web-annotation-utils/src

export type OneOrMore<T> = T | T[];
export type ZeroOrMore<T> = undefined | null | T | T[];

export type OneOrMoreIncluding<Other extends any, RequiredValue extends any> =
  | RequiredValue
  | [RequiredValue, ...Other[]]
  | [...Other[], RequiredValue];
// | [Other, ...OneOrMoreIncluding<Other, RequiredValue>]; // FIXME TypeScript complains about the circular reference..

// OnlyOne<T> extracts the T from a One/ZeroOrMore<T> type
export type OnlyOne<T> = T extends (infer X)[] ? X : T;

export function asArray<T>(value: ZeroOrMore<T>): T[] {
  if (Array.isArray(value)) return value;
  if (value === undefined || value === null) return [];
  return [value];
}

export function asSingleValue<T>(value: ZeroOrMore<T>): T | undefined {
  if (value instanceof Array) return value[0];
  if (value === undefined || value === null) return undefined;
  return value;
}