```ts
/**
 * Flattens an 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 array with the elements flattened up to the 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 readonly unknown[], depth - 1));
    } else {
      acc.push(item);
    }
    return acc;
  }, [] as unknown[]);
}
```