A Metro config can be created in these three ways (ordered by priority):
metro.config.js
metro.config.json
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:
Can be used to generate a key that will invalidate the whole metro cache.
projectRoot
string
The root folder of your project.
watchFolders
Array<string>
Specify any additional (to projectRoot) watch folders, this is used to know which files to watch.
transformerPath
string
The path to the transformer to use.
watch
boolean
Whether we should watch for all files.
reporter
{update: () => void}
Used to report the status of the bundler during the bundling process.
resetCache
boolean
Whether we should reset the cache when starting the build.
stickyWorkers
boolean
Control whether the created workers should stick based on filename or not.
maxWorkers
number
The number of workers we should parallelize the transformer on.
Server Options
These options are used when Metro serves the content.
Option
Type
Description
port
number
Which port to listen on.
useGlobalHotkey
boolean
Whether we should enable CMD+R hotkey for refreshing the bundle.
enhanceMiddleware
(Middleware, Server) => Middleware
The possibility to add custom middleware to the server response chain.
enableVisualizer
boolean
Enable the metro-visualizer middleware (available at /visualizer). This requires the metro-visualizer package to be installed in your project.
Transformer Options
Option
Type
Description
asyncRequireModulePath
string
What module to use for handling async requires.
babelTransformerPath
string
Use a custom babel transformer (only relevant when using the default transformerPath).
dynamicDepsInPackages
string (throwAtRuntime or reject)
What should happen when a dynamic dependency is found.
enableBabelRCLookup
boolean (default: true)
Whether we should use the .babelrc config file.
enableBabelRuntime
boolean (default: true)
Whether the transformer should use the @babel/transform/runtime plugin.
getTransformOptions
GetTransformOptions
Get the transform options.
postMinifyProcess
PostMinifyProcess
What happens after minification.
minifierPath
string
Path to the minifier that minifies the code after transformation.
minifierConfig
{[key: string]: mixed}
Configuration object that will be passed to the minifier (it should be serializable).
optimizationSizeLimit
number
Define a threshold (in bytes) to disable some expensive optimizations for big files.
React Native Only
Option
Type
Description
assetPlugins
Array<string>
List of modules to call to modify Asset data
assetRegistryPath
string
Where to fetch the assets from.
Resolver Options
Option
Type
Description
assetExts
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
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
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
{[name:string]:string}
Which other node_modules to include besides the ones relative to the project directory. This is keyed by dependency name.
resolveRequest
?CustomResolver
An optional function used to resolve requests. Ignored when the request can be resolved through Haste.
useWatchman
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.
Option
Type
Description
blacklistRE
RegExp
A RegEx defining which paths to ignore.
hasteImplModulePath
string
The path to the haste resolver.
platforms
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.
providesModuleNodeModules
Array<string>
Specify any additional node modules that should be processed for providesModule declarations.
Serializer Options
Option
Type
Description
getRunModuleStatement
(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
() => (path: string) => number
Used to generate the module id for require statements.
getPolyfills
({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
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
(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
(module: Array<Module>) => boolean
A filter function to discard specific modules from the output.
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, providesModuleNodeModules or getModulesRunBeforeMainModule that may not be required in your environment.