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typescript/require-readonly-map-parameter-type

Require ReadonlyMap for function and method parameter type annotations.

Targeted pattern scope​

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

Why this rule exists​

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

❌ Incorrect​

function loadIndex(index: Map<string, number>): void {}

const resolver = (index: Map<string, number> | null) => index;

class Registry {
 constructor(private readonly table: Map<string, string>) {}
}

✅ Correct​

function loadIndex(index: ReadonlyMap<string, number>): void {}

const resolver = (index: ReadonlyMap<string, number> | null) => index;

class Registry {
 constructor(private readonly table: 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 parameter types (and top-level union/intersection members), not nested object-property types.

Additional examples​

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

function configure(index?: ReadonlyMap<string, number>): 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-map-parameter-type": "error",
  },
 },
];

When not to use it​

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

Package documentation​

Rule catalog ID: R118

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.