eslint-plugin-functional

eslint-plugin-functional

An ESLint plugin to disable mutation and promote functional programming in JavaScript and TypeScript.

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Enterprise Users

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Getting Started

See our getting started guide.

Rulesets

The following rulesets are made available by this plugin.

Presets:

• Strict (configs.strict)
  Enforce recommended rules designed to strictly enforce functional programming.

• Recommended (configs.recommended)
  Has the same goal as the strict preset but a little more lenient, allowing for functional-like coding styles and nicer integration with non-functional 3rd-party libraries.

• Lite (configs.lite)
  Good if you're new to functional programming or are converting a large codebase.

Categorized:

• Currying (configs.currying)
  JavaScript functions support syntax that is not compatible with curried functions. To enforce currying, this syntax should be prevented.

• No Exceptions (configs.noExceptions)
  Functional programming style does not use run-time exceptions. Instead expressions produces values to indicate errors.

• No Mutations (configs.noMutations)
  Prevent mutating any data as that's not functional

• No Other Paradigms (configs.noOtherParadigms)
  JavaScript is multi-paradigm, allowing not only functional, but object-oriented as well as other programming styles. To promote a functional style, prevent the use of other paradigm styles.

• No Statements (configs.noStatements)
  In functional programming everything is an expression that produces a value. JavaScript has a lot of syntax that is just statements that does not produce a value. That syntax has to be prevented to promote a functional style.

• Stylistic (configs.stylistic)
  Enforce code styles that can be considered to be more functional.

Other:

• All (configs.all)
  Enables all rules defined in this plugin.

• Off (configs.off)
  Disable all rules defined in this plugin.

• Disable Type Checked (configs.disableTypeChecked)
  Disable all rules that require type information.

• External Vanilla Recommended (configs.externalVanillaRecommended)
  Configures recommended vanilla ESLint rules.

• External TypeScript Recommended (configs.externalTypeScriptRecommended)
  Configures recommended TypeScript ESLint rules. Enabling this ruleset will also enable the vanilla one.

The below section gives details on which rules are enabled by each ruleset.

Rules

💼 Configurations enabled in.
⚠️ Configurations set to warn in.
🚫 Configurations disabled in.
☑️ Set in the lite configuration.
✅ Set in the recommended configuration.
🔒 Set in the strict configuration.
🎨 Set in the stylistic configuration.
🔧 Automatically fixable by the --fix CLI option.
💡 Manually fixable by editor suggestions.
💭 Requires type information.
❌ Deprecated.

Currying

Name	Description	💼	⚠️	🚫	🔧	💡	💭	❌
functional-parameters	Enforce functional parameters.	☑️ ✅ 🔒					💭

No Exceptions

Name	Description	💼	⚠️	🚫	🔧	💡	💭	❌
no-promise-reject	Disallow rejecting promises.
no-throw-statements	Disallow throwing exceptions.	☑️ ✅ 🔒					💭
no-try-statements	Disallow try-catch[-finally] and try-finally patterns.	🔒		☑️ ✅

No Mutations

Name	Description	💼	⚠️	🚫	🔧	💡	💭	❌
immutable-data	Enforce treating data as immutable.	☑️ ✅ 🔒					💭
no-let	Disallow mutable variables.	☑️ ✅ 🔒
prefer-immutable-types	Require function parameters to be typed as certain immutability	☑️ ✅ 🔒			🔧	💡	💭
prefer-readonly-type	Prefer readonly types over mutable types.				🔧		💭	❌
type-declaration-immutability	Enforce the immutability of types based on patterns.	☑️ ✅ 🔒			🔧	💡	💭

No Other Paradigms

Name	Description	💼	⚠️	🚫	🔧	💡	💭	❌
no-class-inheritance	Disallow inheritance in classes.	☑️ ✅ 🔒
no-classes	Disallow classes.	✅ 🔒		☑️
no-mixed-types	Restrict types so that only members of the same kind are allowed in them.	☑️ ✅ 🔒					💭
no-this-expressions	Disallow this access.	🔒		☑️ ✅

No Statements

Name	Description	💼	⚠️	🚫	🔧	💡	💭	❌
no-conditional-statements	Disallow conditional statements.	✅ 🔒		☑️			💭
no-expression-statements	Disallow expression statements.	✅ 🔒		☑️			💭
no-loop-statements	Disallow imperative loops.	☑️ ✅ 🔒
no-return-void	Disallow functions that don't return anything.	☑️ ✅ 🔒					💭

Stylistic

Name	Description	💼	⚠️	🚫	🔧	💡	💭	❌
prefer-property-signatures	Prefer property signatures over method signatures.	🎨					💭
prefer-tacit	Replaces x => f(x) with just f.		🎨			💡	💭
readonly-type	Require consistently using either readonly keywords or Readonly<T>	🎨			🔧		💭

External Recommended Rules

In addition to the above rules, there are a few other rules we recommended.

These rules are what are included in the external recommended rulesets.

Vanilla Rules

• no-var
  Without this rule, it is still possible to create mutable var variables.

• no-param-reassign
  Don't allow function parameters to be reassigned, they should be treated as constants.

• prefer-const
  This rule provides a helpful fixer when converting from an imperative code style to a functional one.

TypeScript Rules

• @typescript-eslint/prefer-readonly
  This rule is helpful when working with classes.

• @typescript-eslint/switch-exhaustiveness-check
  Although our no-conditional-statements rule also performs this check, this rule has a fixer that will implement the unimplemented cases which can be useful.

Contributing

See our contributing guide.

Prior work

This project started as a port of tslint-immutable which was originally inspired by eslint-plugin-immutable.

Changelog

9.0.2 (2025-06-02)

Bug Fixes

• mark no-throw-statements rule as requiring type checking (#963) (b06c25c)
• prevent to report prefer-tacit in direct child of getter (#965) (afe34c2)