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typescript/prefer-readonly-property

Require readonly class and interface properties.

Deprecated

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​

class C {
 value: string;
}

✅ Correct​

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​

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

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

ESLint flat config example​

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​

Rule catalog ID: R138

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.