```tsx
export function flatten(items: readonly unknown[], depth?: number): unknown[] {
  if (!Number.isInteger(depth) && depth !== Infinity) {
    throw new Error('Depth must be a non-negative integer or Infinity');
  }
  
  const result: unknown[] = [];
  let _flatten = (arr: readonly unknown[], d: number): void => {
    arr.forEach(item => {
      if (Array.isArray(item) && (d + 1 <= depth)) {
        _flatten([...item], d + 1);
      } else {
        result.push(item);
      }
    });
  };

  _flatten([...items], 0);

  return result;
}
```