![eslint-logo](docs/assets/eslint-functional-logo.png?sanitize=true) # eslint-plugin-functional [![npm version](https://img.shields.io/npm/v/eslint-plugin-functional.svg?style=flat)](https://www.npmjs.com/package/eslint-plugin-functional) [![Release](https://github.com/eslint-functional/eslint-plugin-functional/actions/workflows/release.yml/badge.svg)](https://github.com/eslint-functional/eslint-plugin-functional/actions/workflows/release.yml) [![Coverage Status](https://codecov.io/gh/eslint-functional/eslint-plugin-functional/branch/main/graph/badge.svg)](https://codecov.io/gh/eslint-functional/eslint-plugin-functional) [![semantic-release](https://img.shields.io/badge/%20%20%F0%9F%93%A6%F0%9F%9A%80-semantic--release-e10079.svg?style=flat)](https://github.com/semantic-release/semantic-release) [![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg?style=flat)](https://github.com/prettier/prettier) [![MIT license](https://img.shields.io/github/license/eslint-functional/eslint-plugin-functional.svg?style=flat)](https://opensource.org/licenses/MIT) [![GitHub Discussions](https://img.shields.io/github/discussions/eslint-functional/eslint-plugin-functional)](https://github.com/eslint-functional/eslint-plugin-functional/discussions) An [ESLint](http://eslint.org) plugin to disable mutation and promote functional programming in JavaScript and TypeScript.
## Donate [Any donations would be much appreciated](./DONATIONS.md). πŸ˜„ ### Enterprise Users `eslint-plugin-functional` is available as part of the Tidelift Subscription. Tidelift is working with the maintainers of `eslint-plugin-functional` and a growing network of open source maintainers to ensure your open source software supply chain meets enterprise standards now and into the future. [Learn more.](https://tidelift.com/subscription/pkg/npm-eslint-plugin-functional?utm_source=npm-eslint-plugin-functional&utm_medium=referral&utm_campaign=enterprise&utm_term=repo) ## Getting Started [See our getting started guide](./GETTING_STARTED.md). ## 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](https://www.npmjs.com/package/eslint) rules. - **External TypeScript Recommended** (`configs.externalTypeScriptRecommended`)\ Configures recommended [TypeScript ESLint](https://www.npmjs.com/package/@typescript-eslint/eslint-plugin) rules. Enabling this ruleset will also enable the vanilla one. The [below section](#rules) gives details on which rules are enabled by each ruleset. ## Rules πŸ’Ό Configurations enabled in.\ ⚠️ Configurations set to warn in.\ 🚫 Configurations disabled in.\ β˜‘οΈ Set in the `lite` configuration.\ βœ… Set in the `recommended` configuration.\ πŸ”’ Set in the `strict` configuration.\ 🎨 Set in the `stylistic` configuration.\ πŸ”§ Automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/user-guide/command-line-interface#--fix).\ πŸ’‘ Manually fixable by [editor suggestions](https://eslint.org/docs/latest/use/core-concepts#rule-suggestions).\ πŸ’­ Requires [type information](https://typescript-eslint.io/linting/typed-linting).\ ❌ Deprecated. ### Currying | Name | Description | πŸ’Ό | ⚠️ | 🚫 | πŸ”§ | πŸ’‘ | πŸ’­ | ❌ | | :----------------------------------------------------------- | :----------------------------- | :--------------------------- | :-- | :---------------------------- | :-- | :-- | :-- | :-- | | [functional-parameters](docs/rules/functional-parameters.md) | Enforce functional parameters. | β˜‘οΈ βœ… πŸ”’ ![badge-currying][] | | ![badge-disableTypeChecked][] | | | πŸ’­ | | ### No Exceptions | Name | Description | πŸ’Ό | ⚠️ | 🚫 | πŸ”§ | πŸ’‘ | πŸ’­ | ❌ | | :------------------------------------------------------- | :----------------------------------------------------- | :------------------------------- | :-- | :---- | :-- | :-- | :-- | :-- | | [no-promise-reject](docs/rules/no-promise-reject.md) | Disallow rejecting promises. | | | | | | | | | [no-throw-statements](docs/rules/no-throw-statements.md) | Disallow throwing exceptions. | β˜‘οΈ βœ… πŸ”’ ![badge-noExceptions][] | | | | | | | | [no-try-statements](docs/rules/no-try-statements.md) | Disallow try-catch[-finally] and try-finally patterns. | πŸ”’ ![badge-noExceptions][] | | β˜‘οΈ βœ… | | | | | ### No Mutations | NameΒ Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β  | Description | πŸ’Ό | ⚠️ | 🚫 | πŸ”§ | πŸ’‘ | πŸ’­ | ❌ | | :--------------------------------------------------------------------------- | :-------------------------------------------------------------- | :------------------------------ | :-- | :---------------------------- | :-- | :-- | :-- | :-- | | [immutable-data](docs/rules/immutable-data.md) | Enforce treating data as immutable. | β˜‘οΈ βœ… πŸ”’ ![badge-noMutations][] | | ![badge-disableTypeChecked][] | | | πŸ’­ | | | [no-let](docs/rules/no-let.md) | Disallow mutable variables. | β˜‘οΈ βœ… πŸ”’ ![badge-noMutations][] | | | | | | | | [prefer-immutable-types](docs/rules/prefer-immutable-types.md) | Require function parameters to be typed as certain immutability | β˜‘οΈ βœ… πŸ”’ ![badge-noMutations][] | | ![badge-disableTypeChecked][] | πŸ”§ | πŸ’‘ | πŸ’­ | | | [prefer-readonly-type](docs/rules/prefer-readonly-type.md) | Prefer readonly types over mutable types. | | | ![badge-disableTypeChecked][] | πŸ”§ | | πŸ’­ | ❌ | | [type-declaration-immutability](docs/rules/type-declaration-immutability.md) | Enforce the immutability of types based on patterns. | β˜‘οΈ βœ… πŸ”’ ![badge-noMutations][] | | ![badge-disableTypeChecked][] | πŸ”§ | πŸ’‘ | πŸ’­ | | ### No Other Paradigms | NameΒ Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β  | Description | πŸ’Ό | ⚠️ | 🚫 | πŸ”§ | πŸ’‘ | πŸ’­ | ❌ | | :--------------------------------------------------------- | :------------------------------------------------------------------------ | :----------------------------------- | :-- | :---------------------------- | :-- | :-- | :-- | :-- | | [no-class-inheritance](docs/rules/no-class-inheritance.md) | Disallow inheritance in classes. | β˜‘οΈ βœ… πŸ”’ ![badge-noOtherParadigms][] | | | | | | | | [no-classes](docs/rules/no-classes.md) | Disallow classes. | βœ… πŸ”’ ![badge-noOtherParadigms][] | | β˜‘οΈ | | | | | | [no-mixed-types](docs/rules/no-mixed-types.md) | Restrict types so that only members of the same kind are allowed in them. | β˜‘οΈ βœ… πŸ”’ ![badge-noOtherParadigms][] | | ![badge-disableTypeChecked][] | | | πŸ’­ | | | [no-this-expressions](docs/rules/no-this-expressions.md) | Disallow this access. | πŸ”’ ![badge-noOtherParadigms][] | | β˜‘οΈ βœ… | | | | | ### No Statements | Name | Description | πŸ’Ό | ⚠️ | 🚫 | πŸ”§ | πŸ’‘ | πŸ’­ | ❌ | | :------------------------------------------------------------------- | :--------------------------------------------- | :------------------------------- | :-- | :------------------------------- | :-- | :-- | :-- | :-- | | [no-conditional-statements](docs/rules/no-conditional-statements.md) | Disallow conditional statements. | βœ… πŸ”’ ![badge-noStatements][] | | β˜‘οΈ ![badge-disableTypeChecked][] | | | πŸ’­ | | | [no-expression-statements](docs/rules/no-expression-statements.md) | Disallow expression statements. | βœ… πŸ”’ ![badge-noStatements][] | | β˜‘οΈ ![badge-disableTypeChecked][] | | | πŸ’­ | | | [no-loop-statements](docs/rules/no-loop-statements.md) | Disallow imperative loops. | β˜‘οΈ βœ… πŸ”’ ![badge-noStatements][] | | | | | | | | [no-return-void](docs/rules/no-return-void.md) | Disallow functions that don't return anything. | β˜‘οΈ βœ… πŸ”’ ![badge-noStatements][] | | ![badge-disableTypeChecked][] | | | πŸ’­ | | ### Stylistic | NameΒ Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β  | Description | πŸ’Ό | ⚠️ | 🚫 | πŸ”§ | πŸ’‘ | πŸ’­ | ❌ | | :--------------------------------------------------------------------- | :--------------------------------------------------------------------- | :-- | :-- | :---------------------------- | :-- | :-- | :-- | :-- | | [prefer-property-signatures](docs/rules/prefer-property-signatures.md) | Prefer property signatures over method signatures. | 🎨 | | ![badge-disableTypeChecked][] | | | πŸ’­ | | | [prefer-tacit](docs/rules/prefer-tacit.md) | Replaces `x => f(x)` with just `f`. | | 🎨 | ![badge-disableTypeChecked][] | | πŸ’‘ | πŸ’­ | | | [readonly-type](docs/rules/readonly-type.md) | Require consistently using either `readonly` keywords or `Readonly` | 🎨 | | ![badge-disableTypeChecked][] | πŸ”§ | | πŸ’­ | | [badge-currying]: https://img.shields.io/badge/-currying-red.svg [badge-noExceptions]: https://img.shields.io/badge/-noExceptions-blue.svg [badge-noMutations]: https://img.shields.io/badge/-noMutations-orange.svg [badge-noOtherParadigms]: https://img.shields.io/badge/-noOtherParadigms-yellow.svg [badge-noStatements]: https://img.shields.io/badge/-noStatements-purple.svg [badge-disableTypeChecked]: https://img.shields.io/badge/-disableTypeChecked-navy.svg ## 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](https://eslint.org/docs/rules/no-var)\ Without this rule, it is still possible to create mutable `var` variables. - [no-param-reassign](https://eslint.org/docs/rules/no-param-reassign)\ Don't allow function parameters to be reassigned, they should be treated as constants. - [prefer-const](https://eslint.org/docs/rules/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](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-readonly.mdx)\ This rule is helpful when working with classes. - [@typescript-eslint/switch-exhaustiveness-check](https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/switch-exhaustiveness-check.mdx)\ Although our [no-conditional-statements](./docs/rules/no-conditional-statements.md) 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](./CONTRIBUTING.md). ## Prior work This project started as a port of [tslint-immutable](https://github.com/jonaskello/tslint-immutable) which was originally inspired by [eslint-plugin-immutable](https://github.com/jhusain/eslint-plugin-immutable).