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readonly-keyword

Require readonly modifiers where possible.

Targeted pattern scope​

This rule targets class fields, TypeScript property signatures, and TypeScript index signatures.

What this rule reports​

  • Missing readonly on class properties
  • Missing readonly on interface/type property signatures
  • Missing readonly on index signatures

Why this rule exists​

Readonly properties communicate immutability at the type level and prevent accidental mutation.

❌ Incorrect​

interface State {
  value: number;
}

✅ Correct​

interface State {
  readonly value: number;
}

Additional examples​

// ❌ Mutable class field and index signature
class Settings {
  theme = "dark";
}

interface Cache {
  [key: string]: string;
}

// ✅ Readonly members communicate intent
class Settings {
  readonly theme = "dark";
}

interface Cache {
  readonly [key: string]: string;
}

ESLint flat config example​

import immutable from "eslint-plugin-immutable-2";

export default [
  {
    files: ["**/*.{ts,tsx,mts,cts}"],
    plugins: { immutable },
    rules: {
      "immutable/readonly-keyword": "error",
    },
  },
];

When not to use it​

Avoid this rule in models that are intentionally mutable during their lifecycle (for example form-draft objects, active-record entities, or mutable adapter DTOs). Enforcing readonly in those domains can push developers toward unnecessary type assertions.

Rule catalog ID: R914

Further reading​