Typescript: Omit a property from all interfaces in a union, but keep the union structure

You want to distribute the Omit across a union. Luckily, you can use distributive conditional types to achieve this:

type DistributiveOmit<T, K extends keyof any> = T extends any
  ? Omit<T, K>
  : never;

The T extends any construction looks like it doesn’t do much, but since T is a type parameter, it distributes the conditional type across any union constituents of T.

Let’s test it:

type CC = DistributiveOmit<C, "toRemove">;
// type CC = Pick<A, "key1" | "key2"> | Pick<B, "key1" | "key3">

You can verify that this is equivalent to the CC type you want.

Link to code