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<div align="center">eslint-plugin-functional
An ESLint plugin to disable mutation and promote functional programming in JavaScript and TypeScript.
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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.
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 thestrict
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
<!-- markdownlint-disable --> <!-- begin auto-generated rules list -->πΌ Configurations enabled in.
β οΈ Configurations set to warn in.
π« Configurations disabled in.
βοΈ Set in the lite
configuration.
β
Set in the recommended
configuration.
π Set in the strict
configuration.
π¨ Set in the stylistic
configuration.
π§ Automatically fixable by the --fix
CLI option.
π‘ Manually fixable by editor suggestions.
π Requires type information.
β Deprecated.
Currying
Name | Description | πΌ | β οΈ | π« | π§ | π‘ | π | β |
---|---|---|---|---|---|---|---|---|
functional-parameters | Enforce functional parameters. | βοΈ β
π | ![badge-disableTypeChecked][] | π |
No Exceptions
Name | Description | πΌ | β οΈ | π« | π§ | π‘ | π | β |
---|---|---|---|---|---|---|---|---|
no-promise-reject | Disallow rejecting promises. | |||||||
no-throw-statements | Disallow throwing exceptions. | βοΈ β π ![badge-noExceptions][] | ||||||
no-try-statements | Disallow try-catch[-finally] and try-finally patterns. | π ![badge-noExceptions][] | βοΈ β |
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 | πΌ | β οΈ | π« | π§ | π‘ | π | β |
---|---|---|---|---|---|---|---|---|
immutable-data | Enforce treating data as immutable. | βοΈ β π ![badge-noMutations][] | ![badge-disableTypeChecked][] | π | ||||
no-let | Disallow mutable variables. | βοΈ β π ![badge-noMutations][] | ||||||
prefer-immutable-types | Require function parameters to be typed as certain immutability | βοΈ β π ![badge-noMutations][] | ![badge-disableTypeChecked][] | π§ | π‘ | π | ||
prefer-readonly-type | Prefer readonly types over mutable types. | ![badge-disableTypeChecked][] | π§ | π | β | |||
type-declaration-immutability | Enforce the immutability of types based on patterns. | βοΈ β π ![badge-noMutations][] | ![badge-disableTypeChecked][] | π§ | π‘ | π |
No Other Paradigms
NameΒ Β Β Β Β Β Β Β Β Β Β Β Β Β Β | Description | πΌ | β οΈ | π« | π§ | π‘ | π | β |
---|---|---|---|---|---|---|---|---|
no-classes | Disallow classes. | βοΈ β π ![badge-noOtherParadigms][] | ||||||
no-mixed-types | Restrict types so that only members of the same kind are allowed in them. | βοΈ β π ![badge-noOtherParadigms][] | ![badge-disableTypeChecked][] | π | ||||
no-this-expressions | Disallow this access. | π ![badge-noOtherParadigms][] | βοΈ β |
No Statements
Name | Description | πΌ | β οΈ | π« | π§ | π‘ | π | β |
---|---|---|---|---|---|---|---|---|
no-conditional-statements | Disallow conditional statements. | β π ![badge-noStatements][] | βοΈ ![badge-disableTypeChecked][] | π | ||||
no-expression-statements | Disallow expression statements. | β π ![badge-noStatements][] | βοΈ ![badge-disableTypeChecked][] | π | ||||
no-loop-statements | Disallow imperative loops. | βοΈ β π ![badge-noStatements][] | ||||||
no-return-void | Disallow functions that don't return anything. | βοΈ β π ![badge-noStatements][] | ![badge-disableTypeChecked][] | π |
Stylistic
NameΒ Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β Β | Description | πΌ | β οΈ | π« | π§ | π‘ | π | β |
---|---|---|---|---|---|---|---|---|
prefer-property-signatures | Prefer property signatures over method signatures. | π¨ | ![badge-disableTypeChecked][] | π | ||||
prefer-tacit | Replaces x => f(x) with just f . | π¨ | ![badge-disableTypeChecked][] | π‘ | π | |||
readonly-type | Require consistently using either readonly keywords or Readonly<T> | π¨ | ![badge-disableTypeChecked][] | π§ | π |
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 mutablevar
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
Prior work
This project started as a port of tslint-immutable which was originally inspired by eslint-plugin-immutable.