var peakIndexInMountainArray = function(A) {
  let peak = 0;
  let peakIndex;
  A.forEach((altitude, index) => {
    if (altitude > peak) {
      peak = altitude;
      peakIndex = index;
    }
  });
  return peakIndex;
};

var peakIndexInMountainArray = function(A) {
  const peak = Math.max(...A);
  return A.indexOf(peak);
};