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typescript/prefer-readonly-index-signature

Require readonly index signatures in TypeScript type declarations.

Targeted pattern scope​

This rule targets writable TSIndexSignature nodes.

What this rule reports​

This rule reports index signatures where the readonly modifier is missing.

Why this rule exists​

Readonly index signatures communicate immutability intent and reduce accidental mutation of dictionary-like objects.

❌ Incorrect​

interface UserLookup {
 [id: string]: User;
}

type CountByKey = {
 [key: string]: number;
};

✅ Correct​

interface UserLookup {
 readonly [id: string]: User;
}

type CountByKey = {
 readonly [key: string]: number;
};

Behavior and migration notes​

This rule is autofixable. The fixer inserts readonly before each reported index signature.

Additional examples​

type Cache = {
 [name: string]: string;
};
// ❌ reported

type CacheView = {
 readonly [name: string]: 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-index-signature": "error",
  },
 },
];

When not to use it​

Disable this rule if mutable index signatures are an intentional design choice.

Package documentation​

Rule catalog ID: R109

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.