eslint-plugin-flowtype

8.0.3 verified Sat Apr 25 auth: no javascript

ESLint plugin providing Flow type linting rules. Current stable version is 8.0.3, released October 2021. It adds over 40 rules specific to Flow type annotations, such as enforcing type styles, preventing duplicates, and requiring type declarations. Key differentiators: it is the primary ESLint plugin for Flow, with extensive rule coverage and shareable configs. It requires ESLint 8+ and Node 12+. Maintenance is active but may slow as Flow usage declines. Alternative: use TypeScript with `@typescript-eslint` instead.

Common errors

error Error: Failed to load plugin 'flowtype' - Cannot find module 'eslint-plugin-flowtype' ↓

cause Missing eslint-plugin-flowtype dependency

fix

npm install --save-dev eslint-plugin-flowtype

error Error: ESLint configuration in .eslintrc.json is invalid: "plugins" must be an array ↓

cause Wrong format for plugins field

fix

Use 'plugins': ['flowtype']

error Error: Cannot find module '@babel/plugin-syntax-flow' ↓

cause Missing peer dependency

fix

npm install --save-dev @babel/plugin-syntax-flow @babel/plugin-transform-react-jsx

Warnings

breaking v8.0.0 drops support for ESLint v7 and Node v10. ↓

fix Upgrade to ESLint 8+ and Node 12+.

breaking v6.0.0 contains potentially breaking dependency updates. ↓

fix Review changelog for full list of updated dependencies.

deprecated v7.0.0 dropped Node v10 support. ↓

fix Use Node >=12.

gotcha Adding 'eslint-plugin-flowtype' as plugin name will not work. ↓

fix Use 'flowtype' without prefix.

gotcha Rules must be prefixed with 'flowtype/' in rule configuration. ↓

fix Use 'flowtype/rule-name' instead of just 'rule-name'.

Install

npm install eslint-plugin-flowtype

yarn add eslint-plugin-flowtype

pnpm add eslint-plugin-flowtype

Imports

plugin

wrong

plugins: ['eslint-plugin-flowtype']

correct

// In .eslintrc: 'plugins': ['flowtype']

ESLint automatically adds prefix; just use 'flowtype'.

rules

wrong

rules: { 'boolean-style': 'error' }

correct

// In .eslintrc: 'rules': { 'flowtype/boolean-style': 'error' }

Rules must be prefixed with 'flowtype/'.

configs

wrong

extends: ['eslint-plugin-flowtype/recommended']

correct

// In .eslintrc: 'extends': ['plugin:flowtype/recommended']

Use plugin: prefix for shareable configs.

Quickstart

Installation and basic ESLint configuration with recommended rules and a simple Flow-annotated function.

// Install: npm install --save-dev eslint eslint-plugin-flowtype @babel/plugin-syntax-flow @babel/plugin-transform-react-jsx

// .eslintrc.json
{
  "plugins": ["flowtype"],
  "extends": ["plugin:flowtype/recommended"],
  "rules": {
    "flowtype/boolean-style": [2, "boolean"],
    "flowtype/define-flow-type": 1,
    "flowtype/delimiter-dangle": [1, "never"],
    "flowtype/generic-spacing": [1, "never"],
    "flowtype/no-mixed": 2,
    "flowtype/no-primitive-constructor-types": 2,
    "flowtype/no-weak-types": 2,
    "flowtype/object-type-delimiter": [1, "comma"],
    "flowtype/require-parameter-type": 2,
    "flowtype/require-return-type": [2, "always", { "annotateUndefined": "never" }],
    "flowtype/require-valid-file-annotation": 2,
    "flowtype/semi": [1, "always"],
    "flowtype/space-after-type-colon": [1, "always"],
    "flowtype/space-before-generic-bracket": [1, "never"],
    "flowtype/space-before-type-colon": [1, "never"],
    "flowtype/type-id-match": [2, "^([A-Z][a-z0-9]+)+Type$"],
    "flowtype/union-intersection-spacing": [1, "always"],
    "flowtype/use-flow-type": 1,
    "flowtype/valid-syntax": 1
  }
}

// Example .js file with Flow
// @flow
function square(n: number): number {
  return n * n;
}