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typescript/prefer-readonly-map

Require ReadonlyMap instead of Map in type positions.

Targeted pattern scope​

This rule targets Map identifier references in TypeScript type references.

What this rule reports​

This rule reports Map type references in TypeScript annotations.

Why this rule exists​

ReadonlyMap better communicates read-only intent in APIs that should not mutate incoming maps.

❌ Incorrect​

function f(values: Map<string, string>) {}

✅ Correct​

function f(values: ReadonlyMap<string, string>) {}

Behavior and migration notes​

This rule reports only and does not provide an autofix.

Migration is usually replacing Map<K, V> with ReadonlyMap<K, V> in type positions.

Options​

This rule has no options.

Additional examples​

type Cache = Map<string, number>;
// ❌ reported

type CacheView = ReadonlyMap<string, number>;
// ✅ valid

ESLint flat config example​

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

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/prefer-readonly-map": "error",
  },
 },
];

When not to use it​

Disable this rule if mutable maps are expected throughout your codebase.

Package documentation​

Rule catalog ID: R110

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.