RequireAtLeastOne type

RequireAtLeastOne helps create a type where at least one of the properties of an interface (can be any property) is required to exist. This works because of TypeScript's utility types: https://www.typescriptlang.org/docs/handbook/utility-types.html Let's examine it:

  • [K in keyof T]-? this property (K) is valid only if it has the same name as any property of T.
  • Required<Pick<T, K>> makes a new type from T with just the current property in the iteration, and marks it as required
  • Partial<Pick<T, Exclude<keyof T, K>>> makes a new type with all the properties of T, except from the property K.
  • & is what unites the type with only one required property from Required<...> with all the optional properties from Partial<...>.
  • [keyof T] ensures that only properties of T are allowed.
type RequireAtLeastOne<T> = { [K in keyof T]-?: Required<Pick<T, K>> & Partial<Pick<T, Exclude<keyof T, K>>>; }[keyof T]