Enclose the type definition for a function from a third-party library

Question

Enclose the type definition for a function from a third-party library

I prefer to utilize Typescript for ensuring immutability in my code. Unfortunately, many libraries do not type their exported function parameters as Readonly or DeepReadonly, even if they are not meant to be mutated. This commonly causes issues because a ReadonlyArray is not compatible with an Array.
One way to address this issue is by redefining the function using module augmentation, which is explained in this Stack Overflow post: here. However, I am curious about how we can wrap an existing type definition so that we maintain the current definition but define all parameters as DeepReadonly.

typescript typescript-typings typescript-generics

Answer 1

Answer №1

I suggest utilizing this generic code snippet:

export type DeepReadonlyObject<A> = { readonly [K in keyof A]: DeepReadonlyObject<A[K]> };

This will transform all attributes to readonly and recursively set all values as DeepReadonlyObject values.

Playground

-------------Edited---------------

If you need to override imported types, you can do so using the declare module "Lib" { syntax. However, it's not possible to use iterations in module definition, so each key needs to be declared individually like this:

module "Lib" { 
    export const value = { key: 42 };
    export const value1 = { key: 42 };
}

declare module "Lib" {
    export type value = DeepReadonlyObject<typeof Lib.value>;
}

Unfortunately, each key must be placed separately in the declared module.

Answer 2

I suggest utilizing this generic code snippet:

export type DeepReadonlyObject<A> = { readonly [K in keyof A]: DeepReadonlyObject<A[K]> };

This will transform all attributes to readonly and recursively set all values as DeepReadonlyObject values.

Playground

-------------Edited---------------

If you need to override imported types, you can do so using the declare module "Lib" { syntax. However, it's not possible to use iterations in module definition, so each key needs to be declared individually like this:

module "Lib" { 
    export const value = { key: 42 };
    export const value1 = { key: 42 };
}

declare module "Lib" {
    export type value = DeepReadonlyObject<typeof Lib.value>;
}

Unfortunately, each key must be placed separately in the declared module.

Enclose the type definition for a function from a third-party library

Answer №1

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