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.

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.

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 */  },  watcher: {    /* watcher options */    watchman: {      /* Watchman-specific 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 absolute path of a module (or a package name resolvable from the metro package) exporting a transform function.

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.

fileMapCacheDirectory

Type: string

The path to the metro-file-map cache directory, defaults to os.tmpdir().

Deprecated: hasteMapCacheDirectory

Type: string

Alias of fileMapCacheDirectory

---

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. 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'].

disableHierarchicalLookup

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.

emptyModulePath

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.

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.

nodeModulesPaths

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.

resolveRequest

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);}

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.

requireCycleIgnorePatterns

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($|\/|\\)/].

---

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 filemodule.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 | 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.

getTransformOptions

Type: GetTransformOptions

Get the transform options.

hermesParser

Type: boolean (default: false)

Use the hermes-parser package to use call Hermes parser via WASM instead of the Babel parser.

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.

---

Watcher Options

Options for the filesystem watcher.

note

Dot notation in this section indicates a nested configuration object, e.g. watchman.deferStates → watchman: { deferStates: ... }.

watchman.deferStates

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'].

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 platforms or getModulesRunBeforeMainModule that may not be required in your environment.

Merging Example

// metro.config.jsconst { 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);