 ```ts
// TypeScript function to flatten an array up to a specified depth
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) && depth > 0) {
      acc.push(...flatten(item as unknown[], depth - 1));
    } else {
      acc.push(item);
    }
    return acc;
  }, [] as unknown[]);
}
```