eslint-functional
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 (
configs.strict)\ Enforce recommended rules designed to strictly enforce functional programming.Recommended (
configs.recommended)\ Has the same goal as thestrictpreset 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. | ☑️ ✅ 🔒  |
|  |
| | 💭 |
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-class-inheritance | Disallow inheritance in classes. | ☑️ ✅ 🔒  |
| | | | | | |||
| 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
varvariables.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.