eslint-config-functype

A curated ESLint configuration bundle for functional TypeScript programming. This plugin combines and configures rules from established ESLint plugins to enforce immutability patterns and functional programming best practices.

What This Plugin Does

Instead of recreating functional programming rules, this plugin provides carefully curated configurations that combine rules from:

• eslint-plugin-functional: Core functional programming rules
• @typescript-eslint/eslint-plugin: TypeScript-specific functional patterns
• ESLint core: JavaScript immutability basics

Installation

npm install --save-dev eslint-config-functype @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint-plugin-functional
# or
pnpm add -D eslint-config-functype @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint-plugin-functional

Usage

ESLint 8 (.eslintrc) - Quick Start

// .eslintrc.js
module.exports = {
  extends: ["plugin:functype/recommended"],
  parser: "@typescript-eslint/parser",
  parserOptions: {
    project: "./tsconfig.json",
  },
}

ESLint 8 - Strict Mode

// .eslintrc.js
module.exports = {
  extends: ["plugin:functype/strict"],
  parser: "@typescript-eslint/parser",
  parserOptions: {
    project: "./tsconfig.json",
  },
}

ESLint 9+ (Flat Config)

For ESLint 9+, create your own flat config using our rule selections:

// eslint.config.js
import js from "@eslint/js"
import tseslint from "@typescript-eslint/eslint-plugin"
import functional from "eslint-plugin-functional"
import parser from "@typescript-eslint/parser"

export default [
  js.configs.recommended,
  {
    files: ["**/*.ts", "**/*.tsx"],
    languageOptions: {
      parser,
      parserOptions: {
        ecmaVersion: "latest",
        sourceType: "module",
        project: "./tsconfig.json",
      },
    },
    plugins: {
      "@typescript-eslint": tseslint,
      functional,
    },
    rules: {
      // Core immutability
      "prefer-const": "error",
      "no-var": "error",
      
      // TypeScript functional patterns
      "@typescript-eslint/consistent-type-imports": "error",
      "@typescript-eslint/no-explicit-any": "error",
      "@typescript-eslint/no-unused-vars": ["error", { argsIgnorePattern: "^_" }],
      
      // Functional programming rules
      "functional/no-let": "error",
      "functional/immutable-data": "warn",
      "functional/no-loop-statements": "off", // Enable as "error" for strict mode
    },
  }
]

Individual Rule Usage

You can also use individual rules without our presets:

{
  "plugins": ["@typescript-eslint", "functional"],
  "rules": {
    "functional/no-let": "error",
    "functional/immutable-data": "warn",
    "@typescript-eslint/no-explicit-any": "error",
    "prefer-const": "error"
  }
}

What Rules Are Included

Recommended Configuration

• ✅ prefer-const / no-var - Basic immutability
• ✅ functional/no-let - Disallow let declarations
• ⚠️ functional/immutable-data - Warn on data mutation
• ⚠️ functional/no-mutation - Warn on object/array mutations
• ✅ @typescript-eslint/consistent-type-imports - Clean imports
• ✅ @typescript-eslint/no-explicit-any - Type safety

Strict Configuration

All recommended rules plus:

• ✅ functional/no-loop-statements - Disallow imperative loops
• ✅ functional/immutable-data - Error on data mutation
• ✅ functional/prefer-immutable-types - Encourage readonly types
• ✅ @typescript-eslint/explicit-function-return-type - Explicit return types

Examples

// ❌ Bad (will be flagged)
let x = 1;                    // functional/no-let
arr.push(item);              // functional/immutable-data  
for(let i = 0; i < 10; i++)  // functional/no-loop-statements (strict only)

// ✅ Good  
const x = 1;
const newArr = [...arr, item];
arr.forEach(item => process(item));

Configurations

• recommended: Balanced functional programming rules suitable for most projects
• strict: Maximum functional programming enforcement for pure FP codebases

Philosophy

This plugin follows the principle of composition over recreation. Rather than maintaining custom rules, we curate and combine battle-tested rules from the community, ensuring:

• ✅ Less maintenance burden
• ✅ Better rule quality and edge case handling
• ✅ Automatic updates from upstream plugins
• ✅ Community-driven improvements

CLI Tools

List All Supported Rules

See exactly which rules are configured in each preset:

# Basic rule listing
pnpm run list-rules

# Show rule options/configuration
pnpm run list-rules:verbose  

# Show usage examples
pnpm run list-rules:usage

# Check peer dependency status
pnpm run check-deps

# Show help
pnpm run cli:help

After Installation

Once installed globally or in a project, you can also use:

# If installed globally
functype-list-rules --help

# Or with npx
npx eslint-config-functype functype-list-rules

CI/CD

This plugin includes GitHub Actions workflows for:

• ✅ Testing on Node.js 22
• ✅ Linting with our own rules
• ✅ Building and validation
• ✅ Publishing to npm on version changes

Development

Requirements

• Node.js 22.0.0 or higher
• pnpm (recommended package manager)

# Install dependencies
pnpm install

# Build
pnpm run build

# Lint
pnpm run lint

# List rules during development  
pnpm run list-rules

Troubleshooting

Missing Peer Dependencies

If you see errors like Definition for rule '@typescript-eslint/no-explicit-any' was not found, you're missing peer dependencies.

Quick Check:

pnpm run check-deps

This will show you exactly which dependencies are missing and provide the installation command.

Manual Installation:

pnpm add -D @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint-plugin-functional eslint-plugin-prettier eslint-plugin-simple-import-sort prettier