eslint-plugin-functional

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

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Getting Started

See our getting started guide.

Rulesets

The following rulesets are made available by this plugin.

Presets:

• Strict (plugin:functional/strict)
  Enforce recommended rules designed to strictly enforce functional programming.

• Recommended (plugin:functional/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 (plugin:functional/lite)
  Good if you're new to functional programming or are converting a large codebase.

Categorized:

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

• No Exceptions (plugin:functional/no-exceptions)
  Functional programming style does not use run-time exceptions. Instead expressions produces values to indicate errors.

• No Mutations (plugin:functional/no-mutations)
  Prevent mutating any data as that's not functional

• No Other Paradigms (plugin:functional/no-other-paradigms)
  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 (plugin:functional/no-statements)
  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 (plugin:functional/stylistic)
  Enforce code styles that can be considered to be more functional.

Other:

• All (plugin:functional/all)
  Enables all rules defined in this plugin.

• Off (plugin:functional/off)
  Disable all rules defined in this plugin.

• Disable Type Checked (plugin:functional/disable-type-checked)
  Disable all rules that require type information.

• External Vanilla Recommended (plugin:functional/external-vanilla-recommended)
  Configures recommended vanilla ESLint rules.

• External Typescript Recommended (plugin:functional/external-typescript-recommended)
  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-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.