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typescript/require-readonly-set-parameter-type

Require ReadonlySet for function and method parameter type annotations.

Targeted pattern scope​

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

Why this rule exists​

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

❌ Incorrect​

function loadTags(tags: Set<string>): void {}

const resolver = (tags: Set<string> | null) => tags;

class Registry {
 constructor(private readonly tags: Set<string>) {}
}

✅ Correct​

function loadTags(tags: ReadonlySet<string>): void {}

const resolver = (tags: ReadonlySet<string> | null) => tags;

class Registry {
 constructor(private readonly tags: ReadonlySet<string>) {}
}

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

  • Set<T> is converted to ReadonlySet<T>.
  • 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: { tags: Set<string> }): void {}
// ✅ valid (nested property type is out of scope)

function configure(tags?: ReadonlySet<string>): 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-set-parameter-type": "error",
  },
 },
];

When not to use it​

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

Package documentation​

Rule catalog ID: R126

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.