#prefer-readonly-parameter-types

#Configuration

import { defineConfig, ts } from '@rslint/core';

export default defineConfig([
  ts.configs.recommended,
  {
    rules: {
      '@typescript-eslint/prefer-readonly-parameter-types': 'error',
    },
  },
]);

Requires function parameters to have deeply readonly types, preventing code from accidentally mutating values supplied by callers.

#Rule Details

Examples of incorrect code for this rule:

function consume(items: string[]) {}
function update(options: { enabled: boolean }) {}
function register(value: Set<string>) {}

Examples of correct code for this rule:

function consume(items: readonly string[]) {}
function update(options: Readonly<{ enabled: boolean }>) {}
function register(value: ReadonlySet<string>) {}
function format(value: string) {}

The rule checks nested property and element types, so a readonly outer property whose value is mutable is still reported.

#Options

• allow (default []): type specifiers that the rule should accept without checking their readonlyness. String names and file, lib, or package specifiers are supported.
• checkParameterProperties (default true): check TypeScript constructor parameter properties.
• ignoreInferredTypes (default false): skip parameters without an explicit type annotation.
• treatMethodsAsReadonly (default false): treat method declarations as readonly while checking object types. This is useful for types such as ReadonlySet and ReadonlyMap.

#Original Documentation

• typescript-eslint: prefer-readonly-parameter-types
• Source code