eslint-plugin-redundant-undefined

1.0.0 verified Sat Apr 25 auth: no javascript

An ESLint plugin (v1.0.0, single release so far) that forbids redundant `undefined` in TypeScript optional parameters and properties. It helps enforce cleaner optional types by flagging patterns like `s?: string | undefined` and offering a `followExactOptionalPropertyTypes` option for use with TypeScript's `exactOptionalPropertyTypes` setting. Requires `@typescript-eslint/parser` and ESLint >=8.46.0. Available on npm with MIT license.

Common errors

error Error: Failed to load plugin 'redundant-undefined': Cannot find module 'eslint-plugin-redundant-undefined' ↓

cause Plugin not installed or missing from node_modules.

fix

Run npm install eslint-plugin-redundant-undefined @typescript-eslint/parser --save-dev

error Parsing error: 'parserOptions.project' has been set for @typescript-eslint/parser. The file does not match your project config: .eslintrc.json ↓

cause ESLint is trying to parse the config file itself with TypeScript parser.

fix

Add 'ignorePatterns': ['.eslintrc.json'] to your ESLint config or use a .eslintrc.js file.

error Error: Rule 'redundant-undefined' (or 'redundant-undefined/redundant-undefined') is not defined ↓

cause Plugin not listed in 'plugins' array or rule name misspelled.

fix

Add 'redundant-undefined' to plugins and use 'redundant-undefined/redundant-undefined' as rule name.

Warnings

gotcha Rule only works with `@typescript-eslint/parser`; using default ESLint parser will cause parsing errors. ↓

fix Set parser to '@typescript-eslint/parser' in ESLint config.

gotcha The rule name is 'redundant-undefined/redundant-undefined' – users often try 'redundant-undefined' alone or forget the slash. ↓

fix Use the full prefixed name: 'redundant-undefined/redundant-undefined'.

gotcha Plugin requires ESLint >=8.46.0 and Node.js ^16.0.0 or >=18.0.0; older versions will fail to load. ↓

fix Update ESLint and Node.js to supported versions.

gotcha Option `followExactOptionalPropertyTypes` changes the rule behavior; enabling it without having `exactOptionalPropertyTypes: true` in tsconfig may cause unexpected lint errors. ↓

fix Ensure TypeScript's `exactOptionalPropertyTypes` is enabled if using this option.

Install

npm install eslint-plugin-redundant-undefined

yarn add eslint-plugin-redundant-undefined

pnpm add eslint-plugin-redundant-undefined

Imports

redundant-undefined

wrong

require('eslint-plugin-redundant-undefined') or import syntax in config is not supported

correct

// In .eslintrc: { "plugins": ["redundant-undefined"], "rules": { "redundant-undefined/redundant-undefined": "error" } }

Plugin is added via plugins array in ESLint config; rules are prefixed with 'redundant-undefined/'

default rule

wrong

Setting 'redundant-undefined' as a top-level rule like 'redundant-undefined': 'error' (missing slash) or using wrong rule name

correct

rules: { "redundant-undefined/redundant-undefined": "error" }

Rule name is 'redundant-undefined/redundant-undefined' – the plugin name and rule name are identical but separated by a slash

Quickstart

Minimal ESLint config to enable the plugin and rule, plus an example of incorrect code.

// .eslintrc.json
{
  "parser": "@typescript-eslint/parser",
  "plugins": ["redundant-undefined"],
  "rules": {
    "redundant-undefined/redundant-undefined": "error"
  }
}

// Install
// npm i eslint-plugin-redundant-undefined @typescript-eslint/parser eslint --save-dev

// Example code that will be flagged:
// function f(s?: string | undefined): void {}