Typescript RecursiveRequired generic type

recursive-required.ts

type RecursiveRequired<T> = Required<{
  [P in keyof T]: T[P] extends object | undefined ? RecursiveRequired<Required<T[P]>> : T[P];
}>;

type ExampleType = {
  a?: number;
  b: number;
  c?: {
    d?: {
      e?: number;
      f: boolean;
      g?: {
        h: string
      }
    }
  }
}

type ExampleTypeRequired = RecursiveRequired<ExampleType>

const data: ExampleTypeRequired = {
  a: 1,
  b: 1,
  c: {
    d: {
      e: 1,
      f: false,
      g: {
        h: 'hello'
      }
    }
  }
}

@kennarddh Is there any way to make this work with a generic type that references its own type? For example:

interface TableColumn<T> {
    mappedProperty: keyof T;
    label?: string;
}

This interface could represent a table column definition that requires the name of a valid property from the row item schema. Currently, this produces a type that makes mappedProperty of type DeepRequired<keyof T> when applied to TableColumn<T> instead of leaving it as keyof T.

This creates a problem when attempting to create objects that satisfy this in a mapping function:

function normalizeDefs<T extends object>(defs: TableColumn<T>[]): DeepRequired<TableColumn<T>>[] {
    return defs.map(d => ({
        label: d.mappedProperty.toString().toUpperCase(),
        ...d
    }));
}

TS playground link.

Good catch! What about updating the DeepRequired type?

type DeepRequired<T> = T extends Function
  ? T
  : T extends object
    ? Required<{
        [K in keyof T]-?: T[K] extends PropertyKey ? T[K] : DeepRequired<T[K]>;
      }>
    : T;

Now, if T[K] is not an object, but a PropertyKey (e.g. string | number | symbol) we use that type (keyof T in your case).

What do you think @C-Duxbury ?

Link to playground here.

@gomezcabo Yes, that does the trick nicely! Thanks!

Just for my own academic curiosity, do you know why the TS compiler was saying 'string' is not assignable to type 'DeepRequired<keyof T>'? It seems like they should be compatible.

@gomezcabo Yes, that does the trick nicely! Thanks!

Just for my own academic curiosity, do you know why the TS compiler was saying 'string' is not assignable to type 'DeepRequired<keyof T>'? It seems like they should be compatible.

I guess when passing down the keyof T to the generic, this one could be string, number or symbol and that's where the Type was failing. No way to determine which one of those. That's why the fix was to include the check T[K] extends PropertyKey (PropertyKey === string | number | symbol).