```ts
export function fillBetween(entries: string[], filler: string): string[] {
  const result = [];
  if (entries.length === 0) return result;

  for (let i = 0; i < entries.length - 1; i++) {
    result.push(entries[i]);
    result.push(filler);
  }

  result.push(entries[entries.length - 1]); // Add the last entry without filler

  return result;
}
```