```ts
/**
 * Flattens a nested array up to a specified depth.
 * @param items - The input array that may contain nested arrays.
 * @param depth - The maximum depth to flatten. Defaults to Infinity if not provided.
 * @returns A new flattened array.
 */
export function flatten(items: readonly unknown[], depth: number = Infinity): unknown[] {
  if (!Number.isInteger(depth) || depth < 0) {
    throw new Error('Depth must be a non-negative integer or Infinity');
  }

  return items.reduce((acc, item) => {
    if (Array.isArray(item) && depth > 0) {
      return acc.concat(flatten(item, depth - 1));
    }
    return acc.concat(item);
  }, [] as unknown[]);
}
```