eslint-plugin-flowtype

Flow type linting rules for ESLint.

eslint-plugin-flowtype

Installation

npm install eslint --save-dev
npm install @babel/eslint-parser --save-dev
npm install eslint-plugin-flowtype --save-dev

Configuration

Set parser property to @babel/eslint-parser.
Add plugins section and specify eslint-plugin-flowtype as a plugin.
Enable rules.

{
  "parser": "@babel/eslint-parser",
  "plugins": [
    "flowtype"
  ],
  "rules": {
    "flowtype/boolean-style": [
      2,
      "boolean"
    ],
    "flowtype/define-flow-type": 1,
    "flowtype/delimiter-dangle": [
      2,
      "never"
    ],
    "flowtype/generic-spacing": [
      2,
      "never"
    ],
    "flowtype/interface-id-match": [
      2,
      "^([A-Z][a-z0-9]+)+Type$"
    ],
    "flowtype/no-mixed": 2,
    "flowtype/no-primitive-constructor-types": 2,
    "flowtype/no-types-missing-file-annotation": 2,
    "flowtype/no-weak-types": 2,
    "flowtype/object-type-delimiter": [
      2,
      "comma"
    ],
    "flowtype/require-parameter-type": 2,
    "flowtype/require-readonly-react-props": 0,
    "flowtype/require-return-type": [
      2,
      "always",
      {
        "annotateUndefined": "never"
      }
    ],
    "flowtype/require-valid-file-annotation": 2,
    "flowtype/semi": [
      2,
      "always"
    ],
    "flowtype/space-after-type-colon": [
      2,
      "always"
    ],
    "flowtype/space-before-generic-bracket": [
      2,
      "never"
    ],
    "flowtype/space-before-type-colon": [
      2,
      "never"
    ],
    "flowtype/type-id-match": [
      2,
      "^([A-Z][a-z0-9]+)+Type$"
    ],
    "flowtype/union-intersection-spacing": [
      2,
      "always"
    ],
    "flowtype/use-flow-type": 1,
    "flowtype/valid-syntax": 1
  },
  "settings": {
    "flowtype": {
      "onlyFilesWithFlowAnnotation": false
    }
  }
}

Shareable configurations

Community maintained configurations

The following are third-party submitted/ maintained configurations of eslint-plugin-flowtype:

https://github.com/wemake-services/eslint-config-flowtype-essential

Settings

`onlyFilesWithFlowAnnotation`

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

{
  "settings": {
    "flowtype": {
      "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:

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: {"flowtype":{"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:

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: {"flowtype":{"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: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type X = bool

`define-flow-type`

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: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}

/**
* Copyright 2019 no corp
* @flow
*/
type Foo = $ReadOnly<{}>
// 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: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}

/**
* Copyright 2019 no corp
* @flow
*/
type Foo = $ReadOnly<{}>
// 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:

The first argument is for Object and Tuple annotations. The default value is 'never'.
The second argument is used for Interface annotations. This defaults to the value of the first argument.
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

`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": {
        "flowtype/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: {"flowtype":{"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": {
    "flowtype/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

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';

`no-dupe-keys`

Checks for duplicate properties in Object annotations.

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

{
    "rules": {
        "flowtype/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: {"flowtype":{"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": {
    "flowtype/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": {
    "flowtype/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": {
    "flowtype/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": {
        "flowtype/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-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:

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": {
        "flowtype/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": {
        "flowtype/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: {"flowtype":{"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;
    case 'BAR':
      doBar(state.bar);
      break;
    default:
      (action: empty);  // type error when `Action` is extended with new types
      console.error(`Impossible action: ${action.toString()}`);
  }
}

This rule takes the same arguments as ESLint's no-unused-expressions. See that rule's documentation for details.

The following patterns are considered problems:

foo + 1
// Message: Expected an assignment or function call and instead saw an expression.

x?.y
// Message: Expected an assignment or function call and instead saw an expression.

The following patterns are not considered problems:

(foo: number)

x?.y()

`no-weak-types`

Warns against weak type annotations any, Object and Function. These types can cause flow to silently skip over portions of your code, which would have otherwise caused type errors.

This rule optionally takes one argument, an object to configure which type warnings to enable. By default, all of the warnings are enabled. e.g. to disable the any warning (allowing it to exist in your code), while continuing to warn about Object and Function:

{
    "rules": {
        "flowtype/no-weak-types": [2, {
            "any": false,
            "Object": true,
            "Function": true
        }]
    }
}

// or, the following is equivalent as default is true:

{
    "rules": {
        "flowtype/no-weak-types": [2, {
            "any": false
        }]
    }
}

The following patterns are considered problems:

function foo(thing): any {}
// Message: Unexpected use of weak type "any"

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

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

function foo(thing): Object {}
// Message: Unexpected use of weak type "Object"

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

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

function foo(thing): Function {}
// Message: Unexpected use of weak type "Function"

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

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

(foo: any) => {}
// Message: Unexpected use of weak type "any"

(foo: Function) => {}
// Message: Unexpected use of weak type "Function"

(foo?: any) => {}
// Message: Unexpected use of weak type "any"

(foo?: Function) => {}
// Message: Unexpected use of weak type "Function"

(foo: { a: any }) => {}
// Message: Unexpected use of weak type "any"

(foo: { a: Object }) => {}
// Message: Unexpected use of weak type "Object"

(foo: any[]) => {}
// Message: Unexpected use of weak type "any"

type Foo = any
// Message: Unexpected use of weak type "any"

type Foo = Function
// Message: Unexpected use of weak type "Function"

type Foo = { a: any }
// Message: Unexpected use of weak type "any"

type Foo = { a: Object }
// Message: Unexpected use of weak type "Object"

type Foo = { (a: Object): string }
// Message: Unexpected use of weak type "Object"

type Foo = { (a: string): Function }
// Message: Unexpected use of weak type "Function"

function foo(thing: any) {}
// Message: Unexpected use of weak type "any"

function foo(thing: Object) {}
// Message: Unexpected use of weak type "Object"

var foo: Function
// Message: Unexpected use of weak type "Function"

var foo: Object
// Message: Unexpected use of weak type "Object"

class Foo { props: any }
// Message: Unexpected use of weak type "any"

class Foo { props: Object }
// Message: Unexpected use of weak type "Object"

var foo: any
// Message: Unexpected use of weak type "any"

// Options: [{"Function":false}]
type X = any; type Y = Function; type Z = Object
// Message: Unexpected use of weak type "any"
// Message: Unexpected use of weak type "Object"

// Options: [{"any":false,"Object":false}]
type X = any; type Y = Function; type Z = Object
// Message: Unexpected use of weak type "Function"

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 }

// Options: [{"any":false,"Object":false}]
type X = any; type Y = Object

// Options: [{"Function":false}]
type X = Function

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
function foo(thing): Function {}

`object-type-curly-spacing`

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

This rule enforces consistent spacing inside braces of object types.

Options

The rule has a string option:

"never" (default): disallows spacing inside of braces.
"always": requires spacing inside of braces.

The following patterns are considered problems:

type obj = { "foo": "bar" }
// Message: There must be no space after "{".
// Message: There must be no space before "}".

type obj = {"foo": "bar" }
// Message: There must be no space before "}".

type obj = {"foo": "bar", ... }
// Message: There must be no space before "}".

type obj = {|"foo": "bar" |}
// Message: There must be no space before "|}".

type obj = {"foo": "bar", [key: string]: string }
// Message: There must be no space before "}".

type obj = {
"foo": "bar", [key: string]: string }
// Message: There must be no space before "}".

type obj = { baz: {"foo": "qux"}, bar: 4}
// Message: There must be no space after "{".

// Options: ["always"]
type obj = {"foo": "bar"}
// Message: A space is required after "{".
// Message: A space is required before "}".

// Options: ["always"]
type obj = {"foo": "bar" }
// Message: A space is required after "{".

// Options: ["always"]
type obj = { baz: {"foo": "qux"}, bar: 4}
// Message: A space is required before "}".
// Message: A space is required after "{".
// Message: A space is required before "}".

// Options: ["always"]
type obj = { baz: { "foo": "qux" }, bar: 4}
// Message: A space is required before "}".

// Options: ["always"]
type obj = { "foo": "bar", ...}
// Message: A space is required before "}".

// Options: ["always"]
type obj = {|"foo": "bar" |}
// Message: A space is required after "{|".

// Options: ["always"]
type obj = {"foo": "bar", [key: string]: string }
// Message: A space is required after "{".

The following patterns are not considered problems:

type obj = {baz: {"foo": "qux"}, bar: 4}

type obj = {foo: {"foo": "qux"}}

type obj = {foo: "bar"}

type obj = {foo: "bar"
}

type obj = {
foo: "bar"}

type obj = {
foo: "bar"
}

type obj = {
foo: "bar",
ee: "bar",
}

type obj = {
foo: "bar",
ee: "bar",
             }

type obj = {|"foo": "bar"|}

type obj = {"foo": "bar", [key: string]: string}

// Options: ["always"]
type obj = { baz: { "foo": "qux" }, bar: 4 }

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

// Options: ["always"]
type obj = {
foo: "bar"
}

// Options: ["always"]
type obj = { baz: 4 }

// Options: ["always"]
type obj = {| "foo": "bar" |}

// Options: ["always"]
type obj = { "foo": "bar", [key: string]: string }

// Options: ["always"]
type obj = {  baz: { "foo": "qux" }, bar: 4  }

// Options: ["always"]
type obj = {
  baz: { "foo": "qux" }, bar: 4
}

`object-type-delimiter`

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

Enforces consistent separators between properties in Flow object types.

This rule takes one argument.

If it is 'comma' then a problem is raised when using ; as a separator.

If it is 'semicolon' then a problem is raised when using , as a separator.

The default value is 'comma'.

This rule is ported from babel/flow-object-type, however the default option was changed.

The following patterns are considered problems:

// Options: ["semicolon"]
type Foo = { a: Foo, b: Bar }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
type Foo = { a: Foo; b: Bar }
// Message: Prefer commas to semicolons in object and class types

// Options: ["semicolon"]
type Foo = { [a: string]: Foo, [b: string]: Bar }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
type Foo = { [a: string]: Foo; [b: string]: Bar }
// Message: Prefer commas to semicolons in object and class types

// Options: ["semicolon"]
type Foo = { (): Foo, (): Bar }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
type Foo = { (): Foo; (): Bar }
// Message: Prefer commas to semicolons in object and class types

// Options: ["semicolon"]
declare class Foo { a: Foo, }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
declare class Foo { a: Foo; }
// Message: Prefer commas to semicolons in object and class types

// Options: ["semicolon"]
declare class Foo { [a: string]: Foo, }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
declare class Foo { a: Foo; }
// Message: Prefer commas to semicolons in object and class types

// Options: ["semicolon"]
declare class Foo { (): Foo, }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
declare class Foo { (): Foo; }
// Message: Prefer commas to semicolons in object and class types

// Options: ["semicolon"]
declare class Foo { static (): Foo, }
// Message: Prefer semicolons to commas in object and class types

// Options: ["comma"]
declare class Foo { static (): Foo; }
// Message: Prefer commas to semicolons in object and class types

The following patterns are not considered problems:

// Options: ["semicolon"]
type Foo = { a: Foo; b: Bar }

// Options: ["comma"]
type Foo = { a: Foo, b: Bar }

// Options: ["semicolon"]
type Foo = { [a: string]: Foo; [b: string]: Bar }

// Options: ["comma"]
type Foo = { [a: string]: Foo, [b: string]: Bar }

// Options: ["semicolon"]
type Foo = { (): Foo; (): Bar }

// Options: ["comma"]
type Foo = { (): Foo, (): Bar }

type Foo = { a: Foo, b: Bar }

type Foo = { [a: string]: Foo, [b: string]: Bar }

type Foo = { (): Foo, (): Bar }

// Options: ["semicolon"]
declare class Foo { a: Foo; }

// Options: ["comma"]
declare class Foo { a: Foo, }

// Options: ["semicolon"]
declare class Foo { [a: string]: Foo; }

// Options: ["comma"]
declare class Foo { [a: string]: Foo, }

// Options: ["semicolon"]
declare class Foo { (): Foo; }

// Options: ["comma"]
declare class Foo { (): Foo, }

// Options: ["semicolon"]
// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type Foo = { a: Foo, b: Bar }

`quotes`

Enforces single quotes or double quotes around string literals.

Options

The rule has string options of:

"double" (default) requires double quotes around string literals.
"single" requires single quotes around string literals.

The following patterns are considered problems:

type T = 'hi'
// Message: String literals must use double quote.

// Options: ["double"]
type T = { test: 'hello' | 'test' }
// Message: String literals must use double quote.
// Message: String literals must use double quote.

// Options: ["double"]
type T = { test: "hello" | 'test', t: 'hello' }
// Message: String literals must use double quote.
// Message: String literals must use double quote.

// Options: ["single"]
type T = "hi"
// Message: String literals must use single quote.

// Options: ["single"]
type T = { test: "hello" | "test" }
// Message: String literals must use single quote.
// Message: String literals must use single quote.

// Options: ["single"]
type T = { test: "hello" | 'test', t: 'hello' }
// Message: String literals must use single quote.

The following patterns are not considered problems:

// Options: ["double"]
type T = "hi"

// Options: ["double"]
type T = { test: "hello" | "test" }

// Options: ["double"]
type T = { test: "hello" | "test", t: "hello" }

// Options: ["single"]
type FooType = 'hi'

// Options: ["single"]
type T = { test: 'hello' | 'test' }

// Options: ["single"]
type T = { test: 'hello' | 'test', t: 'hello' }

`require-compound-type-alias`

Requires to make a type alias for all union and intersection types. If these are used in "raw" forms it might be tempting to just copy & paste them around the code. However, this brings sort of a source code pollution and unnecessary changes on several parts when these compound types need to be changed.

Options

The rule has two options:

a string option

"always" (default)
"never"

an object

{
  "rules": {
    "flowtype/require-compound-type-alias": [
      2,
      "always",
      {
        "allowNull": true
      }
    ]
  }
}

allowNull – allows compound types where one of the members is a null, e.g. string | null.

The following patterns are considered problems:

// Options: ["always",{"allowNull":false}]
const foo: string | null = null;
// Message: All union types must be declared with named type alias.

function foo(bar: "A" | "B") {}
// Message: All union types must be declared with named type alias.

const foo: "A" | "B" = "A";
// Message: All union types must be declared with named type alias.

type Foo = { bar: "A" | "B" };
// Message: All union types must be declared with named type alias.

function foo(bar: { n: number } | { s: string }) {}
// Message: All union types must be declared with named type alias.

function foo(bar: { n: number } & { s: string }) {}
// Message: All intersection types must be declared with named type alias.

const foo: { n: number } & { s: string } = { n: 0, s: "" };
// Message: All intersection types must be declared with named type alias.

type Foo = { bar: { n: number } & { s: string } };
// Message: All intersection types must be declared with named type alias.

function foo(bar: { n: number } & { s: string }) {}
// Message: All intersection types must be declared with named type alias.

The following patterns are not considered problems:

const foo: string | null = null;

// Options: ["always",{"allowNull":true}]
const foo: string | null = null;

type Foo = "A" | "B";

type Bar = "A" | "B"; function foo(bar: Bar) {}

type Foo = { disjoint: "A", n: number } | { disjoint: "B", s: string };

type Foo = { n: number } & { s: string };

type Bar = { n: number } & { s: string }; function foo(bar: Bar) {}

// Options: ["never"]
function foo(bar: "A" | "B") {}

// Options: ["never"]
function foo(bar: { n: number } & { s: string }) {}

`require-exact-type`

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

This rule enforces exact object types.

Options

The rule has one string option:

"always" (default): Report all object type definitions that aren't exact.
"never": Report all object type definitions that are exact.

{
  "rules": {
    "flowtype/require-exact-type": [
      2,
      "always"
    ]
  }
}

{
  "rules": {
    "flowtype/require-exact-type": [
      2,
      "never"
    ]
  }
}

The following patterns are considered problems:

type foo = {};
// Message: Object type must be exact.

type foo = { bar: string };
// Message: Object type must be exact.

// Options: ["always"]
type foo = Array<{bar: string}>;
// Message: Object type must be exact.

// Options: ["always"]
(foo: Array<{bar: string}>) => {};
// Message: Object type must be exact.

// Options: ["always"]
interface StackFrame {
          colno?: number;
          lineno?: number;
          filename?: string;
          function?: { name: string };
      }
// Message: Object type must be exact.

// Options: ["never"]
type foo = {| |};
// Message: Object type must not be exact.

// Options: ["never"]
type foo = {| bar: string |};
// Message: Object type must not be exact.

// Options: ["never"]
type foo = { bar: {| baz: string |} };
// Message: Object type must not be exact.

// Options: ["never"]
type foo = Array<{| bar: string |}>;
// Message: Object type must not be exact.

// Options: ["never"]
(foo: Array<{| bar: string |}>) => {};
// Message: Object type must not be exact.

// Options: ["never"]
interface StackFrame {
          colno?: number;
          lineno?: number;
          filename?: string;
          function?: {| name: string |};
      }
// Message: Object type must not be exact.

The following patterns are not considered problems:

type foo = {| |};

type foo = {| bar: string |};

type foo = { [key: string]: string };

type foo = number;

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

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

// Options: ["always"]
type foo = {| bar: {| baz: string |} |};

// Options: ["always"]
type foo = Array<{| bar: string |}>;

// Options: ["always"]
type foo = number;

// Options: ["always"]
interface StackFrame {
          colno?: number;
          lineno?: number;
          filename?: string;
          function?: {| name: string |};
      }

// Options: ["always"]
declare class MyEvent extends Event {
        key: string
      }

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

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

// Options: ["never"]
type foo = { bar: { baz: string } };

// Options: ["never"]
type foo = Array<{bar: string}>;

// Options: ["never"]
type foo = number;

// Options: ["always"]
interface StackFrame {
          colno?: number;
          lineno?: number;
          filename?: string;
          function?: {| name: string |};
      }

type A = { a: string, ... }

`require-indexer-name`

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

This rule validates Flow object indexer name.

Options

The rule has a string option:

"never" (default): Never report files that are missing an indexer key name.
"always": Always report files that are missing an indexer key name.

{
  "rules": {
    "flowtype/require-indexer-name": [
      2,
      "always"
    ]
  }
}

The following patterns are considered problems:

type foo = { [string]: number };
// Message: All indexers must be declared with key name.

The following patterns are not considered problems:

type foo = { [key: string]: number };

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

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

`require-inexact-type`

This rule enforces explicit inexact object types.

Options

The rule has one string option:

"always" (default): Report all object type definitions that aren't explicit inexact, but ignore exact objects.
"never": Report all object type definitions that are explicit inexact.

{
  "rules": {
    "flowtype/require-inexact-type": [
      2,
      "always"
    ]
  }
}

{
  "rules": {
    "flowtype/require-inexact-type": [
      2,
      "never"
    ]
  }
}

The following patterns are considered problems:

type foo = {};
// Message: Type must be explicit inexact.

type foo = { bar: string };
// Message: Type must be explicit inexact.

// Options: ["always"]
type foo = {};
// Message: Type must be explicit inexact.

// Options: ["always"]
type foo = { bar: string };
// Message: Type must be explicit inexact.

// Options: ["never"]
type foo = {...};
// Message: Type must not be explicit inexact.

// Options: ["never"]
type foo = { bar: string, ... };
// Message: Type must not be explicit inexact.

The following patterns are not considered problems:

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

interface Foo { foo: string }

declare class Foo { foo: string }

type foo = {| |};

type foo = {| bar: string |};

type foo = { [key: string]: string, ... };

type foo = number;

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

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

// Options: ["always"]
type foo = { bar: string, ... };

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

// Options: ["always"]
type foo = number;

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

// Options: ["never"]
type foo = {| |};

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

// Options: ["never"]
type foo = {| bar: string |};

// Options: ["never"]
type foo = number;

`require-parameter-type`

Requires that all function parameters have type annotations.

Options

You can skip all arrow functions by providing the excludeArrowFunctions option with true.

Alternatively, you can want to exclude only concise arrow functions (e.g. x => x * 2). Provide excludeArrowFunctions with expressionsOnly for this.

{
    "rules": {
        "flowtype/require-parameter-type": [
            2,
            {
              "excludeArrowFunctions": true
            }
        ]
    }
}

{
    "rules": {
        "flowtype/require-parameter-type": [
            2,
            {
              "excludeArrowFunctions": "expressionsOnly"
            }
        ]
    }
}

You can exclude parameters that match a certain regex by using excludeParameterMatch.

{
    "rules": {
        "flowtype/require-parameter-type": [
            2,
            {
              "excludeParameterMatch": "^_"
            }
        ]
    }
}

This excludes all parameters that start with an underscore (_). The default pattern is a^, which doesn't match anything, i.e., all parameters are checked.

The following patterns are considered problems:

(foo) => {}
// Message: Missing "foo" parameter type annotation.

function x(foo) {}
// Message: Missing "foo" parameter type annotation.

// Options: [{"excludeArrowFunctions":true}]
function x(foo) {}
// Message: Missing "foo" parameter type annotation.

(foo = 'FOO') => {}
// Message: Missing "foo" parameter type annotation.

(...foo) => {}
// Message: Missing "foo" parameter type annotation.

({foo}) => {}
// Message: Missing "{foo}" parameter type annotation.

([foo]) => {}
// Message: Missing "[foo]" parameter type annotation.

({foo = 1} = {}) => {}
// Message: Missing "{foo = 1}" parameter type annotation.

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
// @flow
(foo) => {}
// Message: Missing "foo" parameter type annotation.

// Options: [{"excludeArrowFunctions":"expressionsOnly"}]
(foo) => {}
// Message: Missing "foo" parameter type annotation.

// Options: [{"excludeArrowFunctions":"expressionsOnly"}]
function x(foo) {}
// Message: Missing "foo" parameter type annotation.

// Options: [{"excludeParameterMatch":"^_"}]
(_foo: number, bar) => {}
// Message: Missing "bar" parameter type annotation.

// Options: [{"excludeParameterMatch":"^_"}]
(_foo, bar) => {}
// Message: Missing "bar" parameter type annotation.

The following patterns are not considered problems:

(foo: string) => {}

(foo: string = 'FOO') => {}

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

const f: Foo = (a, b) => 42;

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

([foo]: Array) => {}

type fn = (a: string, b: number) => number;
const f: fn = (a, b) => {}

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
(foo) => {}

// Options: [{"excludeArrowFunctions":true}]
(foo) => {}

// Options: [{"excludeArrowFunctions":"expressionsOnly"}]
(foo) => 3

// Options: [{"excludeParameterMatch":"^_"}]
(_foo, bar: string) => {}

// Options: [{"excludeParameterMatch":"^_"}]
(_foo: number, bar: string) => {}

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
(foo) => {}

`require-readonly-react-props`

This rule validates that React props are marked as $ReadOnly. React props are immutable and modifying them could lead to unexpected results. Marking prop shapes as $ReadOnly avoids these issues.

The rule tries its best to work with both class and functional components. For class components, it does a fuzzy check for one of "Component", "PureComponent", "React.Component" and "React.PureComponent". It doesn't actually infer that those identifiers resolve to a proper React.Component object.

For example, this will NOT be checked:

import MyReact from 'react';
class Foo extends MyReact.Component { }

As a result, you can safely use other classes without getting warnings from this rule:

class MyClass extends MySuperClass { }

React's functional components are hard to detect statically. The way it's done here is by searching for JSX within a function. When present, a function is considered a React component:

// this gets checked
type Props = { };
function MyComponent(props: Props) {
    return <p />;
}

// this doesn't get checked since no JSX is present in a function
type Options = { };
function SomeHelper(options: Options) {
    // ...
}

// this doesn't get checked since no JSX is present directly in a function
function helper() { return <p /> }
function MyComponent(props: Props) {
    return helper();
}

The rule only works for locally defined props that are marked with a $ReadOnly or using covariant notation. It doesn't work with imported props:

// the rule has no way of knowing whether ImportedProps are read-only
import { type ImportedProps } from './somewhere';
class Foo extends React.Component<ImportedProps> { }


// the rule also checks for covariant properties
type Props = {|
    +foo: string
|};
class Bar extends React.Component<Props> { }

// this fails because the object is not fully read-only
type Props = {|
    +foo: string,
    bar: number,
|};
class Bar extends React.Component<Props> { }

// this fails because spreading makes object mutable (as of Flow 0.98)
// https://github.com/gajus/eslint-plugin-flowtype/pull/400#issuecomment-489813899
type Props = {|
    +foo: string,
    ...bar,
|};
class Bar extends React.Component<Props> { }

{
    "rules": {
        "flowtype/require-readonly-react-props": 2
    }
}

Optionally, you can enable support for implicit exact Flow types (useful when using exact_by_default=true Flow option):

{
    "rules": {
        "flowtype/require-readonly-react-props": [
            2,
            {
                "useImplicitExactTypes": true
            }
        ]
    }
}

The following patterns are considered problems:

type Props = { }; class Foo extends React.Component<Props> { }
// Message: Props must be $ReadOnly

type OtherProps = { foo: string }; class Foo extends React.Component<OtherProps> { }
// Message: OtherProps must be $ReadOnly

class Foo extends React.Component<{}> { }
// Message: Foo class props must be $ReadOnly

type Props = { bar: {} }; class Foo extends React.Component<Props, State> { }
// Message: Props must be $ReadOnly

type Props = { }; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

type Props = { }; class Foo extends PureComponent<Props> { }
// Message: Props must be $ReadOnly

export type Props = {}; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

type Props = {| foo: string |}; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

type Props = {| foo: string |} | {| bar: number |}; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

// Options: [{"useImplicitExactTypes":true}]
type Props = { foo: string } | { bar: number }; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

type Props = {| +foo: string, ...bar |}; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

type Props = {| +foo: string, -bar: number |}; class Foo extends Component<Props> { }
// Message: Props must be $ReadOnly

type Props = { }; function Foo(props: Props) { return <p /> }
// Message: Props must be $ReadOnly

type Props = { }; function Foo(props: Props) { return foo ? <p /> : <span /> }
// Message: Props must be $ReadOnly

function Foo(props: {}) { return <p /> }
// Message: Foo component props must be $ReadOnly

export type Props = {}; function Foo(props: Props) { return <p /> }
// Message: Props must be $ReadOnly

The following patterns are not considered problems:

class Foo extends React.Component<$ReadOnly<{}>> { }

type Props = $ReadOnly<{}>; class Foo extends React.Component<Props> { }

type Props = $ReadOnly<{}>; class Foo extends React.PureComponent<Props> { }

class Foo extends React.Component<$ReadOnly<{}, State>> { }

type Props = $ReadOnly<{}>; class Foo extends React.Component<Props, State> { }

type Props = $ReadOnly<{}>; class Foo extends Component<Props> { }

type Props = $ReadOnly<{}>; class Foo extends PureComponent<Props> { }

type FooType = {}; class Foo extends Bar<FooType> { }

class Foo { }

export type Props = $ReadOnly<{}>; class Foo extends Component<Props, State> { }

export type Props = $ReadOnly<{}>; export class Foo extends Component<Props> { }

type Props = {| +foo: string |}; class Foo extends Component<Props> { }

type Props = {| +foo: string, +bar: number |}; class Foo extends Component<Props> { }

type Props = {| +foo: string |} | {| +bar: number |}; class Foo extends Component<Props> { }

// Options: [{"useImplicitExactTypes":true}]
type Props = { +foo: string } | { +bar: number }; class Foo extends Component<Props> { }

type Props = $FlowFixMe; class Foo extends Component<Props> { }

type Props = {||}; class Foo extends Component<Props> { }

// Options: [{"useImplicitExactTypes":true}]
type Props = {||}; class Foo extends Component<Props> { }

// Options: [{"useImplicitExactTypes":true}]
type Props = {}; class Foo extends Component<Props> { }

class Foo extends Component<{||}> { }

// Options: [{"useImplicitExactTypes":true}]
class Foo extends Component<{||}> { }

// Options: [{"useImplicitExactTypes":true}]
class Foo extends Component<{}> { }

class Foo extends React.Component<UnknownProps> { }

import { type Props } from "file"; class Foo extends React.Component<Props> { }

type Props = {}; function Foo() { }

type Props = $ReadOnly<{}>; function Foo(props: Props) { }

type Props = {}; function Foo(props: OtherProps) { }

function Foo() { return <p /> }

function Foo(props: $FlowFixMe) { return <p /> }

function Foo(props: {||}) { return <p /> }

// Options: [{"useImplicitExactTypes":true}]
function Foo(props: {||}) { return <p /> }

// Options: [{"useImplicitExactTypes":true}]
function Foo(props: {}) { return <p /> }

`require-return-type`

Requires that functions have return type annotation.

Options

You can skip all arrow functions by providing the excludeArrowFunctions option with true.

Alternatively, you can exclude a concise arrow function (e.g. () => 2). Provide excludeArrowFunctions with expressionsOnly for this.

{
    "rules": {
        "flowtype/require-return-type": [
            2,
            "always",
            {
              "excludeArrowFunctions": true
            }
        ]
    }
}

{
    "rules": {
        "flowtype/require-return-type": [
            2,
            "always",
            {
              "excludeArrowFunctions": "expressionsOnly"
            }
        ]
    }
}

You can exclude or include specific tests with the includeOnlyMatching and excludeMatching rules.

{
    "rules": {
        "flowtype/require-return-type": [
            2,
            "always",
            {
              "includeOnlyMatching": [
                  "^F.*",
                  "Ba(r|z)"
              ]
            }
        ]
    }
}

{
    "rules": {
        "flowtype/require-return-type": [
            2,
            "always",
            {
              "excludeMatching": [
                  "^F.*",
                  "Ba(r|z)"
              ]
            }
        ]
    }
}

Both rules take an array that can contain either strings or valid RegExp statements.

The following patterns are considered problems:

(foo) => { return "foo"; }
// Message: Missing return type annotation.

// Options: ["always"]
(foo) => { return "foo"; }
// Message: Missing return type annotation.

// Options: ["always"]
(foo) => "foo"
// Message: Missing return type annotation.

(foo) => ({})
// Message: Missing return type annotation.

/* @flow */
(foo) => { return 1; }
// Message: Missing return type annotation.

(foo): undefined => { return; }
// Message: Must not annotate undefined return type.

(foo): void => { return; }
// Message: Must not annotate undefined return type.

(foo): undefined => { return undefined; }
// Message: Must not annotate undefined return type.

(foo): void => { return void 0; }
// Message: Must not annotate undefined return type.

// Options: ["always",{"annotateUndefined":"never"}]
(foo): undefined => { return; }
// Message: Must not annotate undefined return type.

// Options: ["always",{"annotateUndefined":"never"}]
(foo): void => { return; }
// Message: Must not annotate undefined return type.

// Options: ["always",{"annotateUndefined":"always"}]
(foo) => { return; }
// Message: Must annotate undefined return type.

// Options: ["always",{"annotateUndefined":"never"}]
(foo): undefined => { return undefined; }
// Message: Must not annotate undefined return type.

// Options: ["always",{"annotateUndefined":"always"}]
(foo) => { return undefined; }
// Message: Must annotate undefined return type.

// Options: ["always",{"annotateUndefined":"always"}]
(foo) => { return void 0; }
// Message: Must annotate undefined return type.

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
// @flow
(foo) => { return 1; }
// Message: Missing return type annotation.

// Options: ["always",{"annotateUndefined":"always"}]
// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
// @flow
 (foo) => { return undefined; }
// Message: Must annotate undefined return type.

// Options: ["always"]
async () => { return 2; }
// Message: Missing return type annotation.

// Options: ["always",{"annotateUndefined":"always"}]
async () => {}
// Message: Must annotate undefined return type.

// Options: ["always",{"annotateUndefined":"always"}]
async function x() {}
// Message: Must annotate undefined return type.

// Options: ["always",{"annotateUndefined":"never"}]
async (): Promise<void> => { return; }
// Message: Must not annotate undefined return type.

// Options: ["always",{"annotateUndefined":"never"}]
async (): Promise<undefined> => { return; }
// Message: Must not annotate undefined return type.

// Options: ["always",{"annotateUndefined":"always"}]
class Test { constructor() { } }
// Message: Must annotate undefined return type.

class Test { foo() { return 42; } }
// Message: Missing return type annotation.

class Test { foo = () => { return 42; } }
// Message: Missing return type annotation.

class Test { foo = () => 42; }
// Message: Missing return type annotation.

// Options: ["always"]
function* x() {}
// Message: Missing return type annotation.

// Options: ["always",{"excludeArrowFunctions":"expressionsOnly"}]
() => { return 3; }
// Message: Missing return type annotation.

// Options: ["always",{"excludeArrowFunctions":"expressionsOnly"}]
async () => { return 4; }
// Message: Missing return type annotation.

// Options: ["always",{"includeOnlyMatching":["bar"]}]
function foo() { return 42; }
function bar() { return 42; }
// Message: Missing return type annotation.

// Options: ["always",{"includeOnlyMatching":["bar"]}]
const foo = () => { return 42; };
const bar = () => { return 42; }
// Message: Missing return type annotation.

// Options: ["always",{"includeOnlyMatching":["bar"]}]
const foo = { bar() { return 42; }, foobar: function() { return 42; } }
// Message: Missing return type annotation.
// Message: Missing return type annotation.

// Options: ["always",{"excludeMatching":["bar"]}]
const foo = { bar() { return 42; }, baz() { return 42; } }
// Message: Missing return type annotation.

// Options: ["always",{"annotateUndefined":"always"}]
function * foo() { yield 2; }
// Message: Missing return type annotation.

// Options: ["always",{"annotateUndefined":"always"}]
async function * foo() { yield 2; }
// Message: Missing return type annotation.

The following patterns are not considered problems:

return;

(foo): string => {}

const f: Foo = (a, b) => 42;

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

type fn = (a: string, b: number) => number;
const f: fn = (a, b) => { return 42; }

(foo) => { return; }

(foo): Object => ( {} )

(foo) => { return undefined; }

(foo) => { return void 0; }

// Options: ["always",{"annotateUndefined":"always"}]
(foo): undefined => { return; }

// Options: ["always",{"annotateUndefined":"always"}]
(foo): void => { return; }

// Options: ["always",{"annotateUndefined":"never"}]
(foo) => { return; }

// Options: ["always",{"annotateUndefined":"never"}]
(foo) => { return undefined; }

// Options: ["always",{"annotateUndefined":"never"}]
(foo) => { return void 0; }

// Options: ["always",{"annotateUndefined":"always"}]
(foo): undefined => { return undefined; }

// Options: ["always",{"annotateUndefined":"always"}]
(foo): void => { return void 0; }

// Options: ["always"]
// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
(foo) => { return 1; }

// Options: ["always"]
/* @noflow */
(foo) => { return 1; }

// Options: ["always",{"annotateUndefined":"always"}]
// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
(foo) => { return undefined; }

// Options: ["always",{"annotateUndefined":"always"}]
async function doThing(): Promise<void> {}

// Options: ["always",{"annotateUndefined":"ignore"}]
async function doThing(): Promise<void> {}

// Options: ["always",{"annotateUndefined":"ignore"}]
async function doThing() {}

// Options: ["always",{"annotateUndefined":"always"}]
function* doThing(): Generator<number, void, void> { yield 2; }

// Options: ["always",{"annotateUndefined":"always","excludeMatching":["constructor"]}]
class Test { constructor() { } }

class Test { constructor() { } }

// Options: ["always",{"excludeMatching":["foo"]}]
class Test { foo() { return 42; } }

// Options: ["always",{"excludeMatching":["foo"]}]
class Test { foo = () => { return 42; } }

// Options: ["always",{"excludeMatching":["foo"]}]
class Test { foo = () => 42; }

class Test { foo = (): number => { return 42; } }

class Test { foo = (): number => 42; }

async (foo): Promise<number> => { return 3; }

// Options: ["always",{"excludeArrowFunctions":true}]
() => 3

// Options: ["always",{"excludeArrowFunctions":true}]
() => { return 4; }

// Options: ["always",{"excludeArrowFunctions":true}]
() => undefined

// Options: ["always",{"annotateUndefined":"always","excludeArrowFunctions":true}]
() => undefined

// Options: ["always",{"annotateUndefined":"always","excludeArrowFunctions":true}]
() => { return undefined; }

// Options: ["always",{"excludeArrowFunctions":"expressionsOnly"}]
() => 3

// Options: ["always",{"excludeArrowFunctions":"expressionsOnly"}]
async () => 3

// Options: ["always",{"excludeMatching":["foo"]}]
function foo() { return 42; }

// Options: ["always",{"includeOnlyMatching":["bar"]}]
function foo() { return 42; }

// Options: ["always",{"excludeMatching":["bar"]}]
function foo(): number { return 42; }
function bar() { return 42; }

// Options: ["always",{"includeOnlyMatching":["foo","baz"]}]
function foo(): number { return 42; }
function bar() { return 42; }

// Options: ["always",{"excludeMatching":["^b.*","qux"]}]
function foo(): number { return 42; }
function bar() { return 42; }

// Options: ["always",{"includeOnlyMatching":["^f.*"]}]
function foo(): number { return 42; }
function bar() { return 42; }

// Options: ["always",{"includeOnlyMatching":["bar"]}]
const foo = { baz() { return 42; } }

// Options: ["always",{"excludeMatching":["bar"]}]
const foo = { bar() { return 42; } }

// Options: ["always",{"annotateUndefined":"always"}]
function * foo(): Iterable<number> { yield 2; }

// Options: ["always",{"annotateUndefined":"always"}]
async function * foo(): AsyncIterable<number> { yield 2; }

`require-types-at-top`

Requires all type declarations to be at the top of the file, after any import declarations.

Options

The rule has a string option:

"never"
"always"

The default value is "always".

The following patterns are considered problems:

const foo = 3;
type Foo = number;
// Message: All type declaration must be at the top of the file, after any import declarations.

const foo = 3;
opaque type Foo = number;
// Message: All type declaration must be at the top of the file, after any import declarations.

const foo = 3;
export type Foo = number;
// Message: All type declaration must be at the top of the file, after any import declarations.

const foo = 3;
export opaque type Foo = number;
// Message: All type declaration must be at the top of the file, after any import declarations.

const foo = 3;
type Foo = number | string;
// Message: All type declaration must be at the top of the file, after any import declarations.

import bar from "./bar";
const foo = 3;
type Foo = number;
// Message: All type declaration must be at the top of the file, after any import declarations.

The following patterns are not considered problems:

type Foo = number;
const foo = 3;

opaque type Foo = number;
const foo = 3;

export type Foo = number;
const foo = 3;

export opaque type Foo = number;
const foo = 3;

type Foo = number;
const foo = 3;

import bar from "./bar";
type Foo = number;

type Foo = number;
import bar from "./bar";

// Options: ["never"]
const foo = 3;
type Foo = number;

`require-valid-file-annotation`

This rule validates Flow file annotations.

This rule can optionally report missing or missed placed annotations, common typos (e.g. // @floww), and enforce a consistent annotation style.

Options

The rule has a string option:

"never" (default): Never report files that are missing an @flow annotation.
"always": Always report files that are missing an @flow annotation

This rule has an object option:

"annotationStyle" - Enforce a consistent file annotation style.
- "none" (default): Either annotation style is accepted.
- "line": Require single line annotations (i.e. // @flow).
- "block": Require block annotations (i.e. /* @flow */).
"strict" - Enforce a strict flow file annotation.
- false (default): strict flow annotation is not required.
- true: Require strict flow annotation (i.e. // @flow strict).

{
  "rules": {
    "flowtype/require-valid-file-annotation": [
      2,
      "always"
    ]
  }
}

{
  "rules": {
    "flowtype/require-valid-file-annotation": [
      2,
      "always", {
        "annotationStyle": "block",
        "strict": true,
      }
    ]
  }
}

The following patterns are considered problems:

// Options: ["always"]
#!/usr/bin/env node
// Message: Flow file annotation is missing.

// Options: ["always"]
#!/usr/bin/env node
a;
// Message: Flow file annotation is missing.

;// @flow
// Message: Flow file annotation not at the top of the file.

;
// @flow
// Message: Flow file annotation not at the top of the file.

// @Flow
// Message: Malformed Flow file annotation.

// @NoFlow
// Message: Malformed Flow file annotation.

// @Noflow
// Message: Malformed Flow file annotation.

// @floweeeeeee
// Message: Misspelled or malformed Flow file annotation.

// @nofloweeeeeee
// Message: Misspelled or malformed Flow file annotation.

// Options: ["always"]
a;
// Message: Flow file annotation is missing.

// Options: ["always",{"annotationStyle":"line"}]
/* @flow */
// Message: Flow file annotation style must be `// @flow`

// Options: ["always",{"annotationStyle":"block"}]
// @flow
// Message: Flow file annotation style must be `/* @flow */`

// Options: ["always",{"annotationStyle":"block"}]
// @flow
// Message: Flow file annotation style must be `/* @flow */`

// Options: ["always",{"annotationStyle":"line","strict":true}]
// @flow
// Message: Strict Flow file annotation is required, must be `// @flow strict`

// Options: ["always",{"annotationStyle":"line"}]
/* @noflow */
// Message: Flow file annotation style must be `// @noflow`

// Options: ["always",{"annotationStyle":"block"}]
// @noflow
// Message: Flow file annotation style must be `/* @noflow */`

// Options: ["always"]
a;
// Message: Flow file annotation is missing.

// Options: ["always",{"annotationStyle":"block"}]
a;
// Message: Flow file annotation is missing.

// Options: ["always",{"annotationStyle":"line","strict":true}]
a;
// Message: Flow file annotation is missing.

// Options: ["always",{"annotationStyle":"line","strict":true}]
// @flow
a;
b;
// Message: Strict Flow file annotation is required, must be `// @flow strict`

// Options: ["never",{"annotationStyle":"line"}]
/* @flow */
a;
b;
// Message: Flow file annotation style must be `// @flow`

// Options: ["never",{"annotationStyle":"line"}]
/* @flow strict */
a;
b;
// Message: Flow file annotation style must be `// @flow strict`

The following patterns are not considered problems:

a;

// @flow
a;

//@flow
a;

//**@flow
a;

/* foo @flow bar */
a;



// @flow
a;

// @flow
// @FLow

// @noflow
a;

// Options: ["always"]
// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
a;

// Options: ["always",{"annotationStyle":"line"}]
// @flow

// Options: ["always",{"annotationStyle":"line","strict":true}]
// @noflow

// Options: ["always",{"annotationStyle":"line","strict":true}]
// @flow strict

// Options: ["never",{"annotationStyle":"none"}]
// @function

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

// Options: ["always",{"annotationStyle":"block"}]
/* @flow */

`require-variable-type`

Requires that all variable declarators have type annotations.

Options

You can exclude variables that match a certain regex by using excludeVariableMatch.

This excludes all parameters that start with an underscore (_). The default pattern is a^, which doesn't match anything, i.e., all parameters are checked.

{
    "rules": {
        "flowtype/require-variable-type": [
            2,
            {
              "excludeVariableMatch": "^_"
            }
        ]
    }
}

You can choose specific variable types (var, let, and const) to ignore using excludeVariableTypes.

This excludes var and let declarations from needing type annotations, but forces const declarations to have it. By default, all declarations are checked.

{
    "rules": {
        "flowtype/require-variable-type": [
            2,
            {
              "excludeVariableTypes": {
                "var": true,
                "let": true,
                "const": false,
              }
            }
        ]
    }
}

The following patterns are considered problems:

var foo = "bar"
// Message: Missing "foo" variable type annotation.

var foo : string = "bar", bar = 1
// Message: Missing "bar" variable type annotation.

// Options: [{"excludeVariableMatch":"^_"}]
var _foo = "bar", bar = 1
// Message: Missing "bar" variable type annotation.

// Options: [{"excludeVariableTypes":{"let":false,"var":true}}]
var foo = "bar", bar = 1; const oob : string = "oob"; let hey = "yah"
// Message: Missing "hey" variable type annotation.

The following patterns are not considered problems:

var foo : string = "bar"

var foo : string = "bar", bar : number = 1

// Options: [{"excludeVariableMatch":"^_"}]
var _foo = "bar", bar : number = 1

// Options: [{"excludeVariableTypes":{"var":true}}]
var foo = "bar", bar = 1

// Options: [{"excludeVariableTypes":{"let":true,"var":true}}]
var foo = "bar", bar = 1; const oob : string = "oob"; let hey = "yah"

`semi`

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

Enforces consistent use of semicolons after type aliases.

This rule takes one argument. If it is 'never' then a problem is raised when there is a semicolon after a type alias. If it is 'always' then a problem is raised when there is no semicolon after a type alias.

The default value is 'always'.

The following patterns are considered problems:

// Options: ["always"]
class Foo { foo: string }
// Message: Missing semicolon.

// Options: ["never"]
class Foo { foo: string; }
// Message: Extra semicolon.

// Options: []
type FooType = {}
// Message: Missing semicolon.

// Options: ["always"]
type FooType = {}
// Message: Missing semicolon.

// Options: ["never"]
type FooType = {};
// Message: Extra semicolon.

// Options: []
opaque type FooType = {}
// Message: Missing semicolon.

The following patterns are not considered problems:

type FooType = {};

// Options: ["always"]
type FooType = {};

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

// Options: ["always"]
class Foo { foo: string; }

// Options: ["never"]
class Foo { foo: string }

// Options: ["always"]
type FooType = { a: number;
 b: string;
 };

// Options: ["never"]
type FooType = { a: number;
 b: string;
 }

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

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type FooType = {}

opaque type FooType = {};

`sort-keys`

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

Enforces natural, case-insensitive sorting of Object annotations.

Options

The first option specifies sort order.

"asc" (default) - enforce ascending sort order.
"desc" - enforce descending sort order.

{
  "rules": {
    "flowtype/sort-keys": [
      2,
      "asc"
    ]
  }
}

The following patterns are considered problems:

type FooType = { a: number, c: number, b: string }
// Message: Expected type annotations to be in ascending order. "b" must be before "c".

// Options: ["desc"]
type FooType = { a: number, b: number }
// Message: Expected type annotations to be in descending order. "b" must be before "a".

// Options: ["desc"]
type FooType = { b: number, C: number, a: string }
// Message: Expected type annotations to be in descending order. "C" must be before "b".

// Options: ["asc"]
type FooType = { a: number, c: number, C: number, b: string }
// Message: Expected type annotations to be in ascending order. "b" must be before "C".

// Options: ["asc"]
type FooType = { a: number, C: number, c: number, b: string }
// Message: Expected type annotations to be in ascending order. "b" must be before "c".

// Options: ["asc"]
type FooType = { 1: number, 10: number, 2: boolean }
// Message: Expected type annotations to be in ascending order. "2" must be before "10".

type FooType = { a: number, c: number, b: string }
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          a: number,
          c: number,
          b: string,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          a: $ReadOnlyArray<number>,
          c: $ReadOnlyMap<string, number>,
          b: Map<string, Array<Map<string, number>>>,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          ...ErrorsInRecursiveGenericTypeArgsButDoesNotFix<{
            y: boolean,
            x: string,
            z: {
              j: string,
              l: number,
              k: boolean,
            },
          }>,
          a: number,
          c: string,
          b: Map<string, Array<ErrorsInRecursiveGenericTypeArgsButDoesNotFix<{
            y: boolean,
            x: string,
            z: {
              j: string,
              l: number,
              k: boolean,
            },
          }>>>,
        }
      
// Message: Expected type annotations to be in ascending order. "x" must be before "y".
// Message: Expected type annotations to be in ascending order. "k" must be before "l".
// Message: Expected type annotations to be in ascending order. "b" must be before "c".
// Message: Expected type annotations to be in ascending order. "x" must be before "y".
// Message: Expected type annotations to be in ascending order. "k" must be before "l".


        type FooType = {
          ...BPreservesSpreadOrder,
          ...APreservesSpreadOrder,
          c: string,
          b: number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          ...BPreservesSpreadSpans,
          ...APreservesSpreadSpans,
          c: string,
          b: number,
          ...CPreservesSpreadSpans,
          e: string,
          d: number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".
// Message: Expected type annotations to be in ascending order. "d" must be before "e".


        type FooType = {
          ...BPreservesSpreadOrderAndTypeArgs<string, number>,
          ...APreservesSpreadOrderAndTypeArgs<number>,
          c: string,
          b: number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          /* preserves block comment before spread BType */
          // preserves line comment before spread BType
          ... /* preserves comment in spread BType */ BType<Generic> /* preserves trailing comment in spread AType */,
          /* preserves block comment before spread AType */
          // preserves line comment before spread AType
          ... /* preserves comment in spread AType */ AType /* preserves trailing comment in spread AType */,
          /* preserves block comment before reordered key "c" */
          // preserves line comment before reordered key "c"
          c:/* preserves comment and white space or lack of it */string/* preserves trailing comment for key "c" */,
          b: number,
          dWithoutComma: boolean
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          +a: number,
          c: number,
          b: string,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          -a: number,
          c: number,
          b: string,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          a?: number,
          c: ?number,
          b: string,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          a: (number) => void,
          c: number,
          b: (param: string) => number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          a: number | string | boolean,
          c: number,
          b: (param: string) => number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          c: number,
          a: number | string | boolean,
          b: (param: string) => number,
        }
      
// Message: Expected type annotations to be in ascending order. "a" must be before "c".


        type FooType = {
          c: {
            z: number,
            x: string,
            y: boolean,
          },
          a: number | string | boolean,
          b: (param: string) => number,
        }
      
// Message: Expected type annotations to be in ascending order. "x" must be before "z".
// Message: Expected type annotations to be in ascending order. "a" must be before "c".


        type FooType = {
          c: {
            z: {
              j: string,
              l: number,
              k: boolean,
            },
            x: string,
            y: boolean,
          },
          a: number | string | boolean,
          b: (param: string) => number,
        }
      
// Message: Expected type annotations to be in ascending order. "k" must be before "l".
// Message: Expected type annotations to be in ascending order. "x" must be before "z".
// Message: Expected type annotations to be in ascending order. "a" must be before "c".


        type FooType = {
          +c: number,
          -b: number,
          a: number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".
// Message: Expected type annotations to be in ascending order. "a" must be before "b".


        type FooType = {|
          +c: number,
          -b: number,
          a: number,
        |}
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".
// Message: Expected type annotations to be in ascending order. "a" must be before "b".


        type FooType = {
          a(number): void,
          c: number,
          b(param: string): number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          a: number | string | boolean,
          c: number,
          b(param: string): number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        type FooType = {
          c: number,
          a: number | string | boolean,
          b(param: string): number,
        }
      
// Message: Expected type annotations to be in ascending order. "a" must be before "c".


        type FooType = {
          c: {
            z: number,
            x: string,
            y: boolean,
          },
          a: number | string | boolean,
          b(param: string): number,
        }
      
// Message: Expected type annotations to be in ascending order. "x" must be before "z".
// Message: Expected type annotations to be in ascending order. "a" must be before "c".


        type FooType = {
          c: {
            z: {
              j: string,
              l: number,
              k: boolean,
            },
            x: string,
            y: boolean,
          },
          a: number | string | boolean,
          b(param: string): number,
        }
      
// Message: Expected type annotations to be in ascending order. "k" must be before "l".
// Message: Expected type annotations to be in ascending order. "x" must be before "z".
// Message: Expected type annotations to be in ascending order. "a" must be before "c".


        type FooType = {
          /* preserves block comment before a */
          a: number | string | boolean,
          /* preserves block comment before c */
          c: number,
          /* preserves block comment before b */
          b(param: string): number,
        }
      
// Message: Expected type annotations to be in ascending order. "b" must be before "c".


        export type GroupOrdersResponseType = {|
          isSuccess: boolean,
          code: number,
          message?: string,
          errorMessage: string,
          result: {|
            OrderNumber: string,
            Orders: GroupOrderSummaryType[],
            PlacedOn: string,
            Status: string,
            ReturnText: string,
            IncludesLegacyOrder: boolean
          |}
        |};
      
// Message: Expected type annotations to be in ascending order. "code" must be before "isSuccess".
// Message: Expected type annotations to be in ascending order. "errorMessage" must be before "message".
// Message: Expected type annotations to be in ascending order. "ReturnText" must be before "Status".
// Message: Expected type annotations to be in ascending order. "IncludesLegacyOrder" must be before "ReturnText".

The following patterns are not considered problems:

type FooType = { a: number }

type FooType = { a: number, b: number, c: (boolean | number) }

type FooType = { a: string, b: foo, C: number }

type FooType = { 1: number, 2: boolean, 10: number }

// Options: ["desc"]
type FooType = { c: number, b: number, a: number }

// Options: ["desc"]
type FooType = { C: number, b: string, a: {} }

// Options: ["desc"]
type FooType = { 10: number, 2: number, 1: boolean }

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type FooType = { b: number, a: number }

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

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

`sort-type-union-intersection-members`

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

Enforces that members of a type union/intersection are sorted alphabetically.

Options

You can specify the sort order using order.

"asc" (default) - enforce ascending sort order.
"desc" - enforce descending sort order.

{
  "rules": {
    "flowtype/sort-type-union-intersection-members": [
      2,
      {
        "order": "asc"
      }
    ]
  }
}

You can disable checking intersection types using checkIntersections.

true (default) - enforce sort order of intersection members.
false - do not enforce sort order of intersection members.

{
  "rules": {
    "flowtype/sort-type-union-intersection-members": [
      2,
      {
        "checkIntersections": true
      }
    ]
  }
}

You can disable checking union types using checkUnions.

true (default) - enforce sort order of union members.
false - do not enforce sort order of union members.

{
  "rules": {
    "flowtype/sort-type-union-intersection-members": [
      2,
      {
        "checkUnions": true
      }
    ]
  }
}

You can specify the ordering of groups using groupOrder.

Each member of the type is placed into a group, and then the rule sorts alphabetically within each group. The ordering of groups is determined by this option.

keyword - Keyword types (any, string, etc)
named - Named types (A, A['prop'], B[], Array<C>)
literal - Literal types (1, 'b', true, etc)
function - Function types (() => void)
object - Object types ({ a: string }, { [key: string]: number })
tuple - Tuple types ([A, B, C])
intersection - Intersection types (A & B)
union - Union types (A | B)
nullish - null and undefined

{
  "rules": {
    "flowtype/sort-type-union-intersection-members": [
      2,
      {
        "groupOrder": [
          'keyword',
          'named',
          'literal',
          'function',
          'object',
          'tuple',
          'intersection',
          'union',
          'nullish',
        ]
      }
    ]
  }
}

The following patterns are considered problems:

type T1 = B | A;
// Message: Expected union members to be in ascending order. "A" should be before "B".

type T2 = { b: string } & { a: string };
// Message: Expected intersection members to be in ascending order. "{ a: string }" should be before "{ b: string }".

type T3 = [1, 2, 4] & [1, 2, 3];
// Message: Expected intersection members to be in ascending order. "[1, 2, 3]" should be before "[1, 2, 4]".


        type T4 =
          | [1, 2, 4]
          | [1, 2, 3]
          | { b: string }
          | { a: string }
          | (() => void)
          | (() => string)
          | 'b'
          | 'a'
          | 'b'
          | 'a'
          | string[]
          | number[]
          | B
          | A
          | string
          | any;
      
// Message: Expected union members to be in ascending order. "[1, 2, 3]" should be before "[1, 2, 4]".
// Message: Expected union members to be in ascending order. "{ b: string }" should be before "[1, 2, 3]".
// Message: Expected union members to be in ascending order. "{ a: string }" should be before "{ b: string }".
// Message: Expected union members to be in ascending order. "() => void" should be before "{ a: string }".
// Message: Expected union members to be in ascending order. "() => string" should be before "() => void".
// Message: Expected union members to be in ascending order. "'b'" should be before "() => string".
// Message: Expected union members to be in ascending order. "'a'" should be before "'b'".
// Message: Expected union members to be in ascending order. "'b'" should be before "'a'".
// Message: Expected union members to be in ascending order. "'a'" should be before "'b'".
// Message: Expected union members to be in ascending order. "string[]" should be before "'a'".
// Message: Expected union members to be in ascending order. "number[]" should be before "string[]".
// Message: Expected union members to be in ascending order. "B" should be before "number[]".
// Message: Expected union members to be in ascending order. "A" should be before "B".
// Message: Expected union members to be in ascending order. "string" should be before "A".
// Message: Expected union members to be in ascending order. "any" should be before "string".

The following patterns are not considered problems:

type T1 = A | B;

type T2 = { a: string } & { b: string };

type T3 = [1, 2, 3] & [1, 2, 4];


        type T4 =
          | any
          | string
          | A
          | B
          | number[]
          | string[]
          | 'a'
          | 'a'
          | 'b'
          | 'b'
          | (() => string)
          | (() => void)
          | { a: string }
          | { b: string }
          | [1, 2, 3]
          | [1, 2, 4];

`space-after-type-colon`

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

Enforces consistent spacing after the type annotation colon.

Options

This rule has a string argument.

"always" (default): Require a space after the type annotation colon (e.g. foo: BarType).
"never": Require no spaces after the type annotation colon (e.g. foo:BarType).

This rule has an option object.

"allowLineBreak" - Allow a line break to count as a space following the annotation colon.
- "true": Enable
- "false": Disable

{ "rules": { "flowtype/space-after-type-colon": [ 2, "always", { "allowLineBreak": false } ] } }

The following patterns are considered problems:

// Options: ["never"]
(foo: string) => {}
// Message: There must be no space after "foo" parameter type annotation colon.

// Options: ["always"]
(foo:  string) => {}
// Message: There must be 1 space after "foo" parameter type annotation colon.

// Options: ["always"]
(foo:(() => void)) => {}
// Message: There must be a space after "foo" parameter type annotation colon.

// Options: ["never"]
(foo: (() => void)) => {}
// Message: There must be no space after "foo" parameter type annotation colon.

// Options: ["always"]
(foo:  (() => void)) => {}
// Message: There must be 1 space after "foo" parameter type annotation colon.

({ lorem, ipsum, dolor } :   SomeType) => {}
// Message: There must be 1 space after "{ lorem, ipsum, dolor }" parameter type annotation colon.

(foo:{ a: string, b: number }) => {}
// Message: There must be a space after "foo" parameter type annotation colon.

({ a, b } :{ a: string, b: number }) => {}
// Message: There must be a space after "{ a, b }" parameter type annotation colon.

([ a, b ] :string[]) => {}
// Message: There must be a space after "[ a, b ]" parameter type annotation colon.

(i?:number) => {}
// Message: There must be a space after "i" parameter type annotation colon.

(i?:  number) => {}
// Message: There must be 1 space after "i" parameter type annotation colon.

// Options: ["never"]
(i?: number) => {}
// Message: There must be no space after "i" parameter type annotation colon.

(foo:
  { a: string, b: number }) => {}
// Message: There must not be a line break after "foo" parameter type annotation colon.

(foo:
{ a: string, b: number }) => {}
// Message: There must not be a line break after "foo" parameter type annotation colon.

(foo: 
{ a: string, b: number }) => {}
// Message: There must not be a line break after "foo" parameter type annotation colon.

// Options: ["always"]
():Object => {}
// Message: There must be a space after return type colon.

// Options: ["never"]
(): Object => {}
// Message: There must be no space after return type colon.

// Options: ["always"]
():  Object => {}
// Message: There must be 1 space after return type colon.

// Options: ["always"]
():(() => void) => {}
// Message: There must be a space after return type colon.

// Options: ["never"]
(): (() => void) => {}
// Message: There must be no space after return type colon.

// Options: ["always"]
():  (() => void) => {}
// Message: There must be 1 space after return type colon.

// Options: ["never"]
export default function (foo: string) {}
// Message: There must be no space after "foo" parameter type annotation colon.

// Options: ["never"]
function foo (foo: string) {}
// Message: There must be no space after "foo" parameter type annotation colon.

// Options: ["always"]
(foo:string) => {}
// Message: There must be a space after "foo" parameter type annotation colon.

function foo (foo:string) {}
// Message: There must be a space after "foo" parameter type annotation colon.

async function foo({ lorem, ipsum, dolor }:SomeType) {}
// Message: There must be a space after "{ lorem, ipsum, dolor }" parameter type annotation colon.

function x(i?:number) {}
// Message: There must be a space after "i" parameter type annotation colon.

function x(i?:  number) {}
// Message: There must be 1 space after "i" parameter type annotation colon.

// Options: ["never"]
function x(i?: number) {}
// Message: There must be no space after "i" parameter type annotation colon.

function a():x {}
// Message: There must be a space after return type colon.

// Options: ["always"]
function a():  x {}
// Message: There must be 1 space after return type colon.

// Options: ["never"]
function a(): x {}
// Message: There must be no space after return type colon.

type X = (foo:number) => string
// Message: There must be a space after "foo" parameter type annotation colon.

// Options: ["never"]
type X = (foo: number) => string
// Message: There must be no space after "foo" parameter type annotation colon.

type X = (foo:  number) => string
// Message: There must be 1 space after "foo" parameter type annotation colon.

type X = (foo:?number) => string
// Message: There must be a space after "foo" parameter type annotation colon.

type X = (foo:(number)) => string
// Message: There must be a space after "foo" parameter type annotation colon.

type X = (foo:((number))) => string
// Message: There must be a space after "foo" parameter type annotation colon.

type X = (foo:  ((number))) => string
// Message: There must be 1 space after "foo" parameter type annotation colon.

// Options: ["never"]
type X = (foo: ((number))) => string
// Message: There must be no space after "foo" parameter type annotation colon.

type X = (foo:?(number)) => string
// Message: There must be a space after "foo" parameter type annotation colon.

type TArrayPredicate = (el: T, i?:number) => boolean
// Message: There must be a space after "i" parameter type annotation colon.

type TArrayPredicate = (el: T, i?:  number) => boolean
// Message: There must be 1 space after "i" parameter type annotation colon.

// Options: ["never"]
type TArrayPredicate = (el:T, i?: number) => boolean
// Message: There must be no space after "i" parameter type annotation colon.

class X { foo:string }
// Message: There must be a space after "foo" class property type annotation colon.

// Options: ["never"]
class X { foo: string }
// Message: There must be no space after "foo" class property type annotation colon.

class X { foo:?string }
// Message: There must be a space after "foo" class property type annotation colon.

// Options: ["never"]
class X { foo: ?string }
// Message: There must be no space after "foo" class property type annotation colon.

class X { static foo:number }
// Message: There must be a space after "foo" class property type annotation colon.

// Options: ["never"]
class X { static foo: number }
// Message: There must be no space after "foo" class property type annotation colon.

class X { static foo :number }
// Message: There must be a space after "foo" class property type annotation colon.

// Options: ["never"]
class X { static foo : number }
// Message: There must be no space after "foo" class property type annotation colon.

declare class X { static foo:number }
// Message: There must be a space after "foo" type annotation colon.

// Options: ["never"]
declare class X { static foo: number }
// Message: There must be no space after "foo" type annotation colon.

declare class X { static foo :number }
// Message: There must be a space after "foo" type annotation colon.

// Options: ["never"]
declare class X { static foo : number }
// Message: There must be no space after "foo" type annotation colon.

class X { +foo:string }
// Message: There must be a space after "foo" class property type annotation colon.

class X { +foo:  string }
// Message: There must be 1 space after "foo" class property type annotation colon.

// Options: ["never"]
class X { +foo: string }
// Message: There must be no space after "foo" class property type annotation colon.

class X { static +foo:string }
// Message: There must be a space after "foo" class property type annotation colon.

class X { static +foo:  string }
// Message: There must be 1 space after "foo" class property type annotation colon.

// Options: ["never"]
class X { static +foo: string }
// Message: There must be no space after "foo" class property type annotation colon.

type X = { foo:string }
// Message: There must be a space after "foo" type annotation colon.

// Options: ["always"]
type X = { foo:string }
// Message: There must be a space after "foo" type annotation colon.

// Options: ["never"]
type X = { foo: string }
// Message: There must be no space after "foo" type annotation colon.

type X = { foo:  string }
// Message: There must be 1 space after "foo" type annotation colon.

type X = { foo?:string }
// Message: There must be a space after "foo" type annotation colon.

// Options: ["never"]
type X = { foo?: string }
// Message: There must be no space after "foo" type annotation colon.

type X = { foo?:?string }
// Message: There must be a space after "foo" type annotation colon.

type X = { foo?:  ?string }
// Message: There must be 1 space after "foo" type annotation colon.

type Foo = { barType:(string | () => void) }
// Message: There must be a space after "barType" type annotation colon.

type Foo = { barType:(((string | () => void))) }
// Message: There must be a space after "barType" type annotation colon.

// Options: ["never"]
type Foo = { barType: (string | () => void) }
// Message: There must be no space after "barType" type annotation colon.

type Foo = { barType:  (string | () => void) }
// Message: There must be 1 space after "barType" type annotation colon.

type Foo = { barType:  ((string | () => void)) }
// Message: There must be 1 space after "barType" type annotation colon.

type X = { get:() => A; }
// Message: There must be a space after "get" type annotation colon.

type X = { get:<X>() => A; }
// Message: There must be a space after "get" type annotation colon.

// Options: ["never"]
type X = { get: () => A; }
// Message: There must be no space after "get" type annotation colon.

// Options: ["never"]
type X = { get: <X>() => A; }
// Message: There must be no space after "get" type annotation colon.

type X = { get:  () => A; }
// Message: There must be 1 space after "get" type annotation colon.

type X = { get:  <X>() => A; }
// Message: There must be 1 space after "get" type annotation colon.

type X = { +foo:string }
// Message: There must be a space after "foo" type annotation colon.

type X = { +foo:  string }
// Message: There must be 1 space after "foo" type annotation colon.

// Options: ["never"]
type X = { +foo: string }
// Message: There must be no space after "foo" type annotation colon.

type X = { +foo?:string }
// Message: There must be a space after "foo" type annotation colon.

type X = { +foo?:  string }
// Message: There must be 1 space after "foo" type annotation colon.

// Options: ["never"]
type X = { +foo?: string }
// Message: There must be no space after "foo" type annotation colon.

// Options: ["always"]
type X = { [a:b]: c }
// Message: There must be a space after type annotation colon.

// Options: ["never"]
type X = { [a: b]:c }
// Message: There must be no space after type annotation colon.

// Options: ["always"]
type X = { [a:    b]: c }
// Message: There must be 1 space after type annotation colon.

// Options: ["always"]
type X = { +[a:b]: c }
// Message: There must be a space after type annotation colon.

// Options: ["never"]
type X = { +[a: b]:c }
// Message: There must be no space after type annotation colon.

// Options: ["always"]
type X = { +[a:    b]: c }
// Message: There must be 1 space after type annotation colon.

// Options: ["always"]
type X = { [a: b]:c }
// Message: There must be a space after type annotation colon.

// Options: ["never"]
type X = { [a:b]: c }
// Message: There must be no space after type annotation colon.

// Options: ["always"]
type X = { [a: b]:    c }
// Message: There must be 1 space after type annotation colon.

// Options: ["always"]
type X = { [a:b]:c }
// Message: There must be a space after type annotation colon.
// Message: There must be a space after type annotation colon.

// Options: ["never"]
type X = { [a: b]: c }
// Message: There must be no space after type annotation colon.
// Message: There must be no space after type annotation colon.

// Options: ["always"]
type X = { [a:  b]:  c }
// Message: There must be 1 space after type annotation colon.
// Message: There must be 1 space after type annotation colon.

// Options: ["always"]
type X = { [a:(b)]:(c) }
// Message: There must be a space after type annotation colon.
// Message: There must be a space after type annotation colon.

// Options: ["never"]
type X = { [a: (b)]: (c) }
// Message: There must be no space after type annotation colon.
// Message: There must be no space after type annotation colon.

// Options: ["never"]
const x = ({}: {})
// Message: There must be no space after type cast colon.

// Options: ["always"]
const x = ({}:{})
// Message: There must be a space after type cast colon.

// Options: ["always"]
const x = ({}:  {})
// Message: There must be 1 space after type cast colon.

// Options: ["never"]
((x): (string))
// Message: There must be no space after type cast colon.

// Options: ["always"]
((x):(string))
// Message: There must be a space after type cast colon.

// Options: ["always"]
((x):  (string))
// Message: There must be 1 space after type cast colon.

// Options: ["always"]
const x:number = 7;
// Message: There must be a space after const type annotation colon.

// Options: ["always"]
let x:number = 42;
// Message: There must be a space after let type annotation colon.

// Options: ["always"]
var x:number = 42;
// Message: There must be a space after var type annotation colon.

The following patterns are not considered problems:

(foo) => {}

(foo: string) => {}

(foo: (string|number)) => {}

// Options: ["never"]
(foo:string) => {}

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

// Options: ["never"]
(foo:(() => void)) => {}

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

({ lorem, ipsum, dolor }: SomeType) => {}

(foo: { a: string, b: number }) => {}

({ a, b }: ?{ a: string, b: number }) => {}

([ a, b ]: string[]) => {}

(i?: number) => {}

// Options: ["never"]
(i?:number) => {}

// Options: ["always",{"allowLineBreak":true}]
(foo:
  { a: string, b: number }) => {}

// Options: ["always",{"allowLineBreak":true}]
(foo:
  { a: string, b: number }) => {}

// Options: ["never"]
():Object => {}

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

// Options: ["never"]
():(number | string) => {}

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

// Options: ["never"]
():number|string => {}

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

// Options: ["never"]
():(() => void) => {}

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

// Options: ["never"]
():( () => void ) => {}

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

(): { a: number, b: string } => {}

// Options: ["never"]
() :{ a:number, b:string } => {}

function x(foo: string) {}

class Foo { constructor(foo: string) {} }

// Options: ["never"]
function x(foo:string) {}

// Options: ["never"]
class Foo { constructor(foo:string) {} }

async function foo({ lorem, ipsum, dolor }: SomeType) {}

function x({ a, b }: { a: string, b: number }) {}

function x(i?: number) {}

// Options: ["never"]
function x(i?:number) {}

function a(): x {}

// Options: ["never"]
function a():x {}

function a(): (number | string) {}

// Options: ["never"]
function a() :(number | string) {}

type X = (foo: number) => string;

type X = (foo : number) => string;

type X = (foo: ?number) => string;

type X = (foo? : ?number) => string;

type X = (foo: ?{ x: number }) => string;

// Options: ["never"]
type X = (foo:number) => string;

// Options: ["never"]
type X = (foo:?{ x:number }) => string;

type X = (foo: (number)) => string

type X = (foo: ((number))) => string

// Options: ["never"]
type X = (foo:((number))) => string

type X = ?(foo: ((number))) => string

// Options: ["never"]
type X = ?(foo:((number))) => string

type TArrayPredicate = (el: T, i?: number) => boolean

// Options: ["never"]
type TArrayPredicate = (el:T, i?:number) => boolean

type X = (number) => string;

type X = (?number) => string;

type X = number => string;

type X = ?number => string;

type X = ({ foo: bar }) => string;

// Options: ["always"]
type X = (number) => string;

// Options: ["always"]
type X = (?number) => string;

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

// Options: ["always"]
type X = ?number => string;

// Options: ["always"]
type X = ({ foo: bar }) => string;

class Foo { bar }

class Foo { bar = 3 }

class Foo { bar: string }

class Foo { bar: ?string }

// Options: ["never"]
class Foo { bar:string }

// Options: ["never"]
class Foo { bar:?string }

class X { static foo : number }

// Options: ["never"]
class X { static foo :number }

declare class X { static foo : number }

// Options: ["never"]
declare class X { static foo :number }

class X { +foo: string }

class X { static +foo: string }

// Options: ["never"]
class X { +foo:string }

// Options: ["never"]
class X { static +foo:string }

type X = { foo: string }

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

type X = { foo?: string }

type X = { foo?: ?string }

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

type Foo = { barType: (string | () => void) }

type Foo = { barType: ((string | () => void)) }

// Options: ["never"]
type Foo = { barType:(string | () => void) }

// Options: ["never"]
type Foo = { barType:((string | () => void)) }

type X = { get(): A; }

type X = { get<X>(): A; }

// Options: ["never"]
type X = { get(): A; }

// Options: ["never"]
type X = { get<X>(): A; }

type X = { get: () => A; }

type X = { get: <X>() => A; }

// Options: ["never"]
type X = { get:() => A; }

// Options: ["never"]
type X = { get:<X>() => A; }

type X = { +foo: string }

type X = { +foo?: string }

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

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

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

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

// Options: ["always"]
type X = { +[a: b]: c }

// Options: ["never"]
type X = { +[a:b]:c }

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

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

// Options: ["never"]
const x = ({}:{})

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

// Options: ["never"]
((x):(string))

// Options: ["always"]
((x): (string))

// Options: ["always"]
const x: number = 7;

// Options: ["always"]
let x: number = 42;

// Options: ["always"]
var x: number = 42;

`space-before-generic-bracket`

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

Enforces consistent spacing before the opening < of generic type annotation parameters.

This rule takes one argument. If it is 'never' then a problem is raised when there is a space before the <. If it is 'always' then a problem is raised when there is no space before the <.

The default value is 'never'.

The following patterns are considered problems:

type X = Promise <string>
// Message: There must be no space before "Promise" generic type annotation bracket

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

type X = Promise  <string>
// Message: There must be no space before "Promise" generic type annotation bracket

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

// Options: ["always"]
type X = Promise  <string>
// Message: There must be one space before "Promise" generic type annotation bracket

The following patterns are not considered problems:

type X = Promise<string>

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

`space-before-type-colon`

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

Enforces consistent spacing before the type annotation colon.

This rule takes one argument. If it is 'always' then a problem is raised when there is no space before the type annotation colon. If it is 'never' then a problem is raised when there is a space before the type annotation colon. The default value is 'never'.

The following patterns are considered problems:

// Options: ["never"]
(foo : string) => {}
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["never"]
(foo ? : string) => {}
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["always"]
(foo: string) => {}
// Message: There must be a space before "foo" parameter type annotation colon.

// Options: ["always"]
(foo  : string) => {}
// Message: There must be 1 space before "foo" parameter type annotation colon.

// Options: ["always"]
(foo?: string) => {}
// Message: There must be a space before "foo" parameter type annotation colon.

// Options: ["always"]
(foo ?  : string) => {}
// Message: There must be 1 space before "foo" parameter type annotation colon.

// Options: ["always"]
(foo  ?: string) => {}
// Message: There must be a space before "foo" parameter type annotation colon.

({ lorem, ipsum, dolor } : SomeType) => {}
// Message: There must be no space before "{ lorem, ipsum, dolor }" parameter type annotation colon.

(foo : { a: string, b: number }) => {}
// Message: There must be no space before "foo" parameter type annotation colon.

({ a, b } : { a: string, b: number }) => {}
// Message: There must be no space before "{ a, b }" parameter type annotation colon.

([ a, b ] : string[]) => {}
// Message: There must be no space before "[ a, b ]" parameter type annotation colon.

() : x => {}
// Message: There must be no space before return type colon.

// Options: ["always"]
(): x => {}
// Message: There must be a space before return type colon.

// Options: ["always"]
()  : x => {}
// Message: There must be 1 space before return type colon.

function x(foo : string) {}
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["always"]
function x(foo: string) {}
// Message: There must be a space before "foo" parameter type annotation colon.

var x = function (foo : string) {}
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["always"]
var x = function (foo: string) {}
// Message: There must be a space before "foo" parameter type annotation colon.

class Foo { constructor(foo : string ) {} }
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["always"]
class Foo { constructor(foo: string ) {} }
// Message: There must be a space before "foo" parameter type annotation colon.

async function foo({ lorem, ipsum, dolor } : SomeType) {}
// Message: There must be no space before "{ lorem, ipsum, dolor }" parameter type annotation colon.

function a() : x {}
// Message: There must be no space before return type colon.

// Options: ["always"]
function a(): x {}
// Message: There must be a space before return type colon.

// Options: ["always"]
function a()  : x {}
// Message: There must be 1 space before return type colon.

type X = (foo :string) => string;
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["always"]
type X = (foo:string) => string;
// Message: There must be a space before "foo" parameter type annotation colon.

// Options: ["always"]
type X = (foo  :string) => string;
// Message: There must be 1 space before "foo" parameter type annotation colon.

type X = (foo? :string) => string;
// Message: There must be no space before "foo" parameter type annotation colon.

type X = (foo?     :string) => string;
// Message: There must be no space before "foo" parameter type annotation colon.

// Options: ["always"]
type X = (foo?:string) => string;
// Message: There must be a space before "foo" parameter type annotation colon.

type X = (foo? :?string) => string;
// Message: There must be no space before "foo" parameter type annotation colon.

class X { foo :string }
// Message: There must be no space before "foo" class property type annotation colon.

// Options: ["always"]
class X { foo: string }
// Message: There must be a space before "foo" class property type annotation colon.

class X { foo :?string }
// Message: There must be no space before "foo" class property type annotation colon.

// Options: ["always"]
class X { foo: ?string }
// Message: There must be a space before "foo" class property type annotation colon.

class X { static foo : number }
// Message: There must be no space before "foo" class property type annotation colon.

class X { static foo :number }
// Message: There must be no space before "foo" class property type annotation colon.

// Options: ["always"]
class X { static foo: number }
// Message: There must be a space before "foo" class property type annotation colon.

// Options: ["always"]
class X { static foo:number }
// Message: There must be a space before "foo" class property type annotation colon.

declare class Foo { static bar :number; }
// Message: There must be no space before "bar" type annotation colon.

declare class Foo { static bar : number; }
// Message: There must be no space before "bar" type annotation colon.

// Options: ["always"]
declare class Foo { static bar:number; }
// Message: There must be a space before "bar" type annotation colon.

// Options: ["always"]
declare class Foo { static bar: number; }
// Message: There must be a space before "bar" type annotation colon.

// Options: ["always"]
class X { +foo: string }
// Message: There must be a space before "foo" class property type annotation colon.

// Options: ["always"]
class X { +foo  : string }
// Message: There must be 1 space before "foo" class property type annotation colon.

// Options: ["never"]
class X { +foo : string }
// Message: There must be no space before "foo" class property type annotation colon.

// Options: ["always"]
class X { static +foo: string }
// Message: There must be a space before "foo" class property type annotation colon.

// Options: ["always"]
class X { static +foo  : string }
// Message: There must be 1 space before "foo" class property type annotation colon.

// Options: ["never"]
class X { static +foo : string }
// Message: There must be no space before "foo" class property type annotation colon.

type X = { foo : string }
// Message: There must be no space before "foo" type annotation colon.

// Options: ["never"]
type X = { foo : string }
// Message: There must be no space before "foo" type annotation colon.

// Options: ["always"]
type X = { foo: string }
// Message: There must be a space before "foo" type annotation colon.

// Options: ["always"]
type X = { foo  : string }
// Message: There must be 1 space before "foo" type annotation colon.

type X = { foo? : string }
// Message: There must be no space before "foo" type annotation colon.

// Options: ["always"]
type X = { foo?: string }
// Message: There must be a space before "foo" type annotation colon.

// Options: ["always"]
type X = { foo?  : string }
// Message: There must be 1 space before "foo" type annotation colon.

// Options: ["always"]
type X = { foo   ?: string }
// Message: There must be a space before "foo" type annotation colon.

// Options: ["always"]
type X = { +foo: string }
// Message: There must be a space before "foo" type annotation colon.

// Options: ["always"]
type X = { +foo  : string }
// Message: There must be 1 space before "foo" type annotation colon.

// Options: ["never"]
type X = { +foo : string }
// Message: There must be no space before "foo" type annotation colon.

// Options: ["always"]
type X = { +foo?: string }
// Message: There must be a space before "foo" type annotation colon.

// Options: ["always"]
type X = { +foo?  : string }
// Message: There must be 1 space before "foo" type annotation colon.

// Options: ["never"]
type X = { +foo? : string }
// Message: There must be no space before "foo" type annotation colon.

// Options: ["always"]
type X = { [a: b] : c }
// Message: There must be a space before type annotation colon.

// Options: ["never"]
type X = { [a : b]: c }
// Message: There must be no space before type annotation colon.

// Options: ["always"]
type X = { [a  : b] : c }
// Message: There must be 1 space before type annotation colon.

// Options: ["always"]
type X = { +[a:b] : c }
// Message: There must be a space before type annotation colon.

// Options: ["never"]
type X = { +[a : b]: c }
// Message: There must be no space before type annotation colon.

// Options: ["always"]
type X = { +[a  : b] : c }
// Message: There must be 1 space before type annotation colon.

// Options: ["always"]
type X = { [a : b]: c }
// Message: There must be a space before type annotation colon.

// Options: ["never"]
type X = { [a: b] : c }
// Message: There must be no space before type annotation colon.

// Options: ["always"]
type X = { [a : b]  : c }
// Message: There must be 1 space before type annotation colon.

// Options: ["always"]
type X = { [a:b]:c }
// Message: There must be a space before type annotation colon.
// Message: There must be a space before type annotation colon.

// Options: ["never"]
type X = { [a : b] : c }
// Message: There must be no space before type annotation colon.
// Message: There must be no space before type annotation colon.

// Options: ["always"]
type X = { [a  : b]  : c }
// Message: There must be 1 space before type annotation colon.
// Message: There must be 1 space before type annotation colon.

// Options: ["always"]
type X = { [a:(b)]:(c) }
// Message: There must be a space before type annotation colon.
// Message: There must be a space before type annotation colon.

// Options: ["never"]
type X = { [a : (b)] : (c) }
// Message: There must be no space before type annotation colon.
// Message: There must be no space before type annotation colon.

// Options: ["never"]
const x = ({} :{})
// Message: There must be no space before type cast colon.

// Options: ["always"]
const x = ({}:{})
// Message: There must be a space before type cast colon.

// Options: ["always"]
const x = ({}  :{})
// Message: There must be 1 space before type cast colon.

// Options: ["never"]
((x) : string)
// Message: There must be no space before type cast colon.

// Options: ["always"]
((x): string)
// Message: There must be a space before type cast colon.

// Options: ["always"]
((x)  : string)
// Message: There must be 1 space before type cast colon.

// Options: ["always"]
const x:number = 7;
// Message: There must be a space before const type annotation colon.

// Options: ["always"]
let x:number = 42;
// Message: There must be a space before let type annotation colon.

// Options: ["always"]
var x:number = 42;
// Message: There must be a space before var type annotation colon.

The following patterns are not considered problems:

(foo) => {}

(foo: string) => {}

(foo?: string) => {}

(foo ?: string) => {}

// Options: ["never"]
(foo: string) => {}

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

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

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

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

({ lorem, ipsum, dolor }: SomeType) => {}

(foo: { a: string, b: number }) => {}

({ a, b }: ?{ a: string, b: number }) => {}

(): { a: number, b: string } => {}

// Options: ["always"]
() : { a : number, b : string } => {}

([ a, b ]: string[]) => {}

(): x => {}

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

(): (number | string) => {}

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

function x(foo: string) {}

// Options: ["always"]
function x(foo : string) {}

var x = function (foo: string) {}

// Options: ["always"]
var x = function (foo : string) {}

class X { foo({ bar }: Props = this.props) {} }

class Foo { constructor(foo: string ) {} }

// Options: ["always"]
class Foo { constructor(foo : string ) {} }

async function foo({ lorem, ipsum, dolor }: SomeType) {}

function x({ a, b }: { a: string, b: number }) {}

function a(): x {}

// Options: ["always"]
function a() : x {}

function a(): (number | string) {}

// Options: ["always"]
function a() : (number | string) {}

type X = (foo:string) => number;

type X = (foo: string) => number;

type X = (foo: ?string) => number;

type X = (foo?: string) => number;

type X = (foo?: ?string) => number;

type X = (foo   ?: string) => number;

// Options: ["always"]
type X = (foo? : string) => number

// Options: ["always"]
type X = (foo? : ?string) => number

type X = (number) => string;

type X = (?number) => string;

type X = number => string;

type X = ?number => string;

type X = ({ foo: bar }) => string;

// Options: ["always"]
type X = (number) => string;

// Options: ["always"]
type X = (?number) => string;

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

// Options: ["always"]
type X = ?number => string;

// Options: ["always"]
type X = ({ foo : bar }) => string;

class Foo { bar }

class Foo { bar = 3 }

class Foo { bar: string }

class Foo { bar: ?string }

class Foo { bar:?string }

// Options: ["always"]
class Foo { bar : string }

class X { static foo:number }

class X { static foo: number }

// Options: ["always"]
class X { static foo :number }

// Options: ["always"]
class X { static foo : number }

declare class Foo { static bar:number; }

// Options: ["always"]
declare class Foo { static bar :number; }

declare class Foo { static bar: number; }

// Options: ["always"]
declare class Foo { static bar : number; }

class X { +foo: string }

class X { static +foo: string }

// Options: ["always"]
class X { +foo : string }

// Options: ["always"]
class X { static +foo : string }

type X = { foo: string }

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

type X = { foo?: string }

type X = { foo   ?: string }

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

type X = { +foo: string }

type X = { +foo?: string }

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

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

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

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

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

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

// Options: ["always"]
type X = { +[a : b] : c }

// Options: ["never"]
type X = { +[a:b]:c }

// Options: ["always"]
type X = { [a : (b)] : (c) }

// Options: ["never"]
type X = { [a:(b)]:(c) }

// Options: ["never"]
const x = ({}:{})

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

// Options: ["never"]
((x): string)

// Options: ["always"]
((x) : string)

// Options: ["always"]
const x :number = 7;

// Options: ["always"]
let x :number = 42;

// Options: ["always"]
var x :number = 42;

`spread-exact-type`

Enforce object types, that are spread to be exact type explicitly.

The following patterns are considered problems:

type bar = {...{test: string}}
// Message: Use $Exact to make type spreading safe.

type foo = {test: number}; type bar = {...foo}
// Message: Use $Exact to make type spreading safe.

The following patterns are not considered problems:

type bar = {...$Exact<{test: string}>}

type foo = {test: number}; type bar = {...$Exact<foo>}

`type-id-match`

Enforces a consistent naming pattern for type aliases.

Options

This rule requires a text RegExp:

{
    "rules": {
        "flowtype/type-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:

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

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

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

The following patterns are not considered problems:

type FooType = {};

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

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type foo = {};

`type-import-style`

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

Enforces a particular style for type imports:

// 'identifier' style
import {type T, type U, type V} from '...';

// 'declaration' style
import type {T, U, V} from '...';

Options

The rule has a string option:

"identifier" (default): Enforces that type imports are all in the 'identifier' style.
"declaration": Enforces that type imports are all in the 'declaration' style.

This rule has an object option:

ignoreTypeDefault - if true, when in "identifier" mode, default type imports will be ignored. Default is false.

The following patterns are considered problems:

import type {A, B} from 'a';
// Message: Unexpected "import type"

// Options: ["identifier"]
import type {A, B} from 'a';
// Message: Unexpected "import type"

// Options: ["identifier"]
import type {A, B as C} from 'a';
// Message: Unexpected "import type"

// Options: ["identifier"]
import type A from 'a';
// Message: Unexpected "import type"

// Options: ["declaration"]
import {type A, type B} from 'a';
// Message: Unexpected type import
// Message: Unexpected type import

The following patterns are not considered problems:

import {type A, type B} from 'a';

// Options: ["identifier"]
import {type A, type B} from 'a';

// Options: ["declaration"]
import type {A, B} from 'a';

// Options: ["identifier"]
import typeof * as A from 'a';

// Options: ["identifier",{"ignoreTypeDefault":true}]
import type A from 'a';

// Options: ["identifier"]
declare module "m" { import type A from 'a'; }

`union-intersection-spacing`

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

Enforces consistent spacing around union and intersection type separators (| and &).

This rule takes one argument. If it is 'always' then a problem is raised when there is no space around the separator. If it is 'never' then a problem is raised when there is a space around the separator.

The default value is 'always'.

The following patterns are considered problems:

type X = string| number;
// Message: There must be a space before union type annotation separator

// Options: ["always"]
type X = string| number;
// Message: There must be a space before union type annotation separator

type X = string |number;
// Message: There must be a space after union type annotation separator

type X = string|number;
// Message: There must be a space before union type annotation separator
// Message: There must be a space after union type annotation separator

type X = {x: string}|{y: number};
// Message: There must be a space before union type annotation separator
// Message: There must be a space after union type annotation separator

type X = string | number |boolean;
// Message: There must be a space after union type annotation separator

type X = string|number|boolean;
// Message: There must be a space before union type annotation separator
// Message: There must be a space after union type annotation separator
// Message: There must be a space before union type annotation separator
// Message: There must be a space after union type annotation separator

type X = (string)| number;
// Message: There must be a space before union type annotation separator

type X = ((string))|(number | foo);
// Message: There must be a space before union type annotation separator
// Message: There must be a space after union type annotation separator

// Options: ["never"]
type X = string |number;
// Message: There must be no space before union type annotation separator

// Options: ["never"]
type X = string| number;
// Message: There must be no space after union type annotation separator

type X = string& number;
// Message: There must be a space before intersection type annotation separator

// Options: ["always"]
type X = string& number;
// Message: There must be a space before intersection type annotation separator

type X = string &number;
// Message: There must be a space after intersection type annotation separator

type X = {x: string}&{y: number};
// Message: There must be a space before intersection type annotation separator
// Message: There must be a space after intersection type annotation separator

type X = string&number;
// Message: There must be a space before intersection type annotation separator
// Message: There must be a space after intersection type annotation separator

type X = string & number &boolean;
// Message: There must be a space after intersection type annotation separator

type X = string&number&boolean;
// Message: There must be a space before intersection type annotation separator
// Message: There must be a space after intersection type annotation separator
// Message: There must be a space before intersection type annotation separator
// Message: There must be a space after intersection type annotation separator

type X = (string)& number;
// Message: There must be a space before intersection type annotation separator

type X = ((string))&(number & foo);
// Message: There must be a space before intersection type annotation separator
// Message: There must be a space after intersection type annotation separator

// Options: ["never"]
type X = string &number;
// Message: There must be no space before intersection type annotation separator

// Options: ["never"]
type X = string& number;
// Message: There must be no space after intersection type annotation separator

The following patterns are not considered problems:

type X = string | number;

type X = string | number | boolean;

type X = (string) | number;

type X = ((string)) | (number | foo);

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

type X =
| string
| number

function x() {
type X =
| string
| number
}

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type X = string| number;

type X = string & number;

type X = string & number & boolean;

type X = (string) & number;

type X = ((string)) & (number & foo);

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

type X =
& string
& number

function x() {
type X =
& string
& number
}

// Settings: {"flowtype":{"onlyFilesWithFlowAnnotation":true}}
type X = string& number;

`use-flow-type`

Marks Flow type alias declarations as used.

Used to suppress no-unused-vars errors that are triggered by type aliases.

The following patterns are not considered problems:

declare class A {}
// Additional rules: {"no-unused-vars":1}

declare function A(): Y
// Additional rules: {"no-unused-vars":1}

declare module A {}
// Additional rules: {"no-unused-vars":1}

declare module A { declare var a: Y }
// Additional rules: {"no-unused-vars":1}

declare var A: Y
// Additional rules: {"no-unused-vars":1}

import type A from "a"; type X<B = ComponentType<A>> = { b: B }; let x: X; console.log(x);
// Additional rules: {"no-unused-vars":1}

import type A from "a"; type X<B = A<string>> = { b: B }; let x: X; console.log(x);
// Additional rules: {"no-unused-vars":1}

`use-read-only-spread`

Warns against accidentally creating an object which is no longer read-only because of how spread operator works in Flow. Imagine the following code:

type INode = {|
  +type: string,
|};

type Identifier = {|
  ...INode,
  +name: string,
|};

You might expect the identifier name to be read-only, however, that's not true (flow.org/try):

const x: Identifier = { name: '', type: '' };

x.type = 'must NOT be writable!'; // No Flow error
x.name = 'must NOT be writable!'; // No Flow error

This rule suggests to use $ReadOnly<…> to prevent accidental loss of readonly-ness:

type Identifier = $ReadOnly<{|
  ...INode,
  +name: string,
|}>;

const x: Identifier = { name: '', type: '' };

x.type = 'must NOT be writable!'; // $FlowExpectedError[cannot-write]
x.name = 'must NOT be writable!'; // $FlowExpectedError[cannot-write]

The following patterns are considered problems:

type INode = {||};
type Identifier = {|
  ...INode,
  +aaa: string,
|};
// Message: Flow type with spread property and all readonly properties must be wrapped in '$ReadOnly<…>' to prevent accidental loss of readonly-ness.

type INode = {||};
type Identifier = {|
  ...INode,
  +aaa: string,
  +bbb: string,
|};
// Message: Flow type with spread property and all readonly properties must be wrapped in '$ReadOnly<…>' to prevent accidental loss of readonly-ness.

The following patterns are not considered problems:

type INode = {||};
type Identifier = {|
  ...INode,
  name: string,
|};

type INode = {||};
type Identifier = {|
  ...INode,
  name: string, // writable on purpose
  +surname: string,
|};

type Identifier = {|
  +name: string,
|};

type INode = {||};
type Identifier = $ReadOnly<{|
  ...INode,
  +name: string,
|}>;

type INode = {||};
type Identifier = $ReadOnly<{|
  ...INode,
  name: string, // writable on purpose
|}>;

type INode = {||};
type Identifier = $ReadOnly<{|
  ...INode,
  -name: string,
|}>;

`valid-syntax`

Deprecated Babylon (the Babel parser) v6.10.0 fixes parsing of the invalid syntax this plugin warned against.

Checks for simple Flow syntax errors.

The following patterns are not considered problems:

function x(foo: string = "1") {}

function x(foo: Type = bar()) {}