intersperse-flatmap.ts

export const intersperse = <T>(arr: T[], separator: (n: number) => T): T[] => 
  arr.flatMap(arr, (a, i) => i > 0 ? [separator(i-1), a] : [a]))

intersperse.ts

export const intersperse = <T>(arr: T[], separator: (n: number) => T): T[] =>
  arr.reduce<T[]>((acc, currentElement, currentIndex) => {
    const isLast = currentIndex === arr.length - 1;
    return [
      ...acc,
      currentElement,
      ...(isLast ? [] : [separator(currentIndex)]),
    ];
  }, []);