eslint-plugin-functional
v7.1.0
Published
ESLint rules to promote functional programming in TypeScript.
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eslint-plugin-functional
An ESLint plugin to disable mutation and promote functional programming in JavaScript and TypeScript.
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and a growing network of open source maintainers
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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 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 functionalNo 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. | ☑️ ✅ 🔒 | | ![badge-disableTypeChecked][] | | | 💭 | |
No Exceptions
| Name | Description | 💼 | ⚠️ | 🚫 | 🔧 | 💡 | 💭 | ❌ | | :------------------------------------------------------- | :----------------------------------------------------- | :------------------------------- | :-- | :---- | :-- | :-- | :-- | :-- | | no-promise-reject | Disallow rejecting promises. | | | | | | | | | no-throw-statements | Disallow throwing exceptions. | ☑️ ✅ 🔒 ![badge-noExceptions][] | | | | | | | | no-try-statements | Disallow try-catch[-finally] and try-finally patterns. | 🔒 ![badge-noExceptions][] | | ☑️ ✅ | | | | |
No Mutations
| Name | Description | 💼 | ⚠️ | 🚫 | 🔧 | 💡 | 💭 | ❌ | | :--------------------------------------------------------------------------- | :-------------------------------------------------------------- | :------------------------------ | :-- | :---------------------------- | :-- | :-- | :-- | :-- | | immutable-data | Enforce treating data as immutable. | ☑️ ✅ 🔒 ![badge-noMutations][] | | ![badge-disableTypeChecked][] | | | 💭 | | | no-let | Disallow mutable variables. | ☑️ ✅ 🔒 ![badge-noMutations][] | | | | | | | | prefer-immutable-types | Require function parameters to be typed as certain immutability | ☑️ ✅ 🔒 ![badge-noMutations][] | | ![badge-disableTypeChecked][] | 🔧 | 💡 | 💭 | | | prefer-readonly-type | Prefer readonly types over mutable types. | | | ![badge-disableTypeChecked][] | 🔧 | | 💭 | ❌ | | type-declaration-immutability | Enforce the immutability of types based on patterns. | ☑️ ✅ 🔒 ![badge-noMutations][] | | ![badge-disableTypeChecked][] | 🔧 | 💡 | 💭 | |
No Other Paradigms
| Name | Description | 💼 | ⚠️ | 🚫 | 🔧 | 💡 | 💭 | ❌ | | :--------------------------------------------------------- | :------------------------------------------------------------------------ | :----------------------------------- | :-- | :---------------------------- | :-- | :-- | :-- | :-- | | no-class-inheritance | Disallow inheritance in classes. | ☑️ ✅ 🔒 ![badge-noOtherParadigms][] | | | | | | | | no-classes | Disallow classes. | ✅ 🔒 ![badge-noOtherParadigms][] | | ☑️ | | | | | | no-mixed-types | Restrict types so that only members of the same kind are allowed in them. | ☑️ ✅ 🔒 ![badge-noOtherParadigms][] | | ![badge-disableTypeChecked][] | | | 💭 | | | no-this-expressions | Disallow this access. | 🔒 ![badge-noOtherParadigms][] | | ☑️ ✅ | | | | |
No Statements
| Name | Description | 💼 | ⚠️ | 🚫 | 🔧 | 💡 | 💭 | ❌ | | :------------------------------------------------------------------- | :--------------------------------------------- | :------------------------------- | :-- | :------------------------------- | :-- | :-- | :-- | :-- | | no-conditional-statements | Disallow conditional statements. | ✅ 🔒 ![badge-noStatements][] | | ☑️ ![badge-disableTypeChecked][] | | | 💭 | | | no-expression-statements | Disallow expression statements. | ✅ 🔒 ![badge-noStatements][] | | ☑️ ![badge-disableTypeChecked][] | | | 💭 | | | no-loop-statements | Disallow imperative loops. | ☑️ ✅ 🔒 ![badge-noStatements][] | | | | | | | | no-return-void | Disallow functions that don't return anything. | ☑️ ✅ 🔒 ![badge-noStatements][] | | ![badge-disableTypeChecked][] | | | 💭 | |
Stylistic
| Name | Description | 💼 | ⚠️ | 🚫 | 🔧 | 💡 | 💭 | ❌ |
| :--------------------------------------------------------------------- | :--------------------------------------------------------------------- | :-- | :-- | :---------------------------- | :-- | :-- | :-- | :-- |
| prefer-property-signatures | Prefer property signatures over method signatures. | 🎨 | | ![badge-disableTypeChecked][] | | | 💭 | |
| prefer-tacit | Replaces x => f(x)
with just f
. | | 🎨 | ![badge-disableTypeChecked][] | | 💡 | 💭 | |
| readonly-type | Require consistently using either readonly
keywords or Readonly<T>
| 🎨 | | ![badge-disableTypeChecked][] | 🔧 | | 💭 | |
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.