```ts
export function flatten(items: readonly unknown[], depth = Infinity): unknown[] {
  if (typeof depth !== 'number' || depth < 0) {
    throw new Error('depth must be a non-negative integer or Infinity');
  }

  return items.flatMap((item) => {
    if (Array.isArray(item)) {
      if (depth === 1) {
        return item;
      }
      return flatten(item, depth - 1);
    } else {
      return [item];
    }
  });
}
```