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

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

  return items.flatMap(item => {
    if (Array.isArray(item)) {
      return item.length > 0 ? flatten(item, depth - 1) : [];
    }
    return [item];
  });
}
```