Configuring Metro

A Metro config can be created in these three ways (ordered by priority):

  1. metro.config.js
  2. metro.config.json
  3. The metro field in package.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:

module.exports = {
/* general options */
resolver: {
/* resolver options */
},
transformer: {
/* transformer options */
},
serializer: {
/* serializer options */
},
server: {
/* server options */
}
};

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:

resolveRequest: (context, realModuleName, platform, moduleName) => {
// Resolve file path logic...
return {
filePath: "path/to/file",
type: 'sourceFile',
};
}

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:

// in your babelTransformer file
module.exports = ({ filename, options, plugins, src }) => {
// transform file...
return { ast: AST };
}

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.

MethodDescription
mergeConfig(...configs): MergedConfigReturns 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.

Merging Example

// 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);