readonly-type.md

Require consistently using either readonly keywords or Readonly<T> (functional/readonly-type)

💼 This rule is enabled in the 🎨 stylistic config.

🔧 This rule is automatically fixable by the --fix CLI option.

💭 This rule requires type information.

This rule enforces consistently using either readonly keywords or Readonly<T>.

Rule Details

There are two ways to declare type literals as readonly, either by specifying that each property of the type is readonly using the readonly keyword, or by wrapping the type in Readonly.

This rule is designed to enforce a consistent way of doing this.

❌ Incorrect

/* eslint functional/readonly-type: ["error", "keyword"] */

type Foo = Readonly<{
  bar: string;
  baz: number;
}>;

/* eslint functional/readonly-type: ["error", "generic"] */

type Foo = {
  readonly bar: string;
  readonly baz: number;
};

✅ Correct

/* eslint functional/readonly-type: ["error", "keyword"] */

type Foo = {
  readonly bar: string;
  readonly baz: number;
};

type Foo2 = {
  readonly bar: string;
  baz: number;
};

/* eslint functional/readonly-type: ["error", "generic"] */

type Foo = Readonly<{
  bar: string;
  baz: number;
}>;

// No issue as it's not fully readonly.
type Foo2 = {
  readonly bar: string;
  baz: number;
};

Options

This rule takes a single string option, either generic | keyword.

Default Options

const defaults = "generic";

generic

Enforce using Readonly<T> instead of marking each property as readonly with the readonly keyword.

keyword

Enforce using readonly keyword for each property instead of wrapping with Readonly.