Skip to main content

typescript/require-readonly-map-type-alias

Require readonly map types in type alias annotations.

Targeted pattern scope​

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

Why this rule exists​

Type aliases often define reusable API contracts. Using ReadonlyMap in alias annotations communicates immutability expectations to all downstream uses.

❌ Incorrect​

type Lookup = Map<string, number>;

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

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

✅ Correct​

type Lookup = ReadonlyMap<string, number>;

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

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

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

  • Map<K, V> is converted to ReadonlyMap<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<Map<string, number>>;
// ✅ valid (nested generic type is out of scope)

type Config = { lookup: Map<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-map-type-alias": "error",
  },
 },
];

When not to use it​

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

Package documentation​

Rule catalog ID: R121

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.