the following:
  1. Clear watchman watches: `watchman watch-del-all`.
  2. Delete the `node_modules` folder: `rm -rf node_modules && npm install`.
  3. Reset Metro Bundler cache: `rm -rf /tmp/metro-bundler-cache-*` or `npm start -- --reset-cache`.
  4. Remove haste cache: `rm -rf /tmp/haste-map-react-native-packager-*`.

Not extremely helpful but what's happening here is metro is just outright ignoring the symlink and as such, your module is invisible to it.

The workaround here provided by aleclarson in this comment on the metro issue is to use his home-grown get-dev-paths package which searches node_modules for any symlinked dependencies that are referenced as dependencies in your package.json, resolve those links to their real dependencies, and then tell metro to also watch those real directories.

Error #2: Haste module naming collision

That works great with one important caveat: if your linked dependency has any installed dependencies in its node_modules that are identical to any installed dependencies in the root project, you get the following error (this error names react-native as the common dependency).

jest-haste-map: Haste module naming collision: react-native
  The following files share their name; please adjust your hasteImpl:
    * <rootDir>/node_modules/react-native/package.json
    * <rootDir>/../../your-symlinked-module/node_modules/react-native/package.json

When your dependency is installed legitimately (and not linked) any common dependencies are automatically deduped during the install (during yarn install or npm install) and metro (or rather jest-haste-map) seems to rely on this behaviour and can't identify the fact that the two packages are not conflicting with eachother and are legitimately identical. There's unfortunately no way to tell metro this is the case and that it should, as an example, prefer the version of the code in the root project's node_modules so instead we have to manually construct a blacklist of the in-common packages in the linked dependency, construct a regular expression from that, and hand that regular expression to the blacklistRE option of the metro bundler's resolver config.

TODO

  • Remove resolveNodeModulesAtRoot and replace with nodeModulesResolutionStrategy with three options:
    • null: default, don't apply any extraNodeModules config
    • 'peers': apply extraNodeModules that will automatically detect peer dependencies in linked deps and ensure those are resolved in the project root while allowing all other dependencies to resolve naturally.
    • 'root': apply extraNodeModules that will force all node modules to resolve in the project root, equivelant to resolveNodeModulesAtRoot being set to true currently.
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