Skip to main content

typescript/require-readonly-map-return-type

Require ReadonlyMap for function and method return type annotations.

Targeted pattern scope​

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

It checks function declarations, function expressions, arrow functions with return annotations, call/method signatures, and constructor/function type nodes.

What this rule reports​

This rule reports return type annotations that use mutable Map<...>.

Why this rule exists​

Return types define API contracts. Returning ReadonlyMap communicates non-mutating intent to callers and reduces accidental mutation of shared maps.

❌ Incorrect​

function buildLookup(): Map<string, number> {
 return new Map();
}

type Resolver = () => Map<string, string> | null;

interface API {
 run(): Map<string, string>;
}

✅ Correct​

function buildLookup(): ReadonlyMap<string, number> {
 return new Map();
}

type Resolver = () => ReadonlyMap<string, string> | null;

interface API {
 run(): ReadonlyMap<string, string>;
}

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

Additional examples​

function buildConfig(): Promise<Map<string, string>> {
 return Promise.resolve(new Map());
}
// ✅ valid (nested generic type is out of scope)

function buildState(): { values: Map<string, string> } {
 return { values: new Map() };
}
// ✅ valid (nested object-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-return-type": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase intentionally exposes mutable Map return types or if you already enforce broader readonly type policy at the same scope.

Package documentation​

Rule catalog ID: R120

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.