find-low-high-index.js

let findLowIndex = function(arr, key) {
  let low = 0;
  let high = arr.length - 1;
  let mid = Math.floor(high / 2);

  while (low <= high) {
    let mid_elem = arr[mid];

    if (mid_elem < key) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
    mid = low + Math.floor((high - low) / 2);
  }
  
  if (low < arr.length && arr[high] === key) {
    return low;
  }
  return -1;
};

let findHighIndex = function(arr, key) {
  let low = 0;
  let high = arr.length - 1;
  let mid = Math.floor(high / 2);

  while (low <= high) {
    let mid_elem = arr[mid];

    if (mid_elem <= key) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
    mid = low + Math.floor((high - low) / 2);
  }

  if (high === -1){
    return high
  }
  
  if (high < arr.length && arr[high] === key) {
    return high;
  }
  return -1;
};

let array = [1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 6, 6, 6, 6, 6, 6];

var key = 5;
console.log("Low Index of " + key + ": " + (findLowIndex(array, key)));
console.log("High Index of " + key + ": " + (findHighIndex(array, key)));

key = -2;
console.log("Low Index of " + key + ": " + (findLowIndex(array, key)));
console.log("High Index of " + key + ": " + (findHighIndex(array, key)));