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typescript/require-readonly-record-parameter-type

Require Readonly<Record<...>> for function and method parameter type annotations.

Targeted pattern scope​

This rule targets top-level mutable Record<K, V> parameter type annotations, including top-level union/intersection members such as Record<string, string> | null.

It checks function declarations, function expressions, arrow functions, call/method signatures, and constructor/function type nodes (including constructor parameter properties).

What this rule reports​

This rule reports parameter annotations that use mutable Record<...>.

Why this rule exists​

Parameter types define API input contracts. Using Readonly<Record<...>> for parameters communicates non-mutating expectations and helps avoid accidental argument mutation.

❌ Incorrect​

function loadLookup(lookup: Record<string, string>): void {}

const resolver = (lookup: Record<string, string> | null) => lookup;

class Registry {
 constructor(private readonly lookup: Record<string, string>) {}
}

✅ Correct​

function loadLookup(lookup: Readonly<Record<string, string>>): void {}

const resolver = (lookup: Readonly<Record<string, string>> | null) => lookup;

class Registry {
 constructor(private readonly lookup: Readonly<Record<string, string>>) {}
}

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

  • Record<K, V> is converted to Readonly<Record<K, V>>.
  • The rule intentionally checks only top-level parameter types (and top-level union/intersection members), not nested object-property types.

Additional examples​

function configure(options: { lookup: Record<string, string> }): void {}
// ✅ valid (nested property type is out of scope)

function configure(lookup?: Readonly<Record<string, string>>): void {}
// ✅ valid

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/require-readonly-record-parameter-type": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase intentionally mutates record inputs or if you already enforce a broader readonly parameter policy via type-aware linting.

Package documentation​

Rule catalog ID: R122

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.