How to set up eslint-plugin-functional in a Create React App project?

Install the plugin via npm, then extend your ESLint config with a preset like 'lite' to start. Override specific rules for React hooks or state management as needed, since CRA uses functional components but may involve mutable state.

eslint-plugin-functional vs eslint-plugin-immutable, which is better?

eslint-plugin-functional is more comprehensive, covering not just immutability but also statements, exceptions, and paradigms. If you need broad functional enforcement, choose this; for focused immutability rules, eslint-plugin-immutable might be simpler.

Can I use async/await with this plugin?

Yes, but rules like 'no-promise-reject' may restrict promise handling. You might need to configure or disable these rules to allow async patterns while adhering to functional error handling with values.

How to disable the no-classes rule for specific files?

In your ESLint configuration, use overrides to set 'functional/no-classes' to 'off' for files or directories that require classes, such as when integrating with object-oriented libraries.

Does eslint-plugin-functional work with Vue.js or Angular?

It can be integrated, but frameworks like Vue or Angular often use mutable state and classes, so you'll likely need to disable many rules or use the 'lite' preset to avoid constant linting errors.

What are the performance impacts of enabling all rules?

Enabling all rules, especially those requiring type information, can significantly increase linting time. Start with a lighter preset and monitor performance in your development and CI workflows.

functional — ESLint Plugin for FP

What is functional?

eslint-plugin-functional is an ESLint plugin that enforces functional programming principles in JavaScript and TypeScript codebases. It provides linting rules to disable mutation, restrict imperative statements, and promote immutable, side-effect-free coding patterns. The plugin helps developers write more predictable and maintainable code by preventing common anti-patterns in functional programming.

Target Audience

JavaScript and TypeScript developers adopting functional programming, teams enforcing coding standards for immutability, and projects transitioning from imperative to functional styles.

Value Proposition

It offers a comprehensive, configurable set of rules with multiple presets (strict, recommended, lite) to support gradual adoption, integrates seamlessly with existing ESLint setups, and provides TypeScript-specific rules for type-level immutability enforcement.

ESLint rules to disable mutation and promote fp in JavaScript and TypeScript.

Use Cases

Best For

Enforcing immutability in JavaScript and TypeScript projects
Adopting functional programming practices incrementally
Preventing side effects and mutable state in codebases
Linting code to restrict classes and object-oriented patterns
Ensuring error handling without exceptions in functional code
Integrating functional linting with existing ESLint configurations

Not Ideal For

Projects heavily reliant on object-oriented patterns, such as those using class-based React components or extensive inheritance hierarchies.
Codebases that must interface with imperative JavaScript libraries requiring side effects or mutation, like certain DOM manipulation or state management tools.
Teams prioritizing rapid prototyping over code purity, where strict rules could hinder quick iteration and experimentation.

Pros & Cons

Pros

Incremental Adoption Path

Offers lite, recommended, and strict presets, allowing teams to start gently and tighten rules over time, as highlighted in the philosophy section for balanced integration.

Comprehensive Functional Coverage

Includes rulesets for mutations, statements, exceptions, and other paradigms, ensuring holistic enforcement of functional principles across codebases.

TypeScript-First Design

Many rules require type information, providing deep integration with TypeScript for type-level immutability checks, such as prefer-immutable-types.

Active Maintenance and Support

Shown by badges for CI/CD, coverage, and enterprise options via Tidelift, indicating reliable upkeep and community engagement.

Cons

Type Information Dependency

Several rules require TypeScript type checking, which can slow down linting and complicate setup in non-TypeScript or loosely-typed projects.

Strictness Clashes with Imperative Code

The strict preset disallows common patterns like try-catch and classes, making integration with non-functional libraries or legacy code challenging and error-prone.

Limited Auto-Fix Support

Not all rules are auto-fixable; only a few have the 🔧 icon in the rules table, meaning developers often need manual interventions for fixes.

Frequently Asked Questions

What is functional?

Target Audience

JavaScript and TypeScript developers adopting functional programming, teams enforcing coding standards for immutability, and projects transitioning from imperative to functional styles.

Value Proposition

Use Cases

Best For

Enforcing immutability in JavaScript and TypeScript projects
Adopting functional programming practices incrementally
Preventing side effects and mutable state in codebases
Linting code to restrict classes and object-oriented patterns
Ensuring error handling without exceptions in functional code
Integrating functional linting with existing ESLint configurations

Not Ideal For

Projects heavily reliant on object-oriented patterns, such as those using class-based React components or extensive inheritance hierarchies.
Codebases that must interface with imperative JavaScript libraries requiring side effects or mutation, like certain DOM manipulation or state management tools.
Teams prioritizing rapid prototyping over code purity, where strict rules could hinder quick iteration and experimentation.

Pros & Cons

Pros

Incremental Adoption Path

Offers lite, recommended, and strict presets, allowing teams to start gently and tighten rules over time, as highlighted in the philosophy section for balanced integration.

Comprehensive Functional Coverage

Includes rulesets for mutations, statements, exceptions, and other paradigms, ensuring holistic enforcement of functional principles across codebases.

TypeScript-First Design

Many rules require type information, providing deep integration with TypeScript for type-level immutability checks, such as prefer-immutable-types.

Active Maintenance and Support

Shown by badges for CI/CD, coverage, and enterprise options via Tidelift, indicating reliable upkeep and community engagement.

Cons

Type Information Dependency

Several rules require TypeScript type checking, which can slow down linting and complicate setup in non-TypeScript or loosely-typed projects.

Strictness Clashes with Imperative Code

The strict preset disallows common patterns like try-catch and classes, making integration with non-functional libraries or legacy code challenging and error-prone.

Limited Auto-Fix Support

Not all rules are auto-fixable; only a few have the 🔧 icon in the rules table, meaning developers often need manual interventions for fixes.

Frequently Asked Questions

functional

What is functional?

Overview

Use Cases

Best For

Not Ideal For

Pros & Cons

Pros

Cons

Frequently Asked Questions

Related Projects

Found a gem we're missing?

functional

What is functional?

Overview

Use Cases

Best For

Not Ideal For

Pros & Cons

Pros

Cons

Frequently Asked Questions

Related Projects

Found a gem we're missing?