binarySearch.js

function binarySearch(array, index) {
  let start = 0;
  let end = array.length - 1;
  let middle = Math.floor((start + end) / 2);
  while (array[middle] !== index && start <= end) {
    index < array[middle] ?
      end = middle - 1 :
      start = middle + 1;
    middle = Math.floor((start + end) / 2)
  }
  return array[middle] === index ? middle : - 1;
}

console.log(binarySearch([1, 2, 3, 4, 5], 2)) // 1
console.log(binarySearch([1, 2, 3, 4, 5], 3)) // 2
console.log(binarySearch([1, 2, 3, 4, 5], 5)) // 4
console.log(binarySearch([1, 2, 3, 4, 5], 6)) // -1
console.log(binarySearch([
  5, 6, 10, 13, 14, 18, 30, 34, 35, 37,
  40, 44, 64, 79, 84, 86, 95, 96, 98, 99
], 10)) // 2
console.log(binarySearch([
  5, 6, 10, 13, 14, 18, 30, 34, 35, 37,
  40, 44, 64, 79, 84, 86, 95, 96, 98, 99
], 95)) // 16
console.log(binarySearch([
  5, 6, 10, 13, 14, 18, 30, 34, 35, 37,
  40, 44, 64, 79, 84, 86, 95, 96, 98, 99
], 100)) // -1