```ts
/**
 * Flattens a nested array up to a specified depth.
 * @param items - The array to flatten.
 * @param depth - The maximum depth to flatten. Defaults to Infinity.
 * @returns A new flattened array.
 */
export function flatten(items: readonly unknown[], depth: number = Infinity): unknown[] {
  if (!Number.isInteger(depth) || depth < 0) {
    throw new Error("Depth must be a non-negative integer or Infinity.");
  }

  const result: unknown[] = [];

  function recurse(arr: readonly unknown[], currentDepth: number): void {
    for (const item of arr) {
      if (Array.isArray(item) && currentDepth > 0) {
        recurse(item, currentDepth - 1);
      } else {
        result.push(item);
      }
    }
  }

  recurse(items, depth);
  return result;
}
```