Configuring Metro
A Metro config can be created in these three ways (ordered by priority):
metro.config.jsmetro.config.json- The
metrofield inpackage.json
You can also give a custom file to the configuration by specifying --config <path/to/config> when calling the CLI.
Configuration Structure
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:
General Options
cacheStores
Type: Array<CacheStore<TransformResult<>>
List where we store our caches.
cacheVersion
Type: string
Can be used to generate a key that will invalidate the whole metro cache.
projectRoot
Type: string
The root folder of your project.
watchFolders
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)
transformerPath
Type: string
The path to the transformer to use.
reporter
Type: {update: () => void}
Used to report the status of the bundler during the bundling process.
resetCache
Type: boolean
Whether we should reset the cache when starting the build.
stickyWorkers
Type: boolean
Control whether the created workers should stick based on filename or not.
maxWorkers
Type: number
The number of workers we should parallelize the transformer on.
Resolver Options
assetExts
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.
sourceExts
Type: Array<string>
An array of source extensions to include in the bundle. For example, if you would give ['ts'] you would be able to include .ts files in the bundle.
resolverMainFields
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. For example, using ['browser', 'main'] will use the browser field if it exists and will default to main if it doesn't.
extraNodeModules
Type: {[name:string]:string}
Which other node_modules to include besides the ones relative to the project directory. This is keyed by dependency name.
resolveRequest
Type: ?CustomResolver
An optional function used to resolve requests. Ignored when the request can be resolved through Haste. Particularily useful for cases where aliases are used. For example:
useWatchman
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.
blockList
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.
hasteImplModulePath
Type: string
The path to the haste resolver.
platforms
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.
Transformer Options
asyncRequireModulePath
Type: string
What module to use for handling async requires.
babelTransformerPath
Type: string
Use a custom babel transformer (only relevant when using the default transformerPath). For example:
dynamicDepsInPackages
Type: string (throwAtRuntime or reject)
What should happen when a dynamic dependency is found.
enableBabelRCLookup
Type: boolean (default: true)
Whether we should use the .babelrc config file.
enableBabelRuntime
Type: boolean (default: true)
Whether the transformer should use the @babel/transform/runtime plugin.
getTransformOptions
Type: GetTransformOptions
Get the transform options.
postMinifyProcess
Type: PostMinifyProcess
What happens after minification.
minifierPath
Type: string
Path to the minifier that minifies the code after transformation.
minifierConfig
Type: {[key: string]: mixed}
Configuration object that will be passed to the minifier (it should be serializable).
optimizationSizeLimit
Type: number
Define a threshold (in bytes) to disable some expensive optimizations for big files.
React Native Only
assetPlugins
Type: Array<string>
List of modules to call to modify Asset data
assetRegistryPath
Type: string
Where to fetch the assets from.
Serializer Options
getRunModuleStatement
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});.
createModuleIdFactory
Type: () => (path: string) => number
Used to generate the module id for require statements.
getPolyfills
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...
postProcessBundleSourcemap
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.
getModulesRunBeforeMainModule
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.
processModuleFilter
Type: (module: Array<Module>) => boolean
A filter function to discard specific modules from the output.
Server Options
These options are used when Metro serves the content.
port
Type: number
Which port to listen on.
useGlobalHotkey
Type: boolean
Whether we should enable CMD+R hotkey for refreshing the bundle.
enhanceMiddleware
Type: (Middleware, Server) => Middleware
The possibility to add custom middleware to the server response chain.
rewriteRequestUrl
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.
runInspectorProxy
Type: boolean (default: true)
Run Inspector Proxy server inside Metro to be able to inspect React Native code.
Merging Configurations
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
platformsorgetModulesRunBeforeMainModulethat may not be required in your environment.