Peak Index in a Mountain Array

export function peakIndexInMountainArray(arr: number[]): number {
  let [left, right] = [0, arr.length];
 
  while (left < right) {
    const mid = Math.floor((left + right) / 2);
    if (arr[mid] < arr[mid + 1]) {
      left = mid + 1;
    } else {
      right = mid;
    }
  }
  return left;
}