# typescript/prefer-readonly-property

Require readonly class and interface properties.

**Deprecated**

- **Lifecycle:** Deprecated, frozen, and non-recommended.
- **Deprecated since:** `v3.0.0`
- **Available until:** `v4.0.0`
- **Use instead:** [`@typescript-eslint/prefer-readonly`](https://typescript-eslint.io/rules/prefer-readonly/)

The replacement uses type information to report private members that are never
modified outside construction. It intentionally does not impose this rule's
broader syntactic policy on every public, protected, or interface property.

## Targeted pattern scope

This rule targets class properties (`PropertyDefinition`) and interface/type
property signatures (`TSPropertySignature`) where `readonly !== true`.

## What this rule reports

This rule reports writable `PropertyDefinition` and `TSPropertySignature` members.

## Why this rule exists

Readonly property declarations make mutation boundaries explicit and reduce
accidental state changes.

## ❌ Incorrect

```ts
class C {
 value: string;
}
```

## ✅ Correct

```ts
class C {
 readonly value: string;
}
```

## Behavior and migration notes

This rule reports only and does not provide an autofix.

Use it where immutability is the default design policy.

### Options

This rule has no options.

## Additional examples

```ts
interface Settings {
 mode: "dark" | "light";
}
// ❌ reported

interface ReadonlySettings {
 readonly mode: "dark" | "light";
}
// ✅ valid
```

## ESLint flat config example

```ts
import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/prefer-readonly-property": "error",
  },
 },
];
```

## When not to use it

Disable this rule if mutable properties are part of your coding conventions.

## Package documentation

- [eslint-plugin-etc-misc README](https://github.com/Nick2bad4u/eslint-plugin-etc-misc#readme)

> **Rule catalog ID:** R138

## Further reading

- [TypeScript: readonly properties](https://www.typescriptlang.org/docs/handbook/interfaces.html#readonly-properties)

## Adoption resources

- Start at warning level in CI, then move to error after cleanup.
- Use focused codemods/autofix batches per package or directory.