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

  if (!Number.isInteger(depth)) {
    throw new Error('Depth must be a non-negative integer or Infinity');
  }

  const result: unknown[] = [];

  for (const item of items) {
    if (Array.isArray(item)) {
      if (depth > 0) {
        result.push(...flatten(item, depth - 1));
      } else {
        result.push(...item);
      }
    } else {
      result.push(item);
    }
  }

  return result;
}
```