Typescript same key but different type nested (keyof but nested)

Question

I am trying to created a custom type that replaces all types of a Generic parameter T to string[] but keeps all the property names even nested.

Expected behaviour: No error

interface Bar {
    lat: string,
    lng: string
}
interface Foo {
    id: string,
    bar: Bar
}
export type PartialDeepKeyOf<T> = { [id in keyof T]: string[] }

const baz: PartialDeepKeyOf<Foo> = {
    id: ['a','b'],
    bar: {
        lat: ['a','b'],
        lng: ['a','b'],
    }
}

Actual behaviour:

Type '{ lat: string[]; lng: string[]; }' is not assignable to type 'string[]'

Yes but "I am trying to create a custom type that replaces all types of a Generic parameter T to string[] but keeps all the property names even nested." — TSR
– TSR, Commented Jul 27, 2020 at 8:41

Nikita Madeev · Accepted Answer · 2020-07-27 08:46:34Z

2

You need recursive type traversal

type PartialDeepKeyOf<T> = {
    [P in keyof T]: T[P] extends string
    ? string[]
    : PartialDeepKeyOf<T[P]>
};

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answered Jul 27, 2020 at 8:46

Nikita Madeev

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4 Comments

MattTreichel Over a year ago

This works well, but relies on all the leaf properties being of a type string to be converted to the new type. I could extend that to string | number and other primitive types, but then becomes brittle if I have to list out any enums I use as well. Is there any way to change the condition to be something like extends Object and then otherwise have the type converted to the new type? I can't figure out how to do that in TypeScript.

MattTreichel Over a year ago

If I use extends Record<string, any>, this loses the enforcement of the correct key structure.

Nikita Madeev Over a year ago

@MattTreichel If I understand you correctly, then this should work.

MattTreichel Over a year ago

Thanks, that probably does, it turns out my editor's error reporting made me think it wasn't working as intended. I spun this question off into a new thread here because I was impatient: stackoverflow.com/q/72692468/1917171

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