@lint-suite/eslint-plugin-readonly-type-properties

An ESLint rule for TypeScript type and interface property signatures. It requires readonly on primitive-valued properties and writes readonly array syntax as mutable T[], so a type's property mutability is explicit without redundant array immutability.

Install

Requires Node.js 24+, ESLint ^10.9.1, and TypeScript >=6.0.3.

pnpm add -D @lint-suite/eslint-plugin-readonly-type-properties eslint@^10.9.1 typescript@">=6.0.3" typescript-eslint

The plugin does not configure a TypeScript parser. Use typescript-eslint for TypeScript files:

// eslint.config.js
import readonlyTypeProperties from '@lint-suite/eslint-plugin-readonly-type-properties';
import tseslint from 'typescript-eslint';

export default [{
  files: ['**/*.ts'],
  languageOptions: { parser: tseslint.parser },
  plugins: { readonlyTypeProperties },
  rules: { 'readonlyTypeProperties/readonly-type-properties': 'error' }
}];

Options

This rule has no options.

Scope and exemptions

The rule checks TypeScript property signatures only: type aliases, interfaces, and inline object types. It does not check class fields, object-literal properties, methods, mapped types, or index signatures.

Only primitive keyword types (string, number, boolean, bigint, symbol, null, and undefined), literal and template-literal types, and unions or intersections made entirely from those types require readonly. Array, tuple, function, type-reference, unknown, any, object, keyof, typeof, and generic-parameter properties are exempt. readonly T[] and one-argument ReadonlyArray<T> are always reported and autofixed to T[], independently of property signatures.

Examples

Each snippet below is checked by this rule alone, not by an entire ESLint configuration.

Valid examples

1. Readonly primitive properties

type User = { readonly name: string; readonly active: boolean };

2. Readonly optional property

interface Settings { readonly theme?: string }

3. Readonly literal union

type Request = { readonly method: 'GET' | 'POST' };

4. Array-valued property exemption

type Group = { members: string[] };

5. Type-reference property exemption

type Session = { owner: User };

Invalid examples

1. Mutable string property

type User = { name: string };

Add readonly before name.

2. Mutable interface number property

interface Counter { count: number }

Add readonly before count.

3. Mutable literal-union property

type Request = { method: 'GET' | 'POST' };

Add readonly before method.

4. Mutable nullable primitive property

type Record = { id: string | null };

Add readonly before id.

5. Readonly array type syntax

type Names = readonly string[];

Use string[]; the rule autofixes this form.

6. ReadonlyArray syntax

type Group = { members: ReadonlyArray<string> };

Use string[]; the rule autofixes this form.

The named readonlyTypePropertiesRule export is for composed plugin registries.