binarySearch.js

function get_pos(arr, number) {
  var l = 0,
      r = arr.length,
      m;

  while (l < r) {
    m = Math.floor( (l + r) / 2 );
    
    if (number < arr[m]) {
      r = m;
    } else if (number > arr[m]) {
      l = m + 1;
    } else {
      return m + 1;
    }
  }

  return -1;
}

var some = [11, 22, 33, 44, 55, 128, 256];


console.log(get_pos(some, 128));
console.log(get_pos([128], 128));
console.log(get_pos([], 128));