.flowconfig [options]

How to configure Flow's various options

The [options] section in a .flowconfig file can contain several key-value pairs of the form:

[options]
keyA=valueA
keyB=valueB

Any options that are omitted will use their default values. Some options can be overridden with command line flags.

Available options

all (boolean)

Set this to true to check all files, not just those with @flow.

The default value for all is false.

babel_loose_array_spread (boolean)

Set this to true to check that array spread syntax is only used with arrays, not arbitrary iterables (such as Map or Set). This is useful if you transform your code with Babel in loose mode which makes this non-spec-compliant assumption at runtime.

For example:

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const set = new Set();
const values = [...set]; // Valid ES2015, but Set is not compatible with $ReadOnlyArray in Babel loose mode

The default value for babel_loose_array_spread is false.

emoji (boolean)

Set this to true to add emoji to the status messages that Flow outputs when it’s busy checking your project.

The default value for emoji is false.

esproposal.class_instance_fields (enable|ignore|warn)

Set this to warn to indicate that Flow should give a warning on use of instance class fields per the pending spec.

You may also set this to ignore to indicate that Flow should simply ignore the syntax (i.e. Flow will not use this syntax to indicate the presence of a property on instances of the class).

The default value of this option is enable, which allows use of this proposed syntax.

esproposal.class_static_fields (enable|ignore|warn)

Set this to warn to indicate that Flow should give a warning on use of static class fields per the pending spec.

You may also set this to ignore to indicate that Flow should simply ignore the syntax (i.e. Flow will not use this syntax to indicate the presence of a static property on the class).

The default value of this option is enable, which allows use of this proposed syntax.

esproposal.decorators (ignore|warn)

Set this to ignore to indicate that Flow should ignore decorators.

The default value of this option is warn, which gives a warning on use since this proposal is still very early-stage.

esproposal.export_star_as (enable|ignore|warn)

Set this to enable to indicate that Flow should support the export * as syntax from leebyron’s proposal.

You may also set this to ignore to indicate that Flow should simply ignore the syntax. The default value of this option is warn, which gives a warning on use since this proposal is still very early-stage.

esproposal.optional_chaining (enable|ignore|warn)

Set this to enable to indicate that Flow should support the use of optional chaining per the pending spec.

You may also set this to ignore to indicate that Flow should simply ignore the syntax.

The default value of this option is warn, which gives a warning on use since this proposal is still very early-stage.

esproposal.nullish_coalescing (enable|ignore|warn)

Set this to enable to indicate that Flow should support the use of nullish coalescing per the pending spec.

You may also set this to ignore to indicate that Flow should simply ignore the syntax.

The default value of this option is warn, which gives a warning on use since this proposal is still very early-stage.

exact_by_default (boolean)

Set this to true to indicate that Flow should interpret object types as exact by default. When this flag is false, Flow has the following behavior:

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{foo: number} // inexact
{| foo: number |} // exact
{foo: number, ...} // inexact

When this flag is true, Flow has the following behavior:

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{foo: number} // exact
{| foo: number |} // exact
{foo: number, ...} // inexact

The default value is false.

experimental.const_params (boolean)

Setting this to true makes Flow treat all function parameters as const bindings. Reassigning a param is an error which lets Flow be less conservative with refinements.

The default value is false.

include_warnings (boolean)

Setting this to true makes Flow commands include warnings in the error output. Warnings are hidden by default in the CLI to avoid console spew. (An IDE is a much better interface to show warnings.)

The default value is false.

lazy_mode (fs|ide|watchman|none)

For more on lazy modes, see the lazy modes docs.

Setting lazy_mode in the .flowconfig will cause new Flow servers for that root to use that lazy mode (or no lazy mode if set to none). This option can be overridden from the CLI using the --lazy-mode flag.

The default value is none.

log.file (string)

The path to the log file (defaults to /tmp/flow/<escaped root path>.log).

max_header_tokens (integer)

Flow tries to avoid parsing non-flow files. This means Flow needs to start lexing a file to see if it has @flow or @noflow in it. This option lets you configure how much of the file Flow lexes before it decides there is no relevant docblock.

  • Neither @flow nor @noflow - Parse this file with Flow syntax disallowed and do not typecheck it.
  • @flow - Parse this file with Flow syntax allowed and typecheck it.
  • @noflow - Parse this file with Flow syntax allowed and do not typecheck it. This is meant as an escape hatch to suppress Flow in a file without having to delete all the Flow-specific syntax.

The default value of max_header_tokens is 10.

module.file_ext (string)

By default, Flow will look for files with the extensions .js, .jsx, .mjs, .cjs and .json. You can override this behavior with this option.

For example, if you do:

[options]
module.file_ext=.foo
module.file_ext=.bar

Then Flow will instead look for the file extensions .foo and .bar.

Note: you can specify module.file_ext multiple times

module.ignore_non_literal_requires (boolean)

Set this to true and Flow will no longer complain when you use require() with something other than a string literal.

The default value is false.

module.name_mapper (regex -> string)

Specify a regular expression to match against module names, and a replacement pattern, separated by a ->.

For example:

module.name_mapper='^image![a-zA-Z0-9$_]+$' -> 'ImageStub'

This makes Flow treat require('image!foo.jpg') as if it were require('ImageStub').

These are OCaml regular expressions. Use \( and \) (slashes required!) to create a capturing group, which you can refer to in the replacement pattern as \1 (up to \9).

Note: you can specify module.name_mapper multiple times

module.name_mapper.extension (string -> string)

Specify a file extension to match, and a replacement module name, separated by a ->.

Note: This is just shorthand for module.name_mapper='^\(.*\)\.EXTENSION$' -> 'TEMPLATE')

For example:

module.name_mapper.extension='css' -> '<PROJECT_ROOT>/CSSFlowStub.js.flow'

Makes Flow treat require('foo.css') as if it were require(PROJECT_ROOT + '/CSSFlowStub').

Note: You can specify module.name_mapper.extension multiple times for different extensions.

module.system (node|haste)

The module system to use to resolve import and require. Haste is used in React Native.

The default is node.

module.system.node.main_field (string)

Flow reads package.json files for the "name" and "main" fields to figure out the name of the module and which file should be used to provide that module.

So if Flow sees this in the .flowconfig:

[options]
module.system.node.main_field=foo
module.system.node.main_field=bar
module.system.node.main_field=baz

and then it comes across a package.json with

{
  "name": "kittens",
  "main": "main.js",
  "bar": "bar.js",
  "baz": "baz.js"
}

Flow will use bar.js to provide the "kittens" module.

If this option is unspecified, Flow will always use the "main" field.

See this GitHub issue for the original motivation

module.system.node.resolve_dirname (string)

By default, Flow will look in directories named node_modules for node modules. You can configure this behavior with this option.

For example, if you do:

[options]
module.system.node.resolve_dirname=node_modules
module.system.node.resolve_dirname=custom_node_modules

Then Flow will look in directories named node_modules or custom_node_modules.

Note: you can specify module.system.node.resolve_dirname multiple times

module.use_strict (boolean)

Set this to true if you use a transpiler that adds "use strict"; to the top of every module.

The default value is false.

munge_underscores (boolean)

Set this to true to have Flow treat underscore-prefixed class properties and methods as private. This should be used in conjunction with jstransform’s ES6 class transform, which enforces the same privacy at runtime.

The default value is false.

no_flowlib (boolean)

Flow has builtin library definitions. Setting this to true will tell Flow to ignore the builtin library definitions.

The default value is false.

react.runtime (automatic|classic) ≥0.123.0

Set this to automatic if you are using React’s automatic runtime in @babel/plugin-transform-react-jsx. Otherwise, use classic. See the babel documentation for details about the transform.

The default value is automatic.

server.max_workers (integer)

The maximum number of workers the Flow server can start. By default, the server will use all available cores.

sharedmemory.dirs (string)

This affects Linux only.

Flow’s shared memory lives in a memory mapped file. On more modern versions of Linux (3.17+), there is a system call memfd_create which allows Flow to create the file anonymously and only in memory. However, in older kernels, Flow needs to create a file on the file system. Ideally this file lives on a memory-backed tmpfs. This option lets you decide where that file is created.

By default this option is set to /dev/shm and /tmp

Note: You can specify sharedmemory.dirs multiple times.

sharedmemory.minimum_available (unsigned integer)

This affects Linux only.

As explained in the sharedmemory.dirs option’s description, Flow needs to create a file on a filesystem for older kernels. sharedmemory.dirs specifies a list of locations where the shared memory file can be created. For each location, Flow will check to make sure the filesystem has enough space for the shared memory file. If Flow will likely run out of space, it skips that location and tries the next. This option lets you configure the minimum amount of space needed on a filesystem for shared memory.

By default it is 536870912 (2^29 bytes, which is half a gigabyte).

sharedmemory.hash_table_pow (unsigned integer)

The 3 largest parts of the shared memory are a dependency table, a hash table, and a heap. While the heap grows and shrinks, the two tables are allocated in full. This option lets you change the size of the hash table.

Setting this option to X means the table will support up to 2^X elements, which is 16*2^X bytes.

By default, this is set to 19 (Table size is 2^19, which is 8 megabytes)

sharedmemory.heap_size (unsigned integer)

This option configures the maximum possible size for the shared heap. You should most likely not need to configure this, as it doesn’t really affect how much RSS Flow uses. However, if you are working on a massive codebase you might see the following error after init: “Heap init size is too close to max heap size; GC will never get triggered!” In this case, you may need to increase the size of the heap.

By default, this is set to 26843545600 (25 * 2^30 bytes, which is 25GiB)

sharedmemory.log_level (unsigned integer)

Setting this to 1 will cause Flow to output some stats about the data that is serialized into and deserialized out of shared memory.

By default this is 0.

strip_root (boolean) ≤0.48

Obsolete. Set this to true to always strip the root directory from file paths in error messages when using --json, --from emacs, and --from vim. Do not use this option. Instead, pass the command line flag --strip-root.

By default this is false.

suppress_comment (regex) ≤0.126

Defines a magical comment that suppresses any Flow errors on the following line. For example:

suppress_comment= \\(.\\|\n\\)*\\$FlowFixMe

will match a comment like this:

// $FlowFixMe: suppressing this error until we can refactor
var x : string = 123;

and suppress the error. If there is no error on the next line (the suppression is unnecessary), an “Unused suppression” warning will be shown instead.

If no suppression comments are specified in your config, Flow will apply one default: // $FlowFixMe.

Note: You can specify suppress_comment multiple times. If you do define any suppress_comments, the built-in $FlowFixMe suppression will be erased in favor of the regexps you specify. If you wish to use $FlowFixMe with some additional custom suppression comments, you must manually specify \\(.\\|\n\\)*\\$FlowFixMe in your custom list of suppressions.

Note: In version v0.127.0, the option to specify the suppression comment syntax was removed. $FlowFixMe, $FlowIssue, $FlowExpectedError, and $FlowIgnore became the only standard suppressions.

suppress_type (string)

This option lets you alias any with a given string. This is useful for explaining why you’re using any. For example, let’s say you sometimes want to sometimes use any to suppress an error and sometimes to mark a TODO. Your code might look like

var myString: any = 1 + 1;
var myBoolean: any = 1 + 1;

If you add the following to your configuration:

[options]
suppress_type=$FlowFixMe
suppress_type=$FlowTODO

You can update your code to the more readable:

var myString: $FlowFixMe = 1 + 1;
var myBoolean: $FlowTODO = 1 + 1;

Note: You can specify suppress_type multiple times.

temp_dir (string)

Tell Flow which directory to use as a temp directory. Can be overridden with the command line flag --temp-dir.

The default value is /tmp/flow.

traces (integer)

Enables traces on all error output (showing additional details about the flow of types through the system), to the depth specified. This can be very expensive, so is disabled by default.

types_first (boolean) ≥0.125.0

For more on types-first mode, see the types-first docs.

Flow builds intermediate artifacts to represent signatures of modules as they are checked. If this option is set to false, then these artifacts are built using inferred type information. If this option is set to true, then they are built using type annotations at module boundaries.

The default value for types_first is true (as of version 0.134).

well_formed_exports (boolean) ≥0.125.0

Enforce the following restrictions on file exports:

  • Statements manipulating module.exports and the exports alias may only appear as top-level statements.
  • Parts of the source that are visible from a file’s exports need to be annotated unless their type can be trivially inferred (e.g. the exported expression is a numeric literal). This is a requirement for types-first mode to function properly. Failure to properly annotate exports raise signature-verfication-failures.

This option is set to true by default, since it is implied by types_first, but the option is useful on its own when upgrading a project from classic mode to types-first mode.

well_formed_exports.includes (string) ≥0.128.0

Limit the scope of the well_formed_exports requirement to a specific directory of this project. For example

well_formed_exports=true
well_formed_exports.includes=<PROJECT_ROOT>/dirA
well_formed_exports.includes=<PROJECT_ROOT>/dirB

will only report export related errors in files under dirA and dirB. This option requires well_formed_exports to be set to true.

The purpose of this option is to help prepare a codebase for Flow types-first mode. See this section for more.

Between versions v0.125.0 and v0.127.0, this option was named well_formed_exports.whitelist.

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