Type Alias MaximumOneOf<T, K>

MaximumOneOf: K extends keyof T
    ? { [P in K]?: T[K] } & Partial<Record<Exclude<keyof T, K>, never>>
    : never

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

This ensures that:

  • Zero or one property can be provided.
  • If multiple properties are set, TypeScript will produce an error.
  • If no properties are set, it remains valid.

Type Parameters

  • T

    The object type from which at most one property can 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 UserDescriptor = MaximumOneOf<{
  id: string;
  email: string;
  username: string;
}>;

// Valid usage
const user1: UserDescriptor = { id: "123" };
const user2: UserDescriptor = { email: "test@example.com" };
const user3: UserDescriptor = {}; // Allowed (zero properties)

// Invalid usage
const user4: UserDescriptor = { id: "123", email: "test@example.com" }; // Error: Only one property allowed

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