```ts


/** The index of target in a sorted array, or -1. */
export function binarySearch(items: readonly number[], target: number): number {
  if (items.length === 0) return -1;
  
  let low = 0;
  let high = items.length;

  while (low < high) {
    const mid = Math.floor((low + high) / 2);
    if (items[mid] >= target) {
      high = mid; // Target is in the first half, including mid
    } else {
      low = mid + 1;
    }
  }

  return low === items.length || items[low - 1] !== target ? -1 : low - 1;
}
```