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typescript/require-readonly-record-type-alias

Require readonly record types in type alias annotations.

Targeted pattern scope​

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

It checks only the type annotation attached to type aliases.

What this rule reports​

This rule reports type alias annotations that use mutable Record<...>.

Why this rule exists​

Type aliases often define reusable API contracts. Using Readonly<Record<...>> in alias annotations communicates immutability expectations to all downstream uses.

❌ Incorrect​

type Lookup = Record<string, number>;

type MaybeLookup = Record<string, number> | null;

type Combined = Record<string, number> & { readonly kind: "ok" };

✅ Correct​

type Lookup = Readonly<Record<string, number>>;

type MaybeLookup = Readonly<Record<string, number>> | null;

type Combined = Readonly<Record<string, number>> & { readonly kind: "ok" };

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 type alias annotations (and top-level union/intersection members), not nested object-property types.

Additional examples​

type Resolver = Promise<Record<string, number>>;
// ✅ valid (nested generic type is out of scope)

type Config = { lookup: Record<string, number> };
// ✅ valid (nested property type is out of scope)

ESLint flat config example​

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

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

When not to use it​

Disable this rule if your codebase intentionally uses mutable record aliases or if you already enforce immutability through a broader rule strategy.

Package documentation​

Rule catalog ID: R125

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.