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

  return items.flatMap(item => {
    if (!Array.isArray(item)) {
      return item;
    }

    if (depth === 0) {
      return item;
    }

    return item.flatMap(flatten).slice();
  });
}
```