NestedOmit.ts

interface SpacingProps {
    __typename: string,
    property1: {
        __typename: string;
        a: 2;
        nested: {
            __typename: string,
            x: 3
        }
    },
    property2: {
        __typename: string;
        b: 3;
    },
}


type NestedOmit<T,K extends string>=Omit<{
    [key in keyof T]: NestedOmit<Omit<T[key],K>, K>;
}, K>;

const test: NestedOmit<SpacingProps, '__typename'> = {
    property1: { 
        a: 2,
        nested: {
            x: 3
        }
    },
    property2: {
        b: 3
    }
};

More changes!

Omit doesn't distribute unions. This forces it:

type UnionOmit<T, K extends string> = T extends unknown ? Omit<T, K> : never;

type NestedOmitHelper<T, K extends string> = {
  [key in keyof T]: null extends T[key]
    ? NestedOmit<NonNullable<T[key]>, K> | null
    : NestedOmit<T[key], K>;
};
type NestedOmit<T, K extends string> = NonNullable<T> extends {
  [prop in K]: any;
}
  ? UnionOmit<NestedOmitHelper<T, K>, K>
  : NestedOmitHelper<T, K>;

Doesn;t catch the top level yet though :hmm:

https://www.typescriptlang.org/play?#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-lQACMMoAzGYVmZa1GY8A49UdVUplt06nJaSUxAZZns5Bc-mPeVc1py1Xloto3BY-HgBWkW2k92KVBhjk8v26TnlcOwqPlazqKldLRdCzq9OG03GAAWJebUGhYBaDvQDfNJbbpVM-eFyJX1uMcywrCs73+Kc6wsNdnBaWgmjyS5jk3fJrhdfhFGYERoC0PE3FoCBVjXAANY44IQ5psHfdcRjyDhDCAA

Covariants were having issues. This fixes them:

type NullUnionOmit<T, K extends string | number | symbol> = null extends T
  ? UnionOmit<NonNullable<T>, K> | null
  : UnionOmit<T, K>;
type UnionOmit<T, K extends string | number | symbol> = T extends unknown ? Omit<T, K> : never;

type NestedOmitHelper<T, K extends string> = {
  [key in keyof T]: NestedOmit<T[key], K>
};
type NestedOmit<T, K extends string> = NonNullable<T> extends {
  [prop in K]: any;
}
  ? NullUnionOmit<NestedOmitHelper<T, K>, K>
  : NestedOmitHelper<T, K> | T; 

NonNullable is unncessary:

type NullUnionOmit<T, K extends string | number | symbol> = null extends T
  ? UnionOmit<NonNullable<T>, K> | null
  : UnionOmit<T, K>;
type UnionOmit<T, K extends string | number | symbol> = T extends unknown ? Omit<T, K> : never;

type NestedOmitHelper<T, K extends string> = {
  [key in keyof T]: NestedOmit<T[key], K>
};
type NestedOmit<T, K extends string> = T extends {
  [prop in K]: any;
}
  ? NullUnionOmit<NestedOmitHelper<T, K>, K>
  : NestedOmitHelper<T, K> | T; 

Final version for realsies:

type UnionOmit<T, K extends string | number | symbol> = T extends unknown
  ? Omit<T, K>
  : never;
type NullUnionOmit<T, K extends string | number | symbol> = null extends T
  ? UnionOmit<NonNullable<T>, K>
  : UnionOmit<T, K>;
type SafeOmit<T, K extends string | number | symbol> = T extends {
  [P in K]: any;
}
  ? NullUnionOmit<T, K>
  : T;
type RecursiveOmit<T, K extends string | number | symbol> = T extends {
  [P in K]: any;
}
  ? SafeOmit<{ [P in keyof T]: RecursiveOmit<T[P], K> }, K>
  : {
      [P in keyof T]: RecursiveOmit<T[P], K>;
    };

const cleanSolarSystem: RecursiveOmit<SolarSystem, "__typename"> = {
  //__typename: "SolarSystem",
  id: 123,
  name: "The Solar System",
  star: {
    //__typename: "Planet",
    id: 123,
    inhabitants: null,
    name: "Sun",
    size: 9999,
  },
  planets: [
    {
      //__typename: "Planet",
      id: 123,
      name: "Earth",
      size: 12345,
      inhabitants: [
        {
          //__typename: "LifeForm",
          id: 123,
          name: "Human",
        },
      ],
    },
  ],
};

Playground