Type Alias DeepReadonly<T>

DeepReadonly: T extends (...args: never[]) => unknown
    ? T
    : T extends readonly (infer U)[]
        ? readonly DeepReadonly<U>[]
        : T extends object ? { readonly [P in keyof T]: DeepReadonly<T[P]> } : T

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
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

Type Alias DeepReadonly<T>

Type Parameters

Example

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