Improved Merge Sort JavaScript - http://jsbin.com/wiyaja

merge2.js

function sort(array) {
  var len = array.length;
  
  if (len <= 1) {
    return array;
  }
  
  var middle = Math.floor(len*0.5);
  var left = array.slice(0,middle);
  var right = array.slice(middle, len);
  
  return merge(sort(left), sort(right));
}


function merge(left, right) {
  var lenL = left.length;
  var lenR = right.length;
  var total = lenL + lenR;
  var l = 0;
  var r = 0;
  

  var temp = new Array(parseInt(total));

  while ((l+r) < total) {
    var el;
    
    if (l >= lenL) {
      el = right[r];
      r++;
    } else if (r >= lenR) {
      el = left[l];
      l++;
    } else {
      if (left[l] > right[r]) {
        el = left[l];
        l++;
      } else {
        el = right[r];
        r++;
      }
      
    }
    
    
    
    temp[r+l-1] = el;
  
  }
  
  return temp;
  
  
}


var array = [];
for(var i = 0; i < 50; i++) {
   array.push(Math.round(Math.random() * 100));
}
console.log(array);
console.log(sort(array));

http://codereview.stackexchange.com/questions/92020/beginner-implementation-of-merge-sort/92038#92038