DeepReadonly

Makes every property of T readonly recursively. Unlike the built-in Readonly, which only affects the first level, it makes properties readonly all the way down into nested objects.

type Frozen = DeepReadonly<T>;

Usage

DeepReadonly<T>

Handy for expressing immutable state at the type level.

import type { DeepReadonly } from 'es-toolkit/types';

type State = { user: { name: string; tags: string[] } };

type FrozenState = DeepReadonly<State>;
// => { readonly user: { readonly name: string; readonly tags: readonly string[] } }

declare const state: FrozenState;
state.user.name = 'x'; // error: name is readonly.

Recursion rules

• Recurses into: plain objects, arrays/tuples (→ readonly arrays/tuples), and Map/Set (→ ReadonlyMap/ReadonlySet).
• Passes through: functions, Date, and RegExp are left unchanged.

import type { DeepReadonly } from 'es-toolkit/types';

// Arrays/tuples become readonly.
type A = DeepReadonly<number[]>; // readonly number[]

// Map/Set become their readonly versions.
type B = DeepReadonly<Map<string, number>>; // ReadonlyMap<string, number>
type C = DeepReadonly<Set<number>>; // ReadonlySet<number>

Type Parameters

• T: The type to make deeply readonly.