Type Alias ExactlyOneOf<T, K>

ExactlyOneOf: {
    [P in K]: Record<P, T[P]> & Partial<Record<Exclude<K, P>, never>>
}[K]

A utility type that enforces exactly one property from T to be set.

This ensures that:

  • At least one property must be provided.
  • No more than one property can be set at the same time.

If multiple properties are specified, TypeScript will produce an error. If no properties are specified, TypeScript will also produce an error.

Type Parameters

  • T

    The object type from which exactly one property must be set.

  • K extends keyof T = keyof T

    The keys of T that are considered for exclusivity. Defaults to all keys in T.

Example

type UserIdentifier = ExactlyOneOf<{
  id: string;
  email: string;
  username: string;
}>;

// Valid usage
const user1: UserIdentifier = { id: "123" };
const user2: UserIdentifier = { email: "test@example.com" };
const user3: UserIdentifier = { username: "user123" };

// Invalid usage
const user4: UserIdentifier = {}; // Error: At least one property is required
const user5: UserIdentifier = { id: "123", email: "test@example.com" }; // Error: Only one property allowed

Settings

Member Visibility