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typescript/prefer-readonly-property

Require readonly class and interface properties.

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: R111

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.