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typescript/prefer-readonly-set

Require ReadonlySet instead of Set in type positions.

Targeted pattern scope​

This rule targets Set identifier references in TypeScript type references.

What this rule reports​

This rule reports Set type references in TypeScript annotations.

Why this rule exists​

ReadonlySet makes immutability expectations explicit for API consumers.

❌ Incorrect​

function f(values: Set<string>) {}

✅ Correct​

function f(values: ReadonlySet<string>) {}

Behavior and migration notes​

This rule reports only and does not provide an autofix.

Migration is usually replacing Set<T> with ReadonlySet<T> in type positions.

Options​

This rule has no options.

Additional examples​

type AllowedRoles = Set<string>;
// ❌ reported

type AllowedRolesView = ReadonlySet<string>;
// ✅ valid

ESLint flat config example​

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

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

When not to use it​

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

Package documentation​

Rule catalog ID: R113

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.