This project was heavily based off the original flowtype eslint plugin, all credits go to the original maintainers. We duplicated the project with the intention of providing the flowtype community support and maintenance for people that continue to use flowtype.

• Installation
• Configuration
  • Shareable configurations
• Settings
  • onlyFilesWithFlowAnnotation
• Rules
  • array-style-complex-type
  • array-style-simple-type
  • arrow-parens
  • boolean-style
  • define-flow-type
  • delimiter-dangle
  • enforce-line-break
  • enforce-suppression-code
  • generic-spacing
  • interface-id-match
  • newline-after-flow-annotation
  • no-dupe-keys
  • no-duplicate-type-union-intersection-members
  • no-existential-type
  • no-flow-fix-me-comments
  • no-flow-suppressions-in-strict-files
  • no-internal-flow-type
  • no-mixed
  • no-mutable-array
  • no-primitive-constructor-types
  • no-types-missing-file-annotation
  • no-unused-expressions
  • no-weak-types
  • object-type-curly-spacing
  • object-type-delimiter
  • quotes
  • require-compound-type-alias
  • require-exact-type
  • require-indexer-name
  • require-inexact-type
  • require-parameter-type
  • require-readonly-react-props
  • require-return-type
  • require-types-at-top
  • require-valid-file-annotation
  • require-variable-type
  • semi
  • sort-keys
  • sort-type-union-intersection-members
  • space-after-type-colon
  • space-before-generic-bracket
  • space-before-type-colon
  • spread-exact-type
  • type-id-match
  • type-import-style
  • union-intersection-spacing
  • use-flow-type
  • use-read-only-spread
  • valid-syntax

Installation

npm install eslint-plugin-ft-flow eslint hermes-eslint --save-dev

# or with yarn
yarn add -D eslint-plugin-ft-flow eslint hermes-eslint

# or with pnpm
pnpm add -D eslint-plugin-ft-flow eslint hermes-eslint

Configuration

1. Set parser property to hermes-eslint.
2. Add plugins section and specify ft-flow as a plugin.
3. Enable rules.

{
  "parser": "hermes-eslint",
  "plugins": ["ft-flow"],
  "rules": {
    "ft-flow/boolean-style": [2, "boolean"]
    // ... more rules
  },
  "settings": {
    "ft-flow": {
      "onlyFilesWithFlowAnnotation": false
    }
  }
}

Shareable configurations

Recommended

This plugin exports a recommended configuration that enforces Flowtype best practices.

To enable this configuration use the extends property in your .eslintrc config file in place of the above suggested properties:

{
  "extends": ["plugin:ft-flow/recommended"]
}

Babel parser

Alternatively, if you can't yet use hermes-eslint, prior to version 3.0.0 ft-flow shipped a recommended config that used @babel/eslint-parser which is still available under the "plugin:ft-flow/babel-parser" extension.

Though it's recommended to switch to the recommended extension when possible as babel-parser may be removed in a future version.

By default this config also comes preloaded with @babel/eslint-parser which means for eslint to analyze your flow code it relies your babel config (babel.config.js, .babelrc, .babelrc.js). You should already have this setup as part of running/testing your code but if you don't you can learn more here

See ESLint documentation for more information about extending configuration files.

Settings

onlyFilesWithFlowAnnotation

When true, only checks files with a @flow annotation in the first comment.

{
  "settings": {
    "ft-flow": {
      "onlyFilesWithFlowAnnotation": true
    }
  }
}

Rules

array-style-complex-type

The --fix option on the command line automatically fixes problems reported by this rule.

Enforces a particular annotation style of complex types.

Type is considered complex in these cases:

• Maybe type
• Function type
• Object type
• Tuple type
• Union type
• Intersection type

This rule takes one argument.

If it is 'verbose' then a problem is raised when using Type[] instead of Array<Type>.

If it is 'shorthand' then a problem is raised when using Array<Type> instead of Type[].

The default value is 'verbose'.

The following patterns are considered problems:

type X = (?string)[]
// Message: Use "Array<?string>", not "(?string)[]"

// Options: ["verbose"]
type X = (?string)[]
// Message: Use "Array<?string>", not "(?string)[]"

// Options: ["shorthand"]
type X = Array<?string>
// Message: Use "(?string)[]", not "Array<?string>"

// Options: ["shorthand"]
type X = Array<{foo: string}>
// Message: Use "{foo: string}[]", not "Array<{foo: string}>"

type X = (string | number)[]
// Message: Use "Array<string | number>", not "(string | number)[]"

type X = (string & number)[]
// Message: Use "Array<string & number>", not "(string & number)[]"

type X = [string, number][]
// Message: Use "Array<[string, number]>", not "[string, number][]"

type X = {foo: string}[]
// Message: Use "Array<{foo: string}>", not "{foo: string}[]"

type X = (string => number)[]
// Message: Use "Array<string => number>", not "(string => number)[]"

type X = {
    foo: string,
    bar: number
}[]
// Message: Use "Array<{ foo: string, bar: number }>", not "{ foo: string, bar: number }[]"

type X = {
    foo: string,
    bar: number,
    quo: boolean,
    hey: Date
}[]
// Message: Use "Array<Type>", not "Type[]"

The following patterns are not considered problems:

type X = Array<?string>

// Options: ["verbose"]
type X = Array<?string>

// Options: ["shorthand"]
type X = (?string)[]

// Options: ["shorthand"]
type X = Array<string>

// Options: ["shorthand"]
// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}
type X = Array<?string>

array-style-simple-type

The --fix option on the command line automatically fixes problems reported by this rule.

Enforces a particular array type annotation style of simple types.

Type is considered simple in these cases:

• Primitive types
• Literal types
• Mixed type
• Any type
• Class type
• Generic type
• Array type shorthand notation

This rule takes one argument.

If it is 'verbose' then a problem is raised when using Type[] instead of Array<Type>.

If it is 'shorthand' then a problem is raised when using Array<Type> instead of Type[].

The default value is 'verbose'.

The following patterns are considered problems:

type X = string[]
// Message: Use "Array<string>", not "string[]"

// Options: ["verbose"]
type X = string[]
// Message: Use "Array<string>", not "string[]"

// Options: ["shorthand"]
type X = Array<string>
// Message: Use "string[]", not "Array<string>"

type X = Date[]
// Message: Use "Array<Date>", not "Date[]"

type X = Promise<string>[]
// Message: Use "Array<Promise<string>>", not "Promise<string>[]"

type X = $Keys<{foo: string}>[]
// Message: Use "Array<$Keys<{foo: string}>>", not "$Keys<{foo: string}>[]"

type X = any[]
// Message: Use "Array<any>", not "any[]"

type X = mixed[]
// Message: Use "Array<mixed>", not "mixed[]"

type X = void[]
// Message: Use "Array<void>", not "void[]"

type X = null[]
// Message: Use "Array<null>", not "null[]"

type X = Promise<{
    foo: string,
    bar: number
}>[]
// Message: Use "Array<Promise<{ foo: string, bar: number }>>", not "Promise<{ foo: string, bar: number }>[]"

type X = Promise<{
    foo: string,
    bar: number,
    quo: boolean
}>[]
// Message: Use "Array<Type>", not "Type[]"

The following patterns are not considered problems:

type X = Array<string>

// Options: ["verbose"]
type X = Array<string>

// Options: ["shorthand"]
type X = string[]

type X = Array<Array<string>>

// Options: ["verbose"]
type X = (?string)[]

// Options: ["verbose"]
// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}
type X = string[]

type X = Array

type X = typeof Array

arrow-parens

The --fix option on the command line automatically fixes problems reported by this rule.

Enforces the consistent use of parentheses in arrow functions.

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" enforces no braces where they can be omitted.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

The following patterns are considered problems:

a => {}
// Message: undefined

a => a
// Message: undefined

a => {
}
// Message: undefined

a.then(foo => {});
// Message: undefined

a.then(foo => a);
// Message: undefined

a(foo => { if (true) {}; });
// Message: undefined

a(async foo => { if (true) {}; });
// Message: undefined

// Options: ["as-needed"]
(a) => a
// Message: undefined

// Options: ["as-needed"]
(a,) => a
// Message: undefined

// Options: ["as-needed"]
async (a) => a
// Message: undefined

// Options: ["as-needed"]
async(a) => a
// Message: undefined

// Options: ["as-needed",{"requireForBlockBody":true}]
a => {}
// Message: undefined

// Options: ["as-needed",{"requireForBlockBody":true}]
(a) => a
// Message: undefined

// Options: ["as-needed",{"requireForBlockBody":true}]
async a => {}
// Message: undefined

// Options: ["as-needed",{"requireForBlockBody":true}]
async (a) => a
// Message: undefined

// Options: ["as-needed",{"requireForBlockBody":true}]
async(a) => a
// Message: undefined

The following patterns are not considered problems:

() => {}

(a) => {}

(a) => a

(a) => {
}

a.then((foo) => {});

a.then((foo) => { if (true) {}; });

a.then(async (foo) => { if (true) {}; });

// Options: ["always"]
() => {}

// Options: ["always"]
(a) => {}

// Options: ["always"]
(a) => a

// Options: ["always"]
(a) => {
}

// Options: ["always"]
a.then((foo) => {});

// Options: ["always"]
a.then((foo) => { if (true) {}; });

// Options: ["always"]
a.then(async (foo) => { if (true) {}; });

// Options: ["as-needed"]
() => {}

// Options: ["as-needed"]
a => {}

// Options: ["as-needed"]
a => a

// Options: ["as-needed"]
([a, b]) => {}

// Options: ["as-needed"]
({ a, b }) => {}

// Options: ["as-needed"]
(a = 10) => {}

// Options: ["as-needed"]
(...a) => a[0]

// Options: ["as-needed"]
(a, b) => {}

// Options: ["as-needed"]
async ([a, b]) => {}

// Options: ["as-needed"]
async (a, b) => {}

// Options: ["as-needed"]
(a: T) => a

// Options: ["as-needed"]
(a): T => a

// Options: ["as-needed",{"requireForBlockBody":true}]
() => {}

// Options: ["as-needed",{"requireForBlockBody":true}]
a => a

// Options: ["as-needed",{"requireForBlockBody":true}]
([a, b]) => {}

// Options: ["as-needed",{"requireForBlockBody":true}]
([a, b]) => a

// Options: ["as-needed",{"requireForBlockBody":true}]
({ a, b }) => {}

// Options: ["as-needed",{"requireForBlockBody":true}]
({ a, b }) => a + b

// Options: ["as-needed",{"requireForBlockBody":true}]
(a = 10) => {}

// Options: ["as-needed",{"requireForBlockBody":true}]
(...a) => a[0]

// Options: ["as-needed",{"requireForBlockBody":true}]
(a, b) => {}

// Options: ["as-needed",{"requireForBlockBody":true}]
a => ({})

// Options: ["as-needed",{"requireForBlockBody":true}]
async a => ({})

// Options: ["as-needed",{"requireForBlockBody":true}]
async a => a

// Options: ["as-needed",{"requireForBlockBody":true}]
(a: T) => a

// Options: ["as-needed",{"requireForBlockBody":true}]
(a): T => a

// Options: ["always",{"requireForBlockBody":true}]
<T>(a: T) => a

// Options: ["as-needed",{"requireForBlockBody":false}]
<T>(a: T) => { return a; }

// Options: ["always",{"requireForBlockBody":true}]
<T>(a: T) => { return a; }

// Options: ["as-needed",{"requireForBlockBody":true}]
<T>(a: T) => { return a; }

// Options: ["as-needed",{"requireForBlockBody":true}]
(a): %checks => typeof a === "number"

boolean-style

The --fix option on the command line automatically fixes problems reported by this rule.

Enforces a particular style for boolean type annotations. This rule takes one argument.

If it is 'boolean' then a problem is raised when using bool instead of boolean.

If it is 'bool' then a problem is raised when using boolean instead of bool.

The default value is 'boolean'.

The following patterns are considered problems:

type X = bool
// Message: Use "boolean", not "bool"

// Options: ["boolean"]
type X = bool
// Message: Use "boolean", not "bool"

// Options: ["bool"]
type X = boolean
// Message: Use "bool", not "boolean"

The following patterns are not considered problems:

type X = boolean

// Options: ["boolean"]
type X = boolean

// Options: ["bool"]
type X = bool

// Options: ["boolean"]
// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}
type X = bool

define-flow-type

@deprecated rule that is no longer needed with the current recommended config but kept around in case people want to continue using it with @babel/eslint-parser such as the babel-parser extension.

Marks Flow type identifiers as defined.

Used to suppress no-undef reporting of type identifiers.

The following patterns are not considered problems:

var a: AType
// Additional rules: {"no-undef":2}

var a: AType; var b: AType
// Additional rules: {"no-undef":2}

var a; (a: AType)
// Additional rules: {"no-undef":2}

var a: AType<BType>
// Additional rules: {"no-undef":2}

type A = AType
// Additional rules: {"no-undef":2}

declare type A = number
// Additional rules: {"no-undef":2}

opaque type A = AType
// Additional rules: {"no-undef":2}

function f(a: AType) {}
// Additional rules: {"no-undef":2}

function f(a: AType.a) {}
// Additional rules: {"no-undef":2}

function f(a: AType.a.b) {}
// Additional rules: {"no-undef":2}

function f(a): AType {}; var a: AType
// Additional rules: {"no-undef":2}

function f(a): AType {}
// Additional rules: {"no-undef":2}

class C { a: AType }
// Additional rules: {"no-undef":2}

class C { a: AType.a }
// Additional rules: {"no-undef":2}

class C { a: AType.a.b }
// Additional rules: {"no-undef":2}

class C implements AType {}
// Additional rules: {"no-undef":2}

declare interface A {}
// Additional rules: {"no-undef":2}

({ a: ({b() {}}: AType) })
// Additional rules: {"no-undef":2}

type X = {Y<AType>(): BType}
// Additional rules: {"no-undef":2}

// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}

/**
* Copyright 2019 no corp
* @flow
*/
type Foo = $ReadOnly<{}>
// Additional rules: {"no-undef":2}

enum Status { Active, Paused }
// Additional rules: {"no-undef":2}

enum Status { Active = 'active', Paused = 'paused' }
// Additional rules: {"no-undef":2}

enum Status { Active = 1, Paused = 2 }
// Additional rules: {"no-undef":2}

var a: AType
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

var a: AType; var b: AType
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

var a; (a: AType)
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

var a: AType<BType>
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

type A = AType
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

declare type A = number
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

opaque type A = AType
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

function f(a: AType) {}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

function f(a: AType.a) {}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

function f(a: AType.a.b) {}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

function f(a): AType {}; var a: AType
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

function f(a): AType {}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

class C { a: AType }
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

class C { a: AType.a }
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

class C { a: AType.a.b }
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

class C implements AType {}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

declare interface A {}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

({ a: ({b() {}}: AType) })
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

type X = {Y<AType>(): BType}
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}

/**
* Copyright 2019 no corp
* @flow
*/
type Foo = $ReadOnly<{}>
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

enum Status { Active, Paused }
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

enum Status { Active = 'active', Paused = 'paused' }
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

enum Status { Active = 1, Paused = 2 }
// Additional rules: {"no-undef":2,"no-use-before-define":[2,"nofunc"]}

delimiter-dangle

The --fix option on the command line automatically fixes problems reported by this rule.

Enforces consistent use of trailing commas in Object and Tuple annotations.

This rule takes three arguments where the possible values are the same as ESLint's default comma-dangle rule:

1. The first argument is for Object and Tuple annotations. The default value is 'never'.
2. The second argument is used for Interface annotations. This defaults to the value of the first argument.
3. The third argument is used for inexact object notation (trailing ...). The default value is 'never'.

If it is 'never' then a problem is raised when there is a trailing comma.

If it is 'always' then a problem is raised when there is no trailing comma.

If it is 'always-multiline' then a problem is raised when there is no trailing comma on a multi-line definition, or there is a trailing comma on a single-line definition.

If it is 'only-multiline' then a problem is raised when there is a trailing comma on a single-line definition. It allows, but does not enforce, trailing commas on multi-line definitions.

The following patterns are considered problems:

type X = { foo: string, }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = { foo: string, }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = { foo: string; }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = {
foo: string,
}
// Message: Unexpected trailing delimiter

// Options: ["always"]
type X = { foo: string }
// Message: Missing trailing delimiter

// Options: ["always"]
type X = {
foo: string
}
// Message: Missing trailing delimiter

// Options: ["always-multiline"]
type X = { foo: string, }
// Message: Unexpected trailing delimiter

// Options: ["always-multiline"]
type X = {
foo: string
}
// Message: Missing trailing delimiter

// Options: ["only-multiline"]
type X = { foo: string; }
// Message: Unexpected trailing delimiter

// Options: ["always","never"]
interface X { foo: string; }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = { [key: string]: number, }
// Message: Unexpected trailing delimiter

// Options: ["always"]
type X = { [key: string]: number }
// Message: Missing trailing delimiter

// Options: ["always-multiline"]
type X = { [key: string]: number, }
// Message: Unexpected trailing delimiter

// Options: ["always-multiline"]
type X = {
[key: string]: number
}
// Message: Missing trailing delimiter

// Options: ["only-multiline"]
type X = { [key: string]: number; }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = { [key: string]: number, foo: string, }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = {
[key: string]: number,
foo: string,
}
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = {
[key: string]: number,
aReallyLongPropertyNameHere: string,
}
// Message: Unexpected trailing delimiter

// Options: ["always"]
type X = { [key: string]: number, foo: string }
// Message: Missing trailing delimiter

// Options: ["always"]
type X = {
[key: string]: number;
foo: string
}
// Message: Missing trailing delimiter

// Options: ["always-multiline"]
type X = { [key: string]: number, foo: string, }
// Message: Unexpected trailing delimiter

// Options: ["always-multiline"]
type X = {
[key: string]: number,
foo: string
}
// Message: Missing trailing delimiter

// Options: ["only-multiline"]
type X = { [key: string]: number, foo: string, }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = { foo: string, [key: string]: number, }
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = {
foo: string,
[key: string]: number,
}
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = {
aReallyLongPropertyNameHere: string,
[key: string]: number,
}
// Message: Unexpected trailing delimiter

// Options: ["always"]
type X = { foo: string, [key: string]: number }
// Message: Missing trailing delimiter

// Options: ["always"]
type X = { foo: string; [key: string]: number }
// Message: Missing trailing delimiter

// Options: ["always-multiline"]
type X = { foo: string, [key: string]: number; }
// Message: Unexpected trailing delimiter

// Options: ["always-multiline"]
type X = {
foo: string,
[key: string]: number
}
// Message: Missing trailing delimiter

// Options: ["only-multiline"]
type X = { foo: string, [key: string]: number; }
// Message: Unexpected trailing delimiter

type X = { ..., }
// Message: Unexpected trailing delimiter

type X = { ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = { ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = { ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","always"]
type X = { ... }
// Message: Missing trailing delimiter

// Options: ["never","never","always-multiline"]
type X = { ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","always-multiline"]
type X = { ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","only-multiline"]
type X = { ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","only-multiline"]
type X = { ...; }
// Message: Unexpected trailing delimiter

type X = {
...,
}
// Message: Unexpected trailing delimiter

type X = {
...;
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = {
...,
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = {
...;
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","always"]
type X = {
...
}
// Message: Missing trailing delimiter

// Options: ["never","never","always-multiline"]
type X = {
...
}
// Message: Missing trailing delimiter

type X = { foo: string, ..., }
// Message: Unexpected trailing delimiter

type X = { foo: string; ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = { foo: string, ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = { foo: string; ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","always"]
type X = { foo: string, ... }
// Message: Missing trailing delimiter

// Options: ["never","never","always-multiline"]
type X = { foo: string, ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","always-multiline"]
type X = { foo: string; ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","only-multiline"]
type X = { foo: string, ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","only-multiline"]
type X = { foo: string; ...; }
// Message: Unexpected trailing delimiter

type X = {
foo: string,
...,
}
// Message: Unexpected trailing delimiter

type X = {
foo: string;
...;
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = {
foo: string,
...,
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = {
foo: string;
...;
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","always"]
type X = {
foo: string,
...
}
// Message: Missing trailing delimiter

// Options: ["never","never","always-multiline"]
type X = {
foo: string,
...
}
// Message: Missing trailing delimiter

type X = { [key: string]: number, ..., }
// Message: Unexpected trailing delimiter

type X = { [key: string]: number; ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = { [key: string]: number, ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = { [key: string]: number; ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","always"]
type X = { [key: string]: number, ... }
// Message: Missing trailing delimiter

// Options: ["never","never","always-multiline"]
type X = { [key: string]: number, ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","always-multiline"]
type X = { [key: string]: number; ...; }
// Message: Unexpected trailing delimiter

// Options: ["never","never","only-multiline"]
type X = { [key: string]: number, ..., }
// Message: Unexpected trailing delimiter

// Options: ["never","never","only-multiline"]
type X = { [key: string]: number; ...; }
// Message: Unexpected trailing delimiter

type X = {
[key: string]: number,
...,
}
// Message: Unexpected trailing delimiter

type X = {
[key: string]: number;
...;
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = {
[key: string]: number,
...,
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","never"]
type X = {
[key: string]: number;
...;
}
// Message: Unexpected trailing delimiter

// Options: ["never","never","always"]
type X = {
[key: string]: number,
...
}
// Message: Missing trailing delimiter

// Options: ["never","never","always-multiline"]
type X = {
[key: string]: number,
...
}
// Message: Missing trailing delimiter

type X = [string, number,]
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = [string, number,]
// Message: Unexpected trailing delimiter

// Options: ["never"]
type X = [
string,
number,
]
// Message: Unexpected trailing delimiter

// Options: ["always"]
type X = [string, number]
// Message: Missing trailing delimiter

// Options: ["always"]
type X = [
string,
number
]
// Message: Missing trailing delimiter

// Options: ["always-multiline"]
type X = [string, number,]
// Message: Unexpected trailing delimiter

// Options: ["always-multiline"]
type X = [
foo, string
]
// Message: Missing trailing delimiter

// Options: ["only-multiline"]
type X = [ number, string, ]
// Message: Unexpected trailing delimiter

The following patterns are not considered problems:

type X = { foo: string }

// Options: ["never"]
type X = { foo: string }

// Options: ["always"]
type X = { foo: string, }

// Options: ["always"]
type X = { foo: string; }

// Options: ["never"]
type X = {
foo: string
}

// Options: ["always"]
type X = {
foo: string,
}

// Options: ["always-multiline"]
type X = { foo: string }

// Options: ["always-multiline"]
type X = {
foo: string,
}

// Options: ["always-multiline"]
type X = {
foo: string;
}

// Options: ["only-multiline"]
type X = { foo: string }

// Options: ["only-multiline"]
type X = {
foo: string
}

// Options: ["only-multiline"]
type X = {
foo: string,
}

// Options: ["only-multiline"]
type X = {
foo: string;
}

// Options: ["never","always"]
interface X { foo: string; }

// Options: ["never"]
type X = {}

// Options: ["always"]
type X = {}

// Options: ["always-multiline"]
type X = {}

// Options: ["only-multiline"]
type X = {}

// Options: ["never"]
type X = { [key: string]: number }

// Options: ["always"]
type X = { [key: string]: number, }

// Options: ["always"]
type X = { [key: string]: number; }

// Options: ["always-multiline"]
type X = { [key: string]: number }

// Options: ["always-multiline"]
type X = {
[key: string]: number,
}

// Options: ["only-multiline"]
type X = {
[key: string]: number,
}

// Options: ["only-multiline"]
type X = {
[key: string]: number
}

// Options: ["only-multiline"]
type X = { [key: string]: number }

// Options: ["never"]
type X = { [key: string]: number, foo: string }

// Options: ["always"]
type X = { [key: string]: number, foo: string, }

// Options: ["always"]
type X = { [key: string]: number; foo: string; }

// Options: ["always-multiline"]
type X = { [key: string]: number, foo: string }

// Options: ["always-multiline"]
type X = {
[key: string]: number,
foo: string,
}

// Options: ["only-multiline"]
type X = {
[key: string]: number,
foo: string,
}

// Options: ["only-multiline"]
type X = {
[key: string]: number;
foo: string
}

// Options: ["only-multiline"]
type X = { [key: string]: number, foo: string }

// Options: ["never"]
type X = { foo: string, [key: string]: number }

// Options: ["always"]
type X = { foo: string, [key: string]: number, }

// Options: ["always"]
type X = { foo: string; [key: string]: number; }

// Options: ["always-multiline"]
type X = { foo: string, [key: string]: number }

// Options: ["always-multiline"]
type X = {
foo: string,
[key: string]: number,
}

// Options: ["only-multiline"]
type X = {
foo: string,
[key: string]: number,
}

// Options: ["only-multiline"]
type X = {
foo: string;
[key: string]: number
}

// Options: ["only-multiline"]
type X = { foo: string, [key: string]: number }

type X = { ... }

// Options: ["never","never","never"]
type X = { ... }

// Options: ["never","never","always"]
type X = { ..., }

// Options: ["never","never","always-multiline"]
type X = { ... }

// Options: ["never","never","only-multiline"]
type X = { ... }

type X = {
...
}

// Options: ["never","never","never"]
type X = {
...
}

// Options: ["never","never","always"]
type X = {
...,
 }

// Options: ["never","never","always"]
type X = {
...;
 }

// Options: ["never","never","always-multiline"]
type X = {
...,
}

// Options: ["never","never","always-multiline"]
type X = {
...;
}

// Options: ["never","never","only-multiline"]
type X = {
...
}

// Options: ["never","never","only-multiline"]
type X = {
...,
}

// Options: ["never","never","only-multiline"]
type X = {
...;
}

type X = { foo: string, ... }

// Options: ["never","never","never"]
type X = { foo: string, ... }

// Options: ["never","never","always"]
type X = { foo: string, ..., }

// Options: ["never","never","always"]
type X = { foo: string; ...; }

// Options: ["never","never","always-multiline"]
type X = { foo: string, ... }

// Options: ["never","never","only-multiline"]
type X = { foo: string, ... }

type X = {
foo: string,
...
}

// Options: ["never","never","never"]
type X = {
foo: string,
...
}

// Options: ["never","never","always"]
type X = {
foo: string,
...,
}

// Options: ["never","never","always"]
type X = {
foo: string;
...;
}

// Options: ["never","never","always-multiline"]
type X = {
foo: string,
...,
}

// Options: ["never","never","always-multiline"]
type X = {
foo: string;
...;
}

// Options: ["never","never","only-multiline"]
type X = {
foo: string,
...
}

// Options: ["never","never","only-multiline"]
type X = {
foo: string,
...,
}

// Options: ["never","never","only-multiline"]
type X = {
foo: string,
...;
}

// Options: ["never","never","never"]
type X = { [key: string]: number, ... }

// Options: ["never","never","always"]
type X = { [key: string]: number, ..., }

// Options: ["never","never","always"]
type X = { [key: string]: number; ...; }

// Options: ["never","never","always-multiline"]
type X = { [key: string]: number, ... }

// Options: ["never","never","only-multiline"]
type X = { [key: string]: number, ... }

// Options: ["never","never","never"]
type X = {
[key: string]: number,
...
}

// Options: ["never","never","always"]
type X = {
[key: string]: number,
...,
}

// Options: ["never","never","always"]
type X = {
[key: string]: number;
...;
}

// Options: ["never","never","always-multiline"]
type X = {
[key: string]: number,
...,
}

// Options: ["never","never","always-multiline"]
type X = {
[key: string]: number;
...;
}

// Options: ["never","never","only-multiline"]
type X = {
[key: string]: number,
...
}

// Options: ["never","never","only-multiline"]
type X = {
[key: string]: number,
...,
}

// Options: ["never","never","only-multiline"]
type X = {
[key: string]: number;
...;
}

type X = [string, number]

// Options: ["never"]
type X = [string, number]

// Options: ["never"]
type X = [
string,
number
]

// Options: ["always"]
type X = [string, number,]

// Options: ["always"]
type X = [
string,
number,
]

// Options: ["always-multiline"]
type X = [ foo, string ]

// Options: ["always-multiline"]
type X = [
foo, string,
]

// Options: ["only-multiline"]
type X = [ number, string ]

// Options: ["only-multiline"]
type X = [
number,
string
]

// Options: ["only-multiline"]
type X = [
number,
string,
]

// Options: ["never"]
type X = []

// Options: ["always"]
type X = []

// Options: ["always-multiline"]
type X = []

// Options: ["only-multiline"]
type X = []

enforce-line-break

This rule enforces line breaks between type definitions.

The following patterns are considered problems:

type baz = 6;
const hi = 2;
// Message: New line required below type declaration

const foo = 6;
type hi = 2;

// Message: New line required above type declaration

const som = "jes";
// a comment
type fed = "hed";

// Message: New line required above type declaration

type som = "jes";
// a comment
const fed = "hed";

// Message: New line required below type declaration

type hello = 34;
const som = "jes";
type fed = "hed";

// Message: New line required below type declaration
// Message: New line required above type declaration

const a = 5;
export type hello = 34;

// Message: New line required above type declaration

const a = 5;
// a comment
export type hello = 34;

// Message: New line required above type declaration

const a = 5;
/**
 * a jsdoc block
 */
type hello = 34;
// Message: New line required above type declaration

The following patterns are not considered problems:

type gjs = 6;

type gjs = 6;

type hi = 2;


type X = 4;

const red = "serpent";
console.log("hello");

// number or string
type Y = string | number;

// resting + sleep
type snooze = "dreaming" | "";

type Props = {
  accountBalance: string | number,
  accountNumber: string | number,
};

const x = 4;
const y = 489;

// Some Comment
type Props = {
  accountBalance: string | number,
  accountNumber: string | number,
};

type RoadT = "grass" | "gravel" | "cement";

// @flow
type A = string

enforce-suppression-code

This rule enforces a suppression code on flow suppression comments such as $FlowFixMe and $FlowExpectedError.

The following patterns are considered problems:

// $FlowFixMe I am doing something evil here
const text = 'HELLO';
// Message: $FlowFixMe is missing a suppression error code. Please update this suppression to use an error code: $FlowFixMe[…]

// $FlowExpectedError I am doing something evil here
const text = 'HELLO';
// Message: $FlowExpectedError is missing a suppression error code. Please update this suppression to use an error code: $FlowExpectedError[…]

// $FlowIssue I am doing something evil here
const text = 'HELLO';
// Message: $FlowIssue is missing a suppression error code. Please update this suppression to use an error code: $FlowIssue[…]

// $FlowIgnore I am doing something evil here
const text = 'HELLO';
// Message: $FlowIgnore is missing a suppression error code. Please update this suppression to use an error code: $FlowIgnore[…]

/* $FlowIgnore I am doing something evil here */
// Message: $FlowIgnore is missing a suppression error code. Please update this suppression to use an error code: $FlowIgnore[…]

{ /* $FlowIgnore I am doing something evil here */ }
// Message: $FlowIgnore is missing a suppression error code. Please update this suppression to use an error code: $FlowIgnore[…]

/**
  * $FlowIgnore I am doing something evil here
  */
// Message: $FlowIgnore is missing a suppression error code. Please update this suppression to use an error code: $FlowIgnore[…]

The following patterns are not considered problems:

// Just a random comment
//
const text = 'HELLO';

// $FlowFixMe[incompatible-call] TODO 48
const text = 'HELLO';

// $FlowExpectedError[incompatible-call] TODO 48
const text = 'HELLO';

// $FlowIssue[incompatible-call] TODO 48
const text = 'HELLO';

// $FlowIgnore[incompatible-call] TODO 48
const text = 'HELLO';

/* $FlowIgnore[incompatible-call] TODO 48 */

/**
 * $FlowIgnore[incompatible-call] TODO 48
 */

/* $FlowIgnore[incompatible-call] TODO 48 */

generic-spacing

The --fix option on the command line automatically fixes problems reported by this rule.

Enforces consistent spacing within generic type annotation parameters.

This rule takes one argument. If it is 'never' then a problem is raised when there is a space surrounding the generic type parameters. If it is 'always' then a problem is raised when there is no space surrounding the generic type parameters.

The default value is 'never'.

The following patterns are considered problems:

type X = Promise< string>
// Message: There must be no space at start of "Promise" generic type annotation

// Options: ["never"]
type X = Promise<  string>
// Message: There must be no space at start of "Promise" generic type annotation

type X = FooBar<string >
// Message: There must be no space at end of "FooBar" generic type annotation

type X = Promise< string >
// Message: There must be no space at start of "Promise" generic type annotation
// Message: There must be no space at end of "Promise" generic type annotation

type X = Promise< (foo), bar, (((baz))) >
// Message: There must be no space at start of "Promise" generic type annotation
// Message: There must be no space at end of "Promise" generic type annotation

// Options: ["always"]
type X = Promise<string >
// Message: There must be a space at start of "Promise" generic type annotation

// Options: ["always"]
type X = FooBar< string>
// Message: There must be a space at end of "FooBar" generic type annotation

// Options: ["always"]
type X = Promise<string>
// Message: There must be a space at start of "Promise" generic type annotation
// Message: There must be a space at end of "Promise" generic type annotation

// Options: ["always"]
type X = Promise<(foo), bar, (((baz)))>
// Message: There must be a space at start of "Promise" generic type annotation
// Message: There must be a space at end of "Promise" generic type annotation

// Options: ["always"]
type X = FooBar<  string >
// Message: There must be one space at start of "FooBar" generic type annotation

// Options: ["always"]
type X = FooBar< string  >
// Message: There must be one space at end of "FooBar" generic type annotation

// Options: ["always"]
type X = Promise<  (foo), bar, (((baz)))  >
// Message: There must be one space at start of "Promise" generic type annotation
// Message: There must be one space at end of "Promise" generic type annotation

The following patterns are not considered problems:

type X = Promise<string>

type X = Promise<(string)>

type X = Promise<(foo), bar, (((baz)))>

type X = Promise<
  (foo),
  bar,
  (((baz)))
>

type X =  Promise<
    (foo),
    bar,
    (((baz)))
>

// Options: ["always"]
type X = Promise< string >

// Options: ["always"]
type X = Promise< (string) >

// Options: ["always"]
type X = Promise< (foo), bar, (((baz))) >

interface-id-match

Enforces a consistent naming pattern for interfaces.

Options

This rule requires a text RegExp:

{
    "rules": {
        "ft-flow/interface-id-match": [
            2,
            "^([A-Z][a-z0-9]*)+Type$"
        ]
    }
}

'^([A-Z][a-z0-9]*)+Type$' is the default pattern.

The following patterns are considered problems:

interface foo{};
// Message: Interface identifier 'foo' does not match pattern '/^([A-Z][a-z0-9]*)+Type$/u'.

// Options: ["^foo$"]
interface FooType{};
// Message: Interface identifier 'FooType' does not match pattern '/^foo$/u'.

The following patterns are not considered problems:

interface FooType {};

// Options: ["^foo$"]
interface foo {};

// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}
interface foo {};

newline-after-flow-annotation

This rule requires an empty line after the Flow annotation.

Options

The rule has a string option:

• "always" (default): Enforces that @flow annotations be followed by an empty line, separated by newline (LF)
• "always-windows": Identical to "always", but will use a CRLF when autofixing
• "never": Enforces that @flow annotations are not followed by empty lines

{
  "rules": {
    "ft-flow/newline-after-flow-annotation": [
      2,
      "always"
    ]
  }
}

The following patterns are considered problems:

// @flow
import Foo from './foo';
// Message: Expected newline after flow annotation

// Options: ["always"]
// @flow
import Foo from './foo';
// Message: Expected newline after flow annotation

// Options: ["always-windows"]
// @flow
import Foo from './foo';
// Message: Expected newline after flow annotation

// Options: ["never"]
// @flow


// Message: Expected no newline after flow annotation

/*
* @flow
*
* something multi lined
*/
const text: string = 42;
// Message: Expected newline after flow annotation

The following patterns are not considered problems:

// Options: ["always"]
// @flow

import Foo from './foo';

// Options: ["always-windows"]
// @flow

import Foo from './foo';

// Options: ["never"]
// @flow
import Foo from './foo';

// Options: ["always"]
/*
* @flow
*
* something multi lined
*/

const text: string = 42;

no-dupe-keys

Checks for duplicate properties in Object annotations.

This rule mirrors ESLint's no-dupe-keys rule.

{
    "rules": {
        "ft-flow/no-dupe-keys": 2
    }
}

The following patterns are considered problems:

type f = { a: number, b: string, a: number }
// Message: Duplicate property.

type f = { a: number, b: string, a: string }
// Message: Duplicate property.

type f = { get(key: "a"): string, get(key: "a"): string }
// Message: Duplicate property.

type f = { get(key: 1): string, get(key: 1): string }
// Message: Duplicate property.

type f = { get(key: 1.1): string, get(key: 1.1): string }
// Message: Duplicate property.

type f = { get(key: true): string, get(key: true): string }
// Message: Duplicate property.

type f = { get(key: {a: 1}): string, get(key: {a: 1}):string }
// Message: Duplicate property.

var a = "a"; type f = { get(key: a): string, get(key: a): string }
// Message: Duplicate property.

var b = 1; type f = { get(key: b): string, get(key: b): string }
// Message: Duplicate property.

var c = true; type f = { get(key: c): string, get(key: c): string }
// Message: Duplicate property.

var d = {}; type f = { get(key: d): string, get(key: d): string }
// Message: Duplicate property.

var e = []; type f = { get(key: e): string, get(key: e): string }
// Message: Duplicate property.

var e = [1, "a"]; type f = { get(key: e): string, get(key: e): string }
// Message: Duplicate property.

function fn() {}; type f = { get(key: fn): string, get(key: fn): string }
// Message: Duplicate property.

The following patterns are not considered problems:

type FooType = { a: number, b: string, c: number }

// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}
type FooType = { a: number, b: string, a: number }

type f = { get(key: "a"): string, get(key: "b"): string }

type f = { get(key: 1): string, get(key: 2): string }

type f = { get(key: 1.1): string, get(key: 1.2): string }

type f = { get(key: true): string, get(key: false): string }

type f = { get(key: ["a", 1]): string, get(key: ["a", 2]): string }

type f = { get(key: ["a", ["b", 1]]): string, get(key: ["a", ["b", 2]]): string }

type f = { a: number, b: string, c: number }

type f = { get(key: "a"): string, get(key: "b"): string }

type f = { get(key: "a"): string, get(key: "a", key2: "b"): string }

type f = { get(key: "a"): string, get(key: 1): string }

type f = { get(key: { a: 1 }): string, get(key: { a: 2 }): string}

var a = {}; var b = {}; type f = { get(key: a): string, get(key: b): string }

var a = 1; var b = 1; type f = { get(key: a): string, get(key: b): string }

type a = { b: <C>(config: { ...C, key: string}) => C }

export interface Foo { get foo(): boolean; get bar(): string; }

no-duplicate-type-union-intersection-members

The --fix option on the command line automatically fixes problems reported by this rule.

Checks for duplicate members of a type union/intersection.

Options

You can disable checking intersection types using checkIntersections.

• true (default) - check for duplicate members of intersection members.
• false - do not check for duplicate members of intersection members.

{
  "rules": {
    "ft-flow/no-duplicate-type-union-intersection-members": [
      2,
      {
        "checkIntersections": true
      }
    ]
  }
}

You can disable checking union types using checkUnions.

• true (default) - check for duplicate members of union members.
• false - do not check for duplicate members of union members.

{
  "rules": {
    "ft-flow/no-duplicate-type-union-intersection-members": [
      2,
      {
        "checkUnions": true
      }
    ]
  }
}

The following patterns are considered problems:

type A = 1 | 2 | 3 | 1;
// Message: Duplicate union member found "1".

type B = 'foo' | 'bar' | 'foo';
// Message: Duplicate union member found "'foo'".

type C = A | B | A | B;
// Message: Duplicate union member found "A".
// Message: Duplicate union member found "B".

type C = A & B & A & B;
// Message: Duplicate intersection member found "A".
// Message: Duplicate intersection member found "B".

The following patterns are not considered problems:

type A = 1 | 2 | 3;

type B = 'foo' | 'bar';

type C = A | B;

type C = A & B;

no-existential-type

Disallows use of the existential type (*). See more

{
  "rules": {
    "ft-flow/no-existential-type": 2
  }
}

The following patterns are considered problems:

type T = *;
// Message: Unexpected use of existential type (*).

type T = U<*, *>;
// Message: Unexpected use of existential type (*).
// Message: Unexpected use of existential type (*).

const f: (*) => null = () => null;
// Message: Unexpected use of existential type (*).

The following patterns are not considered problems:

type T = string | null

no-flow-fix-me-comments

Disallows $FlowFixMe comment suppressions.

This is especially useful as a warning to ensure instances of $FlowFixMe in your codebase get fixed over time.

Options

This rule takes an optional RegExp that comments a text RegExp that makes the supression valid.

{
    "rules": {
        "ft-flow/no-flow-fix-me-comments": [
            1,
            "TODO\s+[0-9]+"
        ]
    }
}

The following patterns are considered problems:

// $FlowFixMe I am doing something evil here
const text = 'HELLO';
// Message: $FlowFixMe is treated as 'any' and must be fixed.

// Options: ["TODO [0-9]+"]
// $FlowFixMe I am doing something evil here
const text = 'HELLO';
// Message: $FlowFixMe is treated as 'any' and must be fixed. Fix it or match '/TODO [0-9]+/u'.

// Options: ["TODO [0-9]+"]
// $FlowFixMe TODO abc 47 I am doing something evil here
const text = 'HELLO';
// Message: $FlowFixMe is treated as 'any' and must be fixed. Fix it or match '/TODO [0-9]+/u'.

// $$FlowFixMeProps I am doing something evil here
const text = 'HELLO';
// Message: $FlowFixMe is treated as 'any' and must be fixed.

// Options: ["TODO [0-9]+"]
// $FlowFixMeProps I am doing something evil here
const text = 'HELLO';
// Message: $FlowFixMe is treated as 'any' and must be fixed. Fix it or match '/TODO [0-9]+/u'.

The following patterns are not considered problems:

const text = 'HELLO';

// Options: ["TODO [0-9]+"]
// $FlowFixMe TODO 48
const text = 'HELLO';

no-flow-suppressions-in-strict-files

This rule validates that no error suppression comments (e.g. $FlowFixMe) are used in // @flow strict (or // @flow strict-local) files.

This codifies the best practices as documented here:

"Do not add $FlowFixMe to suppress the new errors as they appear; just add @flow strict once all issues have been resolved."

Options

This rule accepts 1 option as an object to disable errors being thrown on specific suppression error types. For example, you don't want $FlowFixMe but $FlowExpectedError you want to allow because they are expected issues that can't be solved.

{
  "rules": {
    "ft-flow/no-flow-suppressions-in-strict-files": [2, {
      "$FlowExpectedError": false
    }]
  }
}

The following patterns are considered problems:

// Options: [{}]
// @flow strict

// $FlowFixMe
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

// Options: [{}]
// @flow strict-local

// $FlowFixMe
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

// Options: [{}]
// @flow strict

// $FlowExpectedError[xxx]
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

// Options: [{}]
/* @flow strict */

// $FlowExpectedError[xxx]
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

// Options: [{}]
/*
* @flow strict
*
* something multi lined
*/

// $FlowExpectedError[xxx]
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

// Options: [{}]
/*
* @flow strict
*
* something multi lined
*/

/* $FlowIgnore[xxx] */
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

// Options: [{"$FlowExpectedError":false}]
// @flow strict

// $FlowFixMe
const text: string = 42;
// Message: No suppression comments are allowed in "strict" Flow files. Either remove the error suppression, or lower the strictness of this module.

The following patterns are not considered problems:

// Options: [{}]
// @flow

// Error suppressions are fine in "normal" Flow files
// $FlowFixMe
const text: string = 42;

// Options: [{}]
// @flow strict

// Definitely nothing to suppress here
// ...

// Options: [{}]
// @flow strict-local

// Definitely nothing to suppress here
// ...

// Options: [{"$FlowExpectedError":false}]
// @flow strict

// $FlowExpectedError
const text: string = 42;

// Options: [{"$FlowExpectedError":false}]
// @flow strict-local

// $FlowExpectedError
const text: string = 42;

no-internal-flow-type

Warns against using internal Flow types such as React$Node, React$Ref and others and suggests using public alternatives instead (React.Node, React.Ref, …).

The following patterns are considered problems:

type X = React$AbstractComponent<Config, Instance>
// Message: Type identifier 'React$AbstractComponent' is not allowed. Use 'React.AbstractComponent' instead.

type X = React$ChildrenArray<string>
// Message: Type identifier 'React$ChildrenArray' is not allowed. Use 'React.ChildrenArray' instead.

type X = React$ComponentType<Props>
// Message: Type identifier 'React$ComponentType' is not allowed. Use 'React.ComponentType' instead.

type X = React$Config<Prosp, DefaultProps>
// Message: Type identifier 'React$Config' is not allowed. Use 'React.Config' instead.

type X = React$Element<typeof Component>
// Message: Type identifier 'React$Element' is not allowed. Use 'React.Element' instead.

type X = React$ElementConfig<typeof Component>
// Message: Type identifier 'React$ElementConfig' is not allowed. Use 'React.ElementConfig' instead.

type X = React$ElementProps<typeof Component>
// Message: Type identifier 'React$ElementProps' is not allowed. Use 'React.ElementProps' instead.

type X = React$ElementRef<typeof Component>
// Message: Type identifier 'React$ElementRef' is not allowed. Use 'React.ElementRef' instead.

type X = React$ElementType
// Message: Type identifier 'React$ElementType' is not allowed. Use 'React.ElementType' instead.

type X = React$Key
// Message: Type identifier 'React$Key' is not allowed. Use 'React.Key' instead.

type X = React$Node
// Message: Type identifier 'React$Node' is not allowed. Use 'React.Node' instead.

type X = React$Ref<typeof Component>
// Message: Type identifier 'React$Ref' is not allowed. Use 'React.Ref' instead.

type X = React$StatelessFunctionalComponent<Props>
// Message: Type identifier 'React$StatelessFunctionalComponent' is not allowed. Use 'React.StatelessFunctionalComponent' instead.

The following patterns are not considered problems:

type X = React.AbstractComponent<Config, Instance>

type X = React.ChildrenArray<string>

type X = React.ComponentType<Props>

type X = React.Config<Props, DefaultProps>

type X = React.Element<typeof Component>

type X = React.ElementConfig<typeof Component>

type X = React.ElementProps<typeof Component>

type X = React.ElementRef<typeof Component>

type X = React.ElementType

type X = React.Key

type X = React.Node

type X = React.Ref<typeof Component>

type X = React.StatelessFunctionalComponent<Props>

type X = React$Rocks

no-mixed

Warns against "mixed" type annotations. These types are not strict enough and could often be made more specific.

The following patterns are considered problems:

The following patterns are considered problems:

function foo(thing): mixed {}
// Message: Unexpected use of mixed type

function foo(thing): Promise<mixed> {}
// Message: Unexpected use of mixed type

function foo(thing): Promise<Promise<mixed>> {}
// Message: Unexpected use of mixed type

The following patterns are not considered problems:

function foo(thing): string {}

function foo(thing): Promise<string> {}

function foo(thing): Promise<Promise<string>> {}

(foo?: string) => {}

(foo: ?string) => {}

(foo: { a: string }) => {}

(foo: { a: ?string }) => {}

(foo: string[]) => {}

type Foo = string

type Foo = { a: string }

type Foo = { (a: string): string }

function foo(thing: string) {}

var foo: string

class Foo { props: string }

no-mutable-array

The --fix option on the command line automatically fixes problems reported by this rule.

Requires use of $ReadOnlyArray instead of just Array or array shorthand notation. $ReadOnlyArray is immutable array collection type and the superclass of Array and tuple types in Flow. Use of $ReadOnlyArray instead of Array can solve some "problems" in typing with Flow (e.g., 1, 2).

General reasons for using immutable data structures:

• They are simpler to construct, test, and use
• They help to avoid temporal coupling
• Their usage is side-effect free (no defensive copies)
• Identity mutability problem is avoided
• They always have failure atomicity
• They are much easier to cache

Note that initialization of a variable with an empty array is considered valid (e.g., const values: Array<string> = [];). This behavior resembles the behavior of Flow's unsealed objects, as it is assumed that empty array is intended to be mutated.

The following patterns are considered problems:

type X = Array<string>
// Message: Use "$ReadOnlyArray" instead of "Array"

type X = string[]
// Message: Use "$ReadOnlyArray" instead of array shorthand notation

const values: Array<Array<string>> = [];
// Message: Use "$ReadOnlyArray" instead of "Array"

let values: Array<Array<string>>;
// Message: Use "$ReadOnlyArray" instead of "Array"
// Message: Use "$ReadOnlyArray" instead of "Array"

The following patterns are not considered problems:

type X = $ReadOnlyArray<string>

const values: Array<$ReadOnlyArray<string>> = [];

const values: $ReadOnlyArray<string>[] = [];

const values: Array<$ReadOnlyArray<string>> = new Array();

const values: Array<$ReadOnlyArray<string>> = Array();

no-primitive-constructor-types

Disallows use of primitive constructors as types, such as Boolean, Number and String. See more.

{
    "rules": {
        "ft-flow/no-primitive-constructor-types": 2
    }
}

The following patterns are considered problems:

type x = Number
// Message: Unexpected use of Number constructor type.

type x = String
// Message: Unexpected use of String constructor type.

type x = Boolean
// Message: Unexpected use of Boolean constructor type.

type x = { a: Number }
// Message: Unexpected use of Number constructor type.

type x = { a: String }
// Message: Unexpected use of String constructor type.

type x = { a: Boolean }
// Message: Unexpected use of Boolean constructor type.

(x: Number) => {}
// Message: Unexpected use of Number constructor type.

(x: String) => {}
// Message: Unexpected use of String constructor type.

(x: Boolean) => {}
// Message: Unexpected use of Boolean constructor type.

The following patterns are not considered problems:

type x = number

type x = string

type x = boolean

type x = { a: number }

type x = { a: string }

type x = { a: boolean }

(x: number) => {}

(x: string) => {}

(x: boolean) => {}

type x = MyNumber

type x = MyString

type x = MyBoolean

no-types-missing-file-annotation

Disallows Flow type imports, aliases, and annotations in files missing a valid Flow file declaration (or a @noflow annotation).

{
    "rules": {
        "ft-flow/no-types-missing-file-annotation": 2
    }
}

The following patterns are considered problems:

const x: number = 42;
// Message: Type annotations require valid Flow declaration.

type FooType = number;
// Message: Type aliases require valid Flow declaration.

import type A from "a"
// Message: Type imports require valid Flow declaration.

import type {A} from "a"
// Message: Type imports require valid Flow declaration.

import {type A} from "a"
// Message: Type imports require valid Flow declaration.

export type {A} from "a"
// Message: Type exports require valid Flow declaration.

function t<T>(): T{}
// Message: Type annotations require valid Flow declaration.

// Settings: {"ft-flow":{"onlyFilesWithFlowAnnotation":true}}
const x: number = 42;
// Message: Type annotations require valid Flow declaration.

The following patterns are not considered problems:

// @flow
const x: number = 42;

/* @flow weak */
type FooType = number;

/* @noflow */
type FooType = number;

/* @noflow */
import type A from "a"

/* @noflow */
import {type A} from "a"

/* @noflow */
export type {A} from "a"

// an unrelated comment
// @flow
export type {A} from "a"

no-unused-expressions

An extension of ESLint's no-unused-expressions. This rule ignores type cast expressions and optional call expressions, but otherwise behaves the same as ESLint's no-unused-expressions.

Bare type casts are useful, for example to assert the exhaustiveness of a switch:

type Action
  = { type: 'FOO', doFoo: (_: number) => void }
  | { type: 'BAR', doBar: (_: string) => void };

type State = { foo: number, bar: string };

function runFooBar(action: Action, state: State): void {
  switch (action.type) {
    case 'FOO':
      doFoo(state.foo);
      break;
    c