Ensure subset interface from interface / PickDeep<>?

Question

I am looking for a way given a interface:

interface Person {
  age: number,
  name: string,
  hometown?: {
    city: string,
    zip: number
  }
}

type SubPerson = EnsureSubInterface<Person, {
  name: string
}>

this would be valid:

const x: SubPerson {
  name: "Tom"
}

this would be invalid:

const x: SubPerson {
  age: 12
}

Look into [the `Partial` type](https://netbasal.com/getting-to-know-the-partial-type-in-typescript-ecfcfbc87cb6). — Heretic Monkey, Jan 31 '19 at 21:00
@HereticMonkey I don't believe that is helpful, that would still allow any part of `T` which I' am trying to restrict. — ThomasReggi, Jan 31 '19 at 21:02
Why do you say `PickDeep` wouldn't regular Pick work? `Pick`? — Titian Cernicova-Dragomir, Jan 31 '19 at 21:02
@TitianCernicova-Dragomir I have deep properties I'd like to also allow for instance `hometown.city` but not `hometown.zip`. — ThomasReggi, Jan 31 '19 at 21:06

score 1 · Accepted Answer · edited Feb 07 '19 at 22:04

If you just want to ensure that the second type parameter is a subset of the original at any give level, you could use DeepPartial (from here) as a constraint on the second parameter to EnsureSubInterface

type DeepPartial<T> = {
    [P in keyof T]?: T[P] extends Array<infer U>
    ? Array<DeepPartial<U>>
    : T[P] extends ReadonlyArray<infer U>
        ? ReadonlyArray<DeepPartial<U>>
        : DeepPartial<T[P]>
};

type EnsureSubInterface<T, U extends DeepPartial<T>> = U

interface Person {
    age: number,
    name: string,
    hometown?: {
        city: string,
        zip: number
    }
}

type SubPerson = EnsureSubInterface<Person, {
    name: string,
    hometown: {
        city: string,
    }
}>


type NotSubPerson = EnsureSubInterface<Person, {
    name: string,
    hometown: {
        city: number, // error
    }
}>


type NotSubPerson = EnsureSubInterface<Person, {
    name: string,
    hometown: {
        City: string, // error
    }
}>

Depending on your tslint config you might want this:

type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends (infer U)[]
  ? DeepPartial<U>[]
  : T[P] extends ReadonlyArray<infer U>
      ? ReadonlyArray<DeepPartial<U>>
      : DeepPartial<T[P]>
};

Ensure subset interface from interface / PickDeep<>?

1 Answers1