Package Exports

eslint-plugin-functional

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Readme

eslint-plugin-functional

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

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.
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

No Mutations Rules

Name	Description	⚜️	🙊	🙈	🙉	💙
`immutable-data`	Disallow mutating objects and arrays	✔️	✔️	✔️	🟢	💙
`no-let`	Disallow mutable variables	✔️	✔️	🟢	🟢
`prefer-immutable-types`	Require types of data to be immutable	✔️	✔️	🟢	🟢	💭
`type-declaration-immutability`	Enforce type alias immutability using patterns	✔️	✔️	✔️	✔️	💭

No Other Paradigms Rules

Name	Description	⚜️	🙊	🙈	🙉	💙
`no-classes`	Disallow classes	✔️	✔️	✔️	✔️
`no-mixed-types`	Disallow types that contain both callable and non-callable members	✔️	✔️	✔️	✔️	💭
`no-this-expressions`	Disallow `this` access	✔️	✔️

No Statements Rules

Name	Description	⚜️	🙊	🙈	🙉	💙
`no-conditional-statements`	Disallow conditional statements (`if` and `switch` statements)	✔️	✔️	🟢		💭
`no-expression-statements`	Disallow expressions to cause side-effects	✔️	✔️			💭
`no-loop-statements`	Disallow imperative loops	✔️	✔️	✔️	✔️
`no-return-void`	Disallow functions that return nothing	✔️	✔️	✔️	✔️	💭

No Exceptions Rules

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	✔️	✔️

Currying Rules

Name	Description	⚜️	🙊	🙈	🙉	🔧	💙
`functional-parameters`	Functions must have functional parameters	✔️	✔️	🟢	🟢

Stylistic Rules

Name	Description	⚜️	🙊	🙈	🙉	🔧	💙
`prefer-property-signatures`	Enforce property signatures over method signatures	✔️					💭
`prefer-tacit`	Tacit/Point-Free style.	✔️				🔧	💙

Key

Symbol	Meaning
⚜️	Ruleset: Current
🙊	Ruleset: Strict
🙈	Ruleset: Recommended
🙉	Ruleset: Lite
✔️	Enabled as Error
🟢	Enabled as Error with Overrides
🔧	Fixable
💭	Only Available for TypeScript
💙	Works better with TypeScript

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 var variables that are mutable.
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.

Prior work

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