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