Interspersing collections in TypeScript

intersperse.ts

type NonEmptyArray<T> = [T, ...T[]];

const hasElements = <T>(collection: T[]): collection is NonEmptyArray<T> =>
  collection.length > 0;

const head = <T>(collection: NonEmptyArray<T>): T =>
  collection['0'];

const tail = <T>(collection: T[]): T[] => {
  const [_, ...tail] = collection;

  return tail;
}
  
const intersperse = <T, U>(collection: T[], separator: U): (T | U)[] => {
  if (!hasElements(collection)) {
    return [];
  }

  return (
    tail(collection)
      .reduce<(T | U)[]>(
        (elements, element) =>
          elements.concat([separator, element]),
        [head(collection)],
      )
  );
};

console.log(
  intersperse(['some', 'uri', 'fragment'], "-")
)