binary search

function binarySearch(list, key) {
  var low = 0
  var high = list.length - 1
  while (low <= high) {
    var mid = parseInt((low + high) / 2)
    var guess = list[mid]
    if (guess === key) {
      return mid
    }
    if (guess > key) {
      high = mid - 1
    } else {
      low = mid + 1
    }
  }
  return -1
}

binarySearch([1, 2, 3, 4, 5, 6, 7, 8, 9], 2)