Registry / testing / eslint-plugin-functional

eslint-plugin-functional

library9.0.4jsnpmunverified

ESLint plugin enforcing functional programming patterns in JavaScript and TypeScript, v9.0.4 (Feb 2026). Requires ESLint ^9.0.0 or ^10.0.0 and TypeScript >=4.7.4. Actively maintained with monthly releases. Provides presets (strict, recommended, lite) and categorized rulesets for immutability, no statements, no exceptions, currying, and stylistic rules. Differentiated by TypeScript-first design, type-aware rules (e.g., type-declaration-immutability), and support for Map/Set mutation detection. Recommended alternative to plugins like eslint-plugin-immutable or writing custom rules.

testing devops

npm install eslint-plugin-functional

github ↗package ↗

INSTALL

IMPORT

SIG · ESLINT-PLUGIN-FUNC

eslint-plugin-functional

testingjavascriptv9.0.4

Install

—

Import

—

Disk

—

Pass rate

0/ 6

Env Coverage0 / 6

glibc

18–22

musl

18–22

Install & Compatibility

Where this runs

tested against v? · npm install

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

node 18–226 runs

build_error

glibc

node 18–226 runs

build_error

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

default

✓ import plugin from 'eslint-plugin-functional'

✗ const plugin = require('eslint-plugin-functional')

ESM-first; require() may fail in some bundler configs.

rules

✓ import { rules } from 'eslint-plugin-functional'

✗ const { rules } = require('eslint-plugin-functional')

Named export for direct rule access; ESM only.

configs

✓ import { configs } from 'eslint-plugin-functional'

✗ import { recommended } from 'eslint-plugin-functional'

configs is a named export containing all rule presets (strict, recommended, lite, etc.). Access via configs.recommended.

Basic ESLint flat config enabling immutable-data, no-let, and prefer-tacit rules with TypeScript parser.

import plugin from 'eslint-plugin-functional'; import tsParser from '@typescript-eslint/parser'; export default [ { files: ['src/**/*.ts'], languageOptions: { parser: tsParser, parserOptions: { project: './tsconfig.json' }, }, plugins: { functional: plugin }, rules: { 'functional/immutable-data': 'error', 'functional/no-let': 'error', 'functional/prefer-tacit': 'warn', }, }, ];

Debug

Known issues

breakingv9.0.0: immutable-data rule now detects Map and Set mutations (e.g., .set(), .delete()). Code that mutates Maps/Sets will be flagged unless ignoreMapsAndSets option is set.

fix

Set 'ignoreMapsAndSets: true' in the immutable-data rule config if you intentionally mutate Maps/Sets.

affects: >=9.0.0

breakingv8.0.0: PrivateIdentifier names are now prefixed with '#'. Rules that reference private fields may report differently.

fix

Update any custom rule logic or inline configs that check private identifiers to include the '#' prefix.

affects: >=8.0.0

gotchano-throw-statements rule requires type checking (marked as requiring type checking since v9.0.2). Ensure parserOptions.project is set and the rule runs only on files included in the TypeScript project.

fix

Add 'parserOptions: { project: "./tsconfig.json" }' to the languageOptions for files using this rule.

affects: >=9.0.2

deprecatedRulesets like 'configs.recommended' are now accessed via named export 'configs'. Old string-based 'extends' patterns (e.g., 'plugin:functional/recommended') may still work but are not the future.

fix

Use the flat config format with import: import { configs } from 'eslint-plugin-functional'; then spread configs.recommended.

affects: >=7.0.0

Errors

Common errors & fixes

Error: ESLint configuration in .eslintrc.json is invalid: - The 'extends' property is not supported in flat config.

Using legacy .eslintrc.* format with new flat config system introduced in v9.

fix

Migrate to eslint.config.js and use import plugin from 'eslint-plugin-functional' with the plugins and rules object.

TypeError: Cannot read properties of undefined (reading 'Functional')

Misconfigured plugin loading: plugin not listed under 'plugins' in flat config.

fix

Add 'plugins: { functional: plugin }' in the flat config object.

Parsing error: The 'parserOptions.project' has been set for 'nonexistent.ts' but the project does not exist.

Missing or incorrect path to tsconfig.json in parserOptions.project.

fix

Ensure parserOptions.project points to a valid tsconfig.json file that includes the file being linted.

Upgrade

Version history

9.0.4latest on npm

Audit

Dependencies

eslintrequiredpeer dependency - plugin requires ESLint to run

typescriptrequiredpeer dependency - many rules require type checking

Agent activity

2 hits · last 30 days

node

Resources

githubhttps://github.com/eslint-functional/eslint-plugin-functional ↗

packagehttps://www.npmjs.com/package/eslint-plugin-functional ↗