```typescript
/** The index of target in a sorted array, or -1 if not found. */
export function binarySearch(items: readonly number[], target: number): number {
  let low = 0;
  let high = items.length; // Changed from high = items.length - 1 to include the last element

  while (low < high) {
    const mid = Math.floor((low + high) / 2);
    if (items[mid] === target) {
      return mid;
    }
    if (items[mid] < target) {
      low = mid + 1; // No change needed as it already handles this case
    } else {
      high = mid; // Changed from high = mid - 1 to include the element at mid in the search window
    }
  }

  return items[low] === target ? low : -1;
}
```