RequiredExactlyOne<T, K>

Overview

Creates a new type where exactly one of the specified keys is made required, and the other specified keys are set to never, while keeping the rest of the type unchanged.

Syntax

type RequiredExactlyOne<T, K extends keyof T> = Simplify<
  Omit<T, K> &
    {
      [P in K]: Required<Pick<T, P>> & Partial<Record<Exclude<K, P>, never>>;
    }[K]
>;

• T: The original type from which keys are being picked.
• K: The keys from the original type T that one of the K is made required, and the other specified keys are set to never.

Example

type T0 = {
  a?: number;
  b: string;
  c: boolean;
};
type E0 = RequiredExactlyOne<T0, 'a'>; // { a: number; b: string; c: boolean }

type T1 = {
  a?: number;
  b: string;
  c: boolean;
};
type E1 = RequiredExactlyOne<T1, 'a' | 'b'>; // { a: number; b?: never; c: boolean } | { a?: never; b: string; c: boolean }

type T2 = {
  a: number;
  b: string;
  c: boolean;
};
type E2 = RequiredExactlyOne<T2, 'a' | 'b'>; // { a: number; b?: never; c: boolean } | { a?: never; b: string; c: boolean }