typescriptTypes.ts

interface A {
  type: 'A';
  j: {
    k: number;
    a: number;
  };
}

interface B {
  type: 'B';
  j: {
    k: number;
    b: number;
  };
}

interface NewA {
  type: 'A';
  j: {
    a: number;
  };
}

interface NewB {
  type: 'B';
  j: {
    b: number;
  };
}

type AorB = A | B;
type NewAorB = NewA | NewB;

function removeK(o: AorB): NewAorB {
  const {
    j: { k, ...j }
  } = o;
  return {
    ...o,
    j
  };
  // Type '{ j: { a: number; } | { b: number; }; type: "A"; } | { j: { a: number; } | { b: number; }; type: "B"; }' is not assignable to type 'NewAorB'.
  //   Type '{ j: { a: number; } | { b: number; }; type: "A"; }' is not assignable to type 'NewAorB'.
  //     Type '{ j: { a: number; } | { b: number; }; type: "A"; }' is not assignable to type 'NewA'.
  //       Types of property 'j' are incompatible.
  //         Type '{ a: number; } | { b: number; }' is not assignable to type '{ a: number; }'.
  // 					Property 'a' is missing in type '{ b: number; }' but required in type '{ a: number; }'.
  // 					ts(2322)
}

function removeK2<T extends AorB>(o: T): NewAorB {
  const {
    j: { k, ...j }
  } = o;
  return {
    ...o,
    j
  };
}