IterativeMergeSort.js

function mergeSort(arr) {
  for (var n = 1; n < 4 ; n++){
    var seg_size = Math.pow(2, n);
    
    for (var seg = 0; seg < Math.ceil(arr.length/seg_size); seg++) {
      var before = arr.slice(0, seg * seg_size);
      var cur = arr.slice(seg * seg_size, (seg+1) * seg_size);
      var after = arr.slice((seg+1) * seg_size);
      
      var left = cur.slice(0, seg_size/2);
      var right = cur.slice(seg_size/2);
            
      var sorted = [];
      while(left.length && right.length) {
        if (left[0] < right[0]) {
          sorted.push(left.shift(1));
        } else {
          sorted.push(right.shift(1));
        }
      }
      
      if (left.length > 0) {
        sorted = sorted.concat(left);
      }
      
      if (right.length > 0) {
        sorted = sorted.concat(right);
      }
      
      arr = before.concat(sorted).concat(after);
    }
  }
  
  return arr;
}

var toSort = [5,9,3,6,2,1,7];

console.log("init: " + toSort);
console.log("final: " + mergeSort(toSort));