Type Alias: DeepReadonly<T>

type DeepReadonly<T> = T extends (...args) => unknown ? T : T extends readonly infer U[] ? readonly DeepReadonly<U>[] : T extends object ? { readonly [P in keyof T]: DeepReadonly<T[P]> } : T;

Defined in: DeepReadonly.ts:19

Utility type that marks every property of a given type as readonly, recursively.

Readonly<T> stops at the top level, so a nested object remains writable and value.nested.field = … still compiles. DeepReadonly applies at every depth, including through arrays.

Functions keep their type, so callbacks on a value remain callable. Primitives keep their type.

Type Parameters

T

T

The type to make deeply readonly.

Example

type PlayerData = {
	requestType: string
	timing: { start: number }
}

const data: DeepReadonly<PlayerData> = {
	requestType: 'segment',
	timing: { start: 0 },
}

// Reading at any depth is unchanged.
ok(data.requestType === 'segment')
ok(data.timing.start === 0)

// @ts-expect-error - top-level properties are readonly
data.requestType = 'manifest'

// @ts-expect-error - nested properties are readonly too
data.timing.start = 1