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typescript/require-readonly-set-return-type

Require ReadonlySet for function and method return type annotations.

Targeted pattern scope​

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

Why this rule exists​

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

❌ Incorrect​

function buildTags(): Set<string> {
 return new Set();
}

type Resolver = () => Set<string> | null;

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

✅ Correct​

function buildTags(): ReadonlySet<string> {
 return new Set();
}

type Resolver = () => ReadonlySet<string> | null;

interface API {
 run(): 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 return types (and top-level union/intersection members), not nested object-property types.

Additional examples​

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

function buildState(): { values: Set<string> } {
 return { values: new Set() };
}
// ✅ 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-set-return-type": "error",
  },
 },
];

When not to use it​

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

Package documentation​

Rule catalog ID: R128

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.