```typescript
export function flatten(items: readonly unknown[], depth: number = Infinity): unknown[] {
    if (depth < 0 || (typeof depth !== 'number' && depth !== Infinity)) {
        throw new Error('Depth must be a non-negative integer or Infinity');
    }

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