swaps.js

var arr = 'abcdefghijklmnopqrstivwxyz'.split('');
var start = 0;
var end = arr.length - 1;
var swaps = 0;

while (start < end) {
  for (var i = start; i < end; i += 2) {
    var tmp = arr[i];
    arr[i] = arr[i + 1];
    arr[i + 1] = tmp;
    swaps++;
  }
  start++;
  end--;
}

console.log(arr);    // b d f h j l n p r t v x z a c e g i k m o q s i w y
console.log(swaps);  // 91