javascript merge sort

JavaScript

function merge_sort(left_part, right_part)
{
	var i = 0;
  var j = 0;
  var results = [];
  
  while (i < left_part.length || j < right_part.length){
  	if (i === left_part.length) {
    // j is the only index left_part
    results.push(right_part[j]);
  
  } else if (j === right_part.length || left_part[i] <= right_part[j]) { results.push(left_part[i]);
  i++;
  } else {
  	results.push(right_part[j]);
    j++;
    }
  } 
  return results;
}
console.log(merge_sort([1,3,4], [3,7,9]));