```ts
export function flatten(items: readonly unknown[], depth?: number): unknown[] {
  if (depth === undefined) depth = Infinity;

  if (typeof depth !== 'number' || !Number.isInteger(depth) && depth < 0) {
    throw new Error('Depth must be a non-negative integer or Infinity.');
  }

  return items.flatMap((item, index) => {
    if (!Array.isArray(item) || depth === 0) {
      return item;
    }
    return flatten(item, depth - 1).map(subItem => (subItem === null || subItem === undefined ? [subItem] : subItem));
  });
}
```