```tsx
/**
 * The index of target in a sorted array, or -1. Handles duplicates.
 */
export function binarySearch(items: readonly number[], target: number): number {
  let low = 0;
  let high = items.length;
  while (low < high) {
    const mid = Math.floor((low + high) / 2);
    if (items[mid] >= target) {
      high = mid;
    } else {
      low = mid + 1;
    }
  }
  return low === items.length || items[low] !== target ? -1 : low;
}
```