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WARNING: This project has been renamed to eslint-plugin-functional. Install using eslint-plugin-functional instead.

eslint-plugin-ts-immutable

ESLint rules to disable mutation in JavaScript and TypeScript.

Background

In some applications it is important to not mutate any data, for example when using Redux to store state in a React application. Moreover immutable data structures has a lot of advantages in general so I want to use them everywhere in my applications.

I originally used immutablejs for this purpose. It is a really nice library but I found it had some drawbacks. Specifically when debugging it was hard to see the structure, creating JSON was not straightforward, and passing parameters to other libraries required converting to regular mutable arrays and objects. The seamless-immutable project seems to have the same conclusions and they use regular objects and arrays and check for immutability at run-time. This solves all the aformentioned drawbacks but introduces a new drawback of only being enforced at run-time. (Altough you loose the structural sharing feature of immutablejs with this solution so you would have to consider if that is something you need).

Then TypeScript 2.0 came along and introduced readonly options for properties, indexers and arrays. TypeScript 3.0 has continued to add support immutability enforcing syntax. This enables us to use regular object and arrays and have the immutability enforced at compile time instead of run-time. Now the only drawback is that there is nothing enforcing the use of readonly in TypeScript.

This can be solved by using linting rules. So the aim of this project is to leverage the type system in TypeScript to enforce immutability at compile-time while still using regular objects and arrays. Additionally, this project will also aim to support vanilla JavaScript where possible.

Installing

npm install eslint eslint-plugin-ts-immutable --save-dev

Note: If you installed ESLint globally (using the -g flag) then you must also install eslint-plugin-ts-immutable globally.

To use this plugin with TypeScript, additionally install @typescript-eslint/parser.

npm install eslint @typescript-eslint/parser eslint-plugin-ts-immutable --save-dev

Usage

Add ts-immutable to the plugins section of your .eslintrc configuration file. Then configure the rules you want to use under the rules section.

{
  "plugins": ["ts-immutable"],
  "rules": {
    "ts-immutable/rule-name": "error"
  }
}

The following rulesets are provided by this plugin. See bellow for what rules are including in each.

• recommended
• functional-lite
• functional

You can enable one of these rulesets like so:

{
  "extends": ["plugin:ts-immutable/recommended"]
}

With TypeScript

@typescript-eslint/parser is needed to parse TypeScript code; add @typescript-eslint/parser to the "parser" filed in your .eslintrc configuration file. Additionally, for this plugin to use type information, you will need to specify a path to your tsconfig.json file in the "project" property of "parserOptions".

{
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "project": "./tsconfig.json"
  },
  "plugins": ["ts-immutable"],
  "rules": {
    "ts-immutable/rule-name": "error"
  }
}

See @typescript-eslint/parser's README.md for more information on the available "parserOptions".

Note: Make sure to use eslint --ext .js,.ts since by default eslint will only search for .js files.

Supported Rules

In addition to immutable rules this project also contains a few rules for enforcing a functional style of programming. The following rules are available:

Key:

Symbol	Meaning
🙈	Ruleset: Recommended This ruleset is designed to enforce immutability in the code.
🙉	Ruleset: Functional Lite This ruleset is designed to enforce a somewhat functional programming code style.
🙊	Ruleset: Functional This ruleset is designed to enforce a functional programming code style.
🔧	Fixable Problems found by this rule are potentially fixable with the --fix option.
💭	Only Avaliable for TypeScript The rule either requires Type Information or only works with TypeScript syntax.
💙	Works better with TypeScript Type Information will be used if available making the rule work in more case.

Immutability rules

Name	Description	🙈	🙉	🙊	🔧	💙
prefer-readonly-types	Use readonly types and readonly modifiers where possible	✔️	✔️	✔️	🔧	💭
no-let	Disallow mutable variables	✔️	✔️	✔️
immutable-data	Disallow mutating objects and arrays	✔️	✔️	✔️		💙
no-method-signature	Enforce property signatures with readonly modifiers over method signatures	✔️	✔️	✔️		💭

Functional style rules

Name	Description	🙈	🙉	🙊	🔧	💙
no-this	Disallow this access		✔️	✔️
no-class	Disallow classes		✔️	✔️
no-mixed-interface	Restrict interfaces so that only members of the same kind are allowed in them		✔️	✔️		💭
no-expression-statement	Disallow expressions to cause side-effects			✔️
no-conditional-statement	Disallow conditional statements (if and switch statements)			✔️
no-loop-statement	Disallow imperative loops		✔️	✔️
no-return-void	Disallow function that return nothing		✔️	✔️		💭
functional-parameters	Functions must have functional parameter		✔️	✔️
no-throw	Disallow throwing exceptions		✔️	✔️
no-try	Disallow try-catch[-finally] and try-finally patterns			✔️
no-reject	Disallow rejecting Promises

Recommended standard rules

In addition to the immutability rules above, there are a few standard rules that needs to be enabled to achieve immutability.

Each of these rules are enabled by default in the rulesets this plugin provides.

no-var

Without this rule, it is still possible to create var variables that are mutable.

no-param-reassign

Without this rule, function parameters are mutable.

prefer-const

This rule is helpful when converting from an imperative code style to a functional one.

@typescript-eslint/explicit-function-return-type

For performance reasons, tslint-immutable does not check implicit return types. So for example this function will return an mutable array but will not be detected:

function foo() {
  return [1, 2, 3];
}

To avoid this situation you can enable @typescript-eslint/explicit-function-return-type. Now the above function is forced to declare the return type and the mutability will be detected.

How to contribute

For new features file an issue. For bugs, file an issue and optionally file a PR with a failing test.

How to develop

To execute the tests run yarn test.

To learn about eslint plugin development se the relevant section of the eslit docs. You can also checkout the typescript-eslint repo which has some more information specific to typescript.

In order to know which AST nodes are created for a snippet of TypeScript code you can use ast explorer with options JavaScript and @typescript-eslint/parser.

How to publish

yarn version --patch
yarn version --minor
yarn version --major

Prior work

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