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typescript/require-readonly-record-property-type

Require Readonly<Record<...>> for property type annotations.

Targeted pattern scope​

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

It checks property signatures in interfaces and type literals.

What this rule reports​

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

Why this rule exists​

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

❌ Incorrect​

interface Config {
 lookup: Record<string, number>;
}

type ApiConfig = {
 lookup: Record<string, number> | null;
};

✅ Correct​

interface Config {
 lookup: Readonly<Record<string, number>>;
}

type ApiConfig = {
 lookup: Readonly<Record<string, number>> | null;
};

Behavior and migration notes​

This rule is autofixable and also provides suggestions.

  • Record<K, V> is converted to Readonly<Record<K, V>>.
  • 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 {
 lookup: Promise<Record<string, number>>;
}
// ✅ valid (nested generic type is out of scope)

type Settings = {
 lookup: { nested: Record<string, number> };
};
// ✅ 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-record-property-type": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase intentionally uses mutable record property types or if you already enforce immutability through broader type-aware rules.

Package documentation​

Rule catalog ID: R123

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.