Merge Sort JavaScript

merge.js

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


function merge(left, right) {
  var a = left.length;
  var b = right.length;

  
  if (a > 0 && b > 0) {
    if (left[0] > right[0]) {
      return [].concat(left[0], merge(left.slice(1,a), right));
    } else {
      return [].concat(right[0], merge(right.slice(1,b), left));
    }
  } else if (a == 0) {
    return right;
  } else of (b == 0)
    return left;
  
}


var array = [];
for(var i = 0; i < 20; i++) {
   array.push(Math.round(Math.random() * 100));
}
console.log(array);
console.log(sort(array));

This one is using too much recursion in the merge function - http://codereview.stackexchange.com/questions/92020/beginner-implementation-of-merge-sort/92038#92038