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Options
options

Options can be passed to Babel in a variety of ways. When passed directly to Babel, you can just pass the objects object. When Babel is used via a wrapper, it may also be necessary, or at least more useful, to pass the options via configuration files.

If passing options via @babel/cli you'll need to kebab-case the names. i.e.

npx babel --root-mode upward file.js # equivalent of passing the rootMode config option

Primary options

These options are only allowed as part of Babel's programmatic options, so they are primarily for use by tools that wrap around Babel, or people calling babel.transform directly. Users of Babel's integrations, like babel-loader or @babel/register are unlikely to use these.

cwd

Type: string
Default: process.cwd()

The working directory that all paths in the programmatic options will be resolved relative to.

caller

Type: Object with a string-typed "name" property.

Utilities may pass a caller object to identify themselves to Babel and pass capability-related flags for use by configs, presets and plugins. For example

babel.transformFileSync("example.js", {
  caller: {
    name: "my-custom-tool",
    supportsStaticESM: true
  },
})

would allow plugins and presets to decide that, since ES modules are supported, they will skip compilation of ES modules into CommonJS modules.

filename

Type: string

The filename associated with the code currently being compiled, if there is one. The filename is optional, but not all of Babel's functionality is available when the filename is unknown, because a subset of options rely on the filename for their functionality.

The three primary cases users could run into are:

  • The filename is exposed to plugins. Some plugins may require the presence of the filename.
  • Options like "test", "exclude", and "ignore" require the filename for string/RegExp matching.
  • .babelrc files are loaded relative to the file being compiled. If this option is omitted, Babel will behave as if babelrc: false has been set.

filenameRelative

Type: string
Default: path.relative(opts.cwd, opts.filename) (if "filename" was passed)

Used as the default value for Babel's sourceFileName option, and used as part of generation of filenames for the AMD / UMD / SystemJS module transforms.

code

Type: boolean
Default: true

Babel's default return value includes code and map properties with the resulting generated code. In some contexts where multiple calls to Babel are being made, it can be helpful to disable code generation and instead use ast: true to get the AST directly in order to avoid doing unnecessary work.

ast

Type: boolean
Default: false

Babel's default is to generate a string and a sourcemap, but in some contexts it can be useful to get the AST itself. The primary use case for this would be a chain of multiple transform passes, along the lines of

const filename = "example.js";
const source = fs.readFileSync(filename, "utf8");

// Load and compile file normally, but skip code generation.
const { ast } = babel.transformSync(source, { filename, ast: true, code: false });

// Minify the file in a second pass and generate the output code here.
const { code, map } = babel.transformFromAstSync(ast, source, {
  filename,
  presets: ["minify"],
  babelrc: false,
  configFile: false,
});

Note: This option is not on by default because the majority of users won't need it and because we'd like to eventually add a caching layer to Babel. Having to cache the AST structure will take significantly more space.

Config Loading options

Loading configuration can get a little complex as environments can have several types of configuration files, and those configuration files can have various nested configuration objects that apply depending on the configuration.

root

Type: string
Default: opts.cwd
Placement: Only allowed in Babel's programmatic options

The initial path that will be processed based on the "rootMode" to determine the conceptual root folder for the current Babel project. This is used in two primary cases:

rootMode

Type: "root" | "upward" | "upward-optional"
Default: "root"
Placement: Only allowed in Babel's programmatic options
Version: ^7.1.0

This option, combined with the "root" value, defines how Babel chooses its project root. The different modes define different ways that Babel can process the "root" value to get the final project root.

"root" is the default mode because it avoids the risk that Babel will accidentally load a babel.config.js that is entirely outside of the current project folder. If you use "upward-optional", be aware that it will walk up the directory structure all the way to the filesystem root, and it is always possible that someone will have a forgotten babel.config.js in their home directory, which could cause unexpected errors in your builds.

Users with monorepo project structures that run builds/tests on a per-package basis may well want to use "upward" since monorepos often have a babel.config.js in the project root. Running Babel in a monorepo subdirectory without "upward", will cause Babel to skip loading any babel.config.js files in the project root, which can lead to unexpected errors and compilation failure.

envName

Type: string
Default: process.env.BABEL_ENV || process.env.NODE_ENV || "development"
Placement: Only allowed in Babel's programmatic options

The current active environment used during configuration loading. This value is used as the key when resolving "env" configs, and is also available inside configuration functions, plugins, and presets, via the api.env() function.

configFile

Type: string | boolean
Default: path.resolve(opts.root, "babel.config.js"), if it exists, false otherwise
Placement: Only allowed in Babel's programmatic options

Defaults to searching for a default babel.config.js file, but can be passed the path of any JS or JSON5 config file.

NOTE: This option does not affect loading of .babelrc files, so while it may be tempting to do configFile: "./foo/.babelrc", it is not recommended. If the given .babelrc is loaded via the standard file-relative logic, you'll end up loading the same config file twice, merging it with itself. If you are linking a specific config file, it is recommended to stick with a naming scheme that is independent of the "babelrc" name.

babelrc

Type: boolean
Default: true as long as the filename option has been specificed
Placement: Allowed in Babel's programmatic options, or inside of the loaded "configFile". A programmatic option will override a config file one.

true will enable searching for configuration files relative to the "filename" provided to Babel.

A babelrc value passed in the programmatic options will override one set within a configuration file.

Note: .babelrc files are only loaded if the current "filename" is inside of a package that matches one of the "babelrcRoots" packages.

babelrcRoots

Type: boolean | MatchPattern | Array<MatchPattern>
Default: opts.root
Placement: Allowed in Babel's programmatic options, or inside of the loaded configFile. A programmatic option will override a config file one.

By default, Babel will only search for .babelrc files within the "root" package because otherwise Babel cannot know if a given .babelrc is meant to be loaded, or if it's "plugins" and "presets" have even been installed, since the file being compiled could be inside node_modules, or have been symlinked into the project.

This option allows users to provide a list of other packages that should be considered "root" packages when considering whether to load .babelrc files.

For example, a monorepo setup that wishes to allow individual packages to have their own configs might want to do

babelrcRoots: [
  // Keep the root as a root
  ".",

  // Also consider monorepo packages "root" and load their .babelrc files.
  "./packages/*"
]

Plugin and Preset options

plugins

Type: Array<PluginEntry | Plugin> (PluginEntry)
Default: []

An array of plugins to activate when processing this file. For more information on how individual entries interact, especially when used across multiple nested "env" and "overrides" configs, see merging.

Note: The option also allows Plugin instances from Babel itself, but using these directly is not recommended. If you need to create a persistent representation of a plugin or preset, you should use babel.createConfigItem().

presets

Type: Array<PresetEntry> (PresetEntry)
Default: []

An array of presets to activate when processing this file. For more information on how individual entries interact, especially when used across multiple nested "env" and "overrides" configs, see merging.

Note: The format of presets is identical to plugins, except for the fact that name normalization expects "preset-" instead of "plugin-", and presets cannot be instances of Plugin.

passPerPreset

Type: boolean
Default: false
Status: Deprecated

Instructs Babel to run each of the presets in the presets array as an independent pass. This option tends to introduce a lot of confusion around the exact ordering of plugins, but can be useful if you absolutely need to run a set of operations as independent compilation passes.

Note: This option may be removed in future Babel versions as we add better support for defining ordering between plugins.

Config Merging options

extends

Type: string
Placement: Not allowed inside of presets

Configs may "extend" other configuration files. Config fields in the current config will be merged on top of the extended file's configuration.

env

Type: { [envKey: string]: Options }
Placement: May not be nested inside of another env block.

Allows for entire nested configuration options that will only be enabled if the envKey matches the envName option.

Note: env[envKey] options will be merged on top of the options specified in the root object.

overrides

Type: Array<Options>
Placement: May not be nested inside of another overrides object, or within an env block.

Allows users to provide an array of options that will be merged into the current configuration one at a time. This feature is best used alongside the "test"/"include"/"exclude" options to provide conditions for which an override should apply. For example:

overrides: [{
  test: "./vendor/large.min.js",
  compact: true,
}],

could be used to enable the compact option for one specific file that is known to be large and minified, and tell Babel not to bother trying to print the file nicely.

test

Type: MatchPattern | Array<MatchPattern> (MatchPattern)

If all patterns fail to match, the current configuration object is considered inactive and is ignored during config processing. This option is most useful when used within an overrides option object, but it's allowed anywhere.

Note: These toggles do not affect the programmatic and config-loading options in earlier sections, since they are taken into account long before the configuration that is prepared for merging.

include

Type: MatchPattern | Array<MatchPattern> (MatchPattern)

This option is a synonym for "test".

exclude

Type: MatchPattern | Array<MatchPattern> (MatchPattern)

If any of patterns match, the current configuration object is considered inactive and is ignored during config processing. This option is most useful when used within an overrides option object, but it's allowed anywhere.

Note: These toggles do not affect the programmatic and config-loading options in earlier sections, since they are taken into account long before the configuration that is prepared for merging.

ignore

Type: Array<MatchPattern> (MatchPattern)
Placement: Not allowed inside of presets

If any of the patterns match, Babel will immediately stop all processing of the current build. For example, a user may want to do something like

ignore: [
  "./lib",
]

to explicitly disable Babel compilation of files inside the lib directory.

Note: This option disables all Babel processing of a file. While that has its uses, it is also worth considering the "exclude" option as a less aggressive alternative.

only

Type: Array<MatchPattern> (MatchPattern)
Placement: Not allowed inside of presets

If all of the patterns fail to match, Babel will immediately stop all processing of the current build. For example, a user may want to do something like

only: [
  "./src",
]

to explicitly enable Babel compilation of files inside the src directory while disabling everything else.

Note: This option disables all Babel processing of a file. While that has its uses, it is also worth considering the "test"/"include" options as a less aggressive alternative.

Source Map options

inputSourceMap

Type: boolean | SourceMap
Default: true

true will attempt to load an input sourcemap from the file itself, if it contains a //# sourceMappingURL=... comment. If no map is found, or the map fails to load and parse, it will be silently discarded.

If an object is provided, it will be treated as the source map object itself.

sourceMaps

Type: boolean | "inline" | "both"
Default: false

  • true to generate a sourcemap for the code and include it in the result object.
  • "inline" to generate a sourcemap and append it as a data URL to the end of the code, but not include it in the result object.
  • "both" is the same as inline, but will include the map in the result object.

@babel/cli overloads some of these to also affect how maps are written to disk:

  • true will write the map to a .map file on disk
  • "inline" will write the file directly, so it will have a data: containing the map
  • "both" will write the file with a data: URL and also a .map.

Note: These options are bit weird, so it may make the most sense to just use true and handle the rest in your own code, depending on your use case.

sourceMap

This is an synonym for sourceMaps. Using sourceMaps is recommended.

sourceFileName

Type: string
Default: path.basename(opts.filenameRelative) when available, or "unknown"

The name to use for the file inside the source map object.

sourceRoot

Type: string

The sourceRoot fields to set in the generated source map, if one is desired.

Misc options

sourceType

Type: "script" | "module" | "unambiguous"
Default: "module"

  • "script" - Parse the file using the ECMAScript Script grammar. No import/export statements allowed, and files are not in strict mode.
  • "module" - Parse the file using the ECMAScript Module grammar. Files are automatically strict, and import/export statements are allowed.
  • "unambiguous" - Consider the file a "module" if import/export statements are present, or else consider it a "script".

unambiguous can be quite useful in contexts where the type is unknown, but it can lead to false matches because it's perfectly valid to have a module file that does not use import/export statements.

This option is important because the type of the current file affects both parsing of input files, and certain transforms that may wish to add import/require usage to the current file.

For instance, @babel/plugin-transform-runtime relies on the type of the current document to decide whether to insert an import declaration, or a require() call. @babel/preset-env also does the same for its "useBuiltIns" option. Since Babel defaults to treating files are ES modules, generally these plugins/presets will insert import statements. Setting the correct sourceType can be important because having the wrong type can lead to cases where Babel would insert import statements into files that are meant to be CommonJS files. This can be particularly important in projects where compilation of node_modules dependencies is being performed, because inserting an import statements can cause Webpack and other tooling to see a file as an ES module, breaking what would otherwise be a functional CommonJS file.

Note: This option will not affect parsing of .mjs files, as they are currently hard-coded to always parse as "module" files.

highlightCode

Type: boolean
Default: true

Highlight tokens in code snippets in Babel's error messages to make them easier to read.

wrapPluginVisitorMethod

Type: (key: string, nodeType: string, fn: Function) => Function

Allows users to add a wrapper on each visitor in order to inspect the visitor process as Babel executes the plugins.

  • key is a simple opaque string that represents the plugin being executed.
  • nodeType is the type of AST node currently being visited.
  • fn is the visitor function itself.

Users can return a replacement function that should call the original function after performing whatever logging and analysis they wish to do.

parserOpts

Type: {}

An opaque object containing options to pass through to the parser being used.

generatorOpts

Type: {}

An opaque object containing options to pass through to the code generator being used.

Code Generator options

retainLines

Type: boolean
Default: false

Babel will make an effort to generate code such that items are printed on the same line that they were on in the original file. This option exists so that users who cannot use source maps can get vaguely useful error line numbers, but it is only a best-effort, and is not guaranteed in all cases with all plugins.

compact

Type: boolean | "auto"
Default: "auto"

"auto" will set the value by evaluating code.length > 500_000

All optional newlines and whitespace will be omitted when generating code in compact mode.

minified

Type: boolean
Default: false

Includes compact: true, omits block-end semicolons, omits () from new Foo() when possible, and may output shorter versions of literals.

auxiliaryCommentBefore

Type: string

Allows specifying a prefix comment to insert before pieces of code that were not present in the original file.

Note: The definition of what is and isn't present in the original file can get a little ugly, so usage of this option is not recommended. If you need to annotate code somehow, it is better to do so using a Babel plugin.

auxiliaryCommentAfter

Type: string

Allows specifying a prefix comment to insert after pieces of code that were not present in the original file.

Note: The definition of what is and isn't present in the original file can get a little ugly, so usage of this option is not recommended. If you need to annotate code somehow, it is better to do so using a Babel plugin.

comments

Type: boolean
Default: true

Provides a default comment state for shouldPrintComment if no function is given. See the default value of that option for more info.

shouldPrintComment

Type: (value: string) => boolean
Default without minified: (val) => opts.comments || /@license|@preserve/.test(val)
Default with minified: () => opts.comments

A function that can decide whether a given comment should be included in the output code from Babel.

AMD / UMD / SystemJS module options

moduleIds

Type: boolean
Default: !!opts.moduleId

Enables module ID generation.

moduleId

Type: string

A hard-coded ID to use for the module. Cannot be used alongside getModuleId.

getModuleId

Type: (name: string) => string

Given the babel-generated module name, return the name to use. Returning a falsy value will use the original name.

moduleRoot

Type: string

A root path to include on generated module names.

Options Concepts

MatchPattern

Type: string | RegExp | (filename: string | void, context: { callee: { name: string } | void, envName: string ) => boolean

Several Babel options perform tests against file paths. In general, these options support a common pattern approach where each pattern can be

  • string - A file path with simple support for * and ** as full slug matches. Any file or parent folder matching the pattern counts as a match. The path follow's Node's normal path logic, so on POSIX is must be /-separated, but on Windows both / and \ are supported.
  • RegExp - A regular expression to match against the normalized filename. On POSIX the path RegExp will run against a /-separated path, and on Windows it will be on a \-separated path.

Importantly, if either of these are used, Babel requires that the filename option be present, and will consider it an error otherwise.

  • (filename: string | void, context: { callee: { name: string } | void, envName: string }) => boolean is a general callback that should return a boolean to indicate whether it is a match or not. The function is passed the filename or undefined if one was not given to Babel. It is also passed the current envName and callee options that were specified by the top-level call to Babel.

Merging

Babel's configuration merging is relatively straightforward. Options will overwrite existing options when they are present, and their value is not undefined, with a few special cases:

  • parserOpts objects are merged, rather than replaced, using the same logic as top-level options.
  • generatorOpts objects are merged, rather than replaced, using the same logic as top-level options.
  • plugins and presets are replaced based on the identity of the plugin/preset object/function itself combined with the name of the entry.

Plugin/Preset merging

As an example, consider a config with:

plugins: [
  './other',
  ['./plug', { thing: true, field1: true }]
],
overrides: [{
  plugins: [
    ['./plug', { thing: false, field2: true }],
  ]
}]

The overrides item will be merged on top of the top-level plugins. Importantly, the plugins array as a whole doesn't just replace the top-level one. The merging logic will see that "./plug" is the same plugin in both cases, and { thing: false, field2: true } will replace the original options, resulting in a config as

plugins: [
  './other',
  ['./plug', { thing: false, field2: true }],
],

Since merging is based on identity + name, it is considered an error to use the same plugin with the same name twice in the same plugins/presets array. For example

plugins: [
  './plug',
  './plug',
]

is considered an error, because it's identical to plugins: ['./plug']. Additionally, even

plugins: [
  ['./plug', {one: true}],
  ['./plug', {two: true}]
]

is considered an error, because the second one would just always replace the first one.

If you actually do want to instantiate two separate instances of a plugin, you must assign each one a name to disambiguate them. For example:

plugins: [
  ['./plug', {one: true}, "first-instance-name"],
  ['./plug', {two: true}, "second-instance-name"]
]

because each instance has been given a unique name and this a unique identity.

Plugin/Preset entries

PluginEntry / PresetEntry

Individual plugin/preset items can have several different structures:

  • EntryTarget - Individual plugin
  • [EntryTarget, EntryOptions] - Individual plugin w/ options
  • [EntryTarget, EntryOptions, string] - Individual plugin with options and name (see merging for more info on names)
  • ConfigItem - A plugin configuration item created by babel.createConfigItem().

The same EntryTarget may be used multiple times unless each one is given a different name, and doing so will result in a duplicate-plugin/preset error.

That can be a little hard to read, so as an example:

plugins: [
  // EntryTarget
  '@babel/plugin-transform-classes',

  // [EntryTarget, EntryOptions]
  ['@babel/plugin-transform-arrow-functions', { spec: true }],

  // [EntryTarget, EntryOptions, string]
  ['@babel/plugin-transform-for-of', { loose: true }, "some-name"],

  // ConfigItem
  babel.createConfigItem(require("@babel/plugin-transform-spread")),
],

EntryTarget

Type: string | {} | Function

A plugin/preset target can come from a few different sources:

  • string - A require-style path or plugin/preset identifier. Identifiers will be passed through name normalization.
  • {} | Function - An actual plugin/preset object or function after it has been require()ed.

EntryOptions

Type: undefined | {} | false

Options are passed through to each plugin/preset when they are executed. undefined will be normalized to an empty object.

false indicates that an entry is entirely disabled. This can be useful in contexts where ordering is important, but a separate condition is needed to decide if something is enabled. For instance:

plugins: [
  'one',
  ['two', false],
  'three',
],
overrides: [{
  test: "./src",
  plugins: [
    'two',
  ]
}]

would enable the two plugin for files in src, but two would still execute between one and three.

Name Normalization

By default, Babel expects plugins to have a babel-plugin- or babel-preset- prefix in their name. To avoid repetition, Babel has a name normalization phase will automatically add these prefixes when loading items. This boils down to a few primary rules:

  • Absolute paths pass through untouched.
  • Relative paths starting with ./ pass through untouched.
  • References to files within a package are untouched.
  • Any identifier prefixed with module: will have the prefix removed but otherwise be untouched.
  • plugin-/preset- will be injected at the start of any @babel-scoped package that doesn't have it as a prefix.
  • babel-plugin-/babel-preset- will be injected as a prefix any unscoped package that doesn't have it as a prefix.
  • babel-plugin-/babel-preset- will be injected as a prefix any @-scoped package that doesn't have it anywhere in their name.
  • babel-plugin/babel-preset will be injected as the package name if only the @-scope name is given.

Here are some examples, when applied in a plugin context:

Input Normalized
"/dir/plugin.js" "/dir/plugin.js"
"./dir/plugin.js" "./dir/plugin.js"
"mod" "babel-plugin-mod"
"mod/plugin" "mod/plugin"
"babel-plugin-mod" "babel-plugin-mod"
"@babel/mod" "@babel/plugin-mod"
"@babel/plugin-mod" "@babel/plugin-mod"
"@babel/mod/plugin" "@babel/mod/plugin"
"@scope" "@scope/babel-plugin"
"@scope/babel-plugin" "@scope/babel-plugin"
"@scope/mod" "@scope/babel-plugin-mod"
"@scope/babel-plugin-mod" "@scope/babel-plugin-mod"
"@scope/prefix-babel-plugin-mod" "@scope/prefix-babel-plugin-mod"
"@scope/mod/plugin" "@scope/mod/plugin"
"module:foo" "foo"