id | title |
---|---|
configuration |
Configuring Metro |
A Metro config can be created in these three ways (ordered by priority):
metro.config.js
metro.config.json
- The
metro
field inpackage.json
You can also give a custom file to the configuration by specifying --config <path/to/config>
when calling the CLI.
:::note
When Metro is started via the React Native CLI, some defaults are different from those mentioned below. See the React Native repository for details.
:::
The configuration is based on our concepts, which means that for every module we have a separate config option. A common configuration structure in Metro looks like this:
module.exports = {
/* general options */
resolver: {
/* resolver options */
},
transformer: {
/* transformer options */
},
serializer: {
/* serializer options */
},
server: {
/* server options */
},
watcher: {
/* watcher options */
watchman: {
/* Watchman-specific options */
}
}
};
Type: Array<CacheStore<TransformResult<>>>
List where we store our caches.
Type: string
Can be used to generate a key that will invalidate the whole metro cache.
Type: string
The root folder of your project.
Type: Array<string>
Specify any additional (to projectRoot) watch folders, this is used to know which files to watch.
(By default the file watching is disabled in CI environments. Also it can be manually disabled by setting the env variable CI=true
)
Type: string
The absolute path of a module (or a package name resolvable from the metro
package) exporting a transform
function.
Type: {update: () => void}
Used to report the status of the bundler during the bundling process.
Type: boolean
Whether we should reset the cache when starting the build.
Type: boolean
Control whether the created workers should stick based on filename or not.
Type: number
The number of workers we should parallelize the transformer on.
Type: string
The path to the metro-file-map
cache directory, defaults to os.tmpdir()
.
Type: string
Alias of fileMapCacheDirectory
Type: Array<string>
An array of asset extensions to include in the bundle. For example, if you would give ['ttf']
you would be able to include .ttf
files in the bundle.
Type: Array<string>
The list of source file extensions to include in the bundle. For example, including 'ts'
allows Metro to include .ts
files in the bundle.
The order of these extensions defines the order to match files on disk. For more info, see Module Resolution.
Defaults to ['js', 'jsx', 'json', 'ts', 'tsx']
.
Type: Array<string>
Specify the fields in package.json files that will be used by the module resolver to do redirections when requiring certain packages. The default is ['browser', 'main']
, so the resolver will use the browser
field if it exists and main
otherwise.
:::note
When Metro is started via the React Native CLI this will default to ['react-native', 'browser', 'main']
.
:::
Type: boolean
Whether to disable looking up modules in node_modules
folders. This only affects the default search through the directory tree, not other Metro options like extraNodeModules
or nodeModulesPaths
. Defaults to false
.
Type: string
What module to use as the canonical "empty" module when one is needed. Defaults to using the one included in metro-runtime
. You only need to change this if Metro is installed outside of your project.
Type: {[name:string]:string}
Which other node_modules
to include besides the ones relative to the project directory. This is keyed by dependency name.
Type: Array<string>
This option can be used to add additional node_modules
folders for Metro to locate third-party dependencies when resolving modules. This is useful if third-party dependencies are installed in a different location outside of the direct path of source files.
This option works similarly to how $NODE_PATH works for Node.js based tooling, except that nodeModulesPaths
takes precedence over hierarchical node_modules
lookup.
Type: ?CustomResolver
An optional function used to resolve requests. Particularly useful for cases where aliases or custom protocols are used. For example:
resolveRequest: (context, moduleName, platform) => {
if (moduleName.startsWith('my-custom-resolver:')) {
// Resolve file path logic...
// NOTE: Throw an error if there is no resolution.
return {
filePath: "path/to/file",
type: 'sourceFile',
};
}
// Optionally, chain to the standard Metro resolver.
return context.resolveRequest(context, moduleName, platform);
}
Type: boolean
If set to false
, it'll prevent Metro from using watchman (even if it's installed)
These options are only useful with React Native projects.
Type: RegExp
or Array<RegExp>
A RegEx defining which paths to ignore, however if a blocklisted file is required it will be brought into the dependency graph.
Type: string
The path to the haste resolver.
Type: Array<string>
Additional platforms to look out for, For example, if you want to add a "custom" platform, and use modules ending in .custom.js, you would return ['custom'] here.
Type: Array<RegExp>
In development mode, suppress require cycle warnings for any cycle involving a module that matches any of these expressions. This is useful for third-party code and first-party expected cycles.
Note that if you specify your own value for this config option it will replace (not concatenate with) Metro's default.
Defaults to [/(^|\/|\\)node_modules($|\/|\\)/]
.
Type: string
What module to use for handling async requires.
Type: string
Use a custom babel transformer (only relevant when using the default transformerPath). For example:
// in your babelTransformer file
module.exports = ({ filename, options, plugins, src }) => {
// transform file...
return { ast: AST };
}
Type: string
(throwAtRuntime
or reject
)
What should happen when a dynamic dependency is found.
Type: boolean
(default: true
)
Whether we should use the .babelrc
config file.
Type: boolean | string
(default: true
)
Whether the transformer should use the @babel/transform/runtime
plugin.
If the value is a string, it is treated as a runtime version number and passed as version
to the @babel/plugin-transform-runtime
configuration. This allows you to optimize the generated babel runtime based on the
runtime in the app's node modules confugration.
Type: GetTransformOptions
Get the transform options.
Type: boolean
(default: false
)
Use the hermes-parser package to use call Hermes parser via WASM instead of the Babel parser.
Type: string
(default: 'metro-minify-terser'
)
Path, or package name resolvable from metro-transform-worker
, to the minifier that minifies the code after transformation.
Type: {[key: string]: mixed}
Configuration object that will be passed to the minifier (it should be serializable).
Type: number
Define a threshold (in bytes) to disable some expensive optimizations for big files.
Type: Array<string>
List of modules to call to modify Asset data
Type: string
Where to fetch the assets from.
Type: (number
| string) => string
Specify the format of the initial require statements that are appended at the end of the bundle. By default is __r(${moduleId});
.
Type: () => (path: string) => number
Used to generate the module id for require
statements.
Type: ({platform: ?string}) => $ReadOnlyArray<string>
An optional list of polyfills to include in the bundle. The list defaults to a set of common polyfills for Number, String, Array, Object...
Type: PostProcessBundleSourcemap
An optional function that can modify the code and source map of the bundle before it is written. Applied once for the entire bundle.
Type: (entryFilePath: string) => Array<string>
An array of modules to be required before the entry point. It should contain the absolute path of each module. Note that this will add the additional require statements only if the passed modules are already included as part of the bundle.
Type: (module: Array<Module>) => boolean
A filter function to discard specific modules from the output.
These options are used when Metro serves the content.
Type: number
Which port to listen on.
Type: boolean
Whether we should enable CMD+R hotkey for refreshing the bundle.
Type: (Middleware, Server) => Middleware
The possibility to add custom middleware to the server response chain.
Type: string => string
A function that will be called every time Metro processes a URL. Metro will use the return value of this function as if it were the original URL provided by the client. This applies to all incoming HTTP requests (after any custom middleware), as well as bundle URLs in /symbolicate
request payloads and within the hot reloading protocol.
Type: boolean
(default: true
)
Run Inspector Proxy server inside Metro to be able to inspect React Native code.
Options for the filesystem watcher.
:::note
Dot notation in this section indicates a nested configuration object, e.g. watchman.deferStates
β watchman: { deferStates: ... }
.
:::
Type: Array<string>
The extensions which Metro should watch in addition to sourceExts
, but which will not be automatically tried by the resolver.
Therefore, the two behaviour differences from resolver.sourceExts
when importing a module are:
- Modules can only be required when fully specified (e.g.
import moduleA from 'moduleA.mjs'
). - No platform-specific resolution is performed.
Defaults to ['cjs', 'mjs']
.
Type: Array<string>
Applies when using Watchman. Metro will defer processing filesystem updates while these states are asserted in the watch. This is useful for debouncing builds while the filesystem hasn't settled, e.g. during large source control operations.
The default value is ['hg.update']
.
Using the metro-config
package it is possible to merge multiple configurations together.
Method | Description |
---|---|
mergeConfig(...configs): MergedConfig |
Returns the merged configuration of two or more configuration objects. |
:::note
Arrays and function based config parameters do not deeply merge and will instead override any pre-existing config parameters.
This allows overriding and removing default config parameters such as platforms
or getModulesRunBeforeMainModule
that may not be required in your environment.
:::
// metro.config.js
const { mergeConfig } = require("metro-config");
const configA = {
/* general options */
resolver: {
/* resolver options */
},
transformer: {
/* transformer options */
},
serializer: {
/* serializer options */
},
server: {
/* server options */
}
};
const configB = {
/* general options */
resolver: {
/* resolver options */
},
transformer: {
/* transformer options */
},
serializer: {
/* serializer options */
},
server: {
/* server options */
}
};
module.exports = mergeConfig(configA, configB);