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typescript/require-readonly-set-property-type

Require ReadonlySet for top-level property type annotations.

Targeted pattern scope​

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

It checks property signatures in interfaces and type literals.

What this rule reports​

This rule reports property annotations that use mutable Set<...>.

Why this rule exists​

Property signatures define object contracts consumed across your codebase. Using ReadonlySet for these property types communicates immutability intent and helps avoid accidental mutation through shared set-like data.

❌ Incorrect​

interface Config {
 values: Set<string>;
}

type ApiConfig = {
 values: Set<string> | null;
};

✅ Correct​

interface Config {
 values: ReadonlySet<string>;
}

type ApiConfig = {
 values: ReadonlySet<string> | null;
};

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 property type annotations (and top-level union/intersection members), not nested object-property types.

Additional examples​

interface Config {
 values: Promise<Set<string>>;
}
// ✅ valid (nested generic type is out of scope)

type Settings = {
 values: { nested: Set<string> };
};
// ✅ valid (nested 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-property-type": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase intentionally models property sets as mutable or if broader readonly policies already enforce your preferred constraints.

Package documentation​

Rule catalog ID: R127

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.