merge_sort.js

function mergeSort(array) {
  // Determine the size of the input array.
  var arraySize = array.length;
 
  // If the array being passed in has only one element
  // within it, it is considered to be a sorted array.
  if (arraySize === 1) { 
    return; 
  }
 
  // If array contains more than one element,
  // split it into two parts (left and right arrays).
  var midpoint = Math.floor(arraySize / 2);
  var leftArray = array.slice(0, midpoint);
  var rightArray = array.slice(midpoint);
 
  // Recursively call mergeSort() on
  // leftArray and rightArray sublists.
  mergeSort(leftArray);
  mergeSort(rightArray);
  
  // After the mergeSort functions above finish executing,
  // merge the sorted leftArray and rightArray together.
  merge(leftArray, rightArray, array);
  
  // Return the fully sorted array.
  return array;
}

function merge(leftArray, rightArray, array) {
  var index = 0;
 
  while (leftArray.length && rightArray.length) {
    console.log('array is: ', array);
    if (rightArray[0] < leftArray[0]) {
      array[index++] = rightArray.shift();
    } else {
      array[index++] = leftArray.shift();
    }
  }
  
  while (leftArray.length) {
    console.log('left array is: ', leftArray);
    array[index++] = leftArray.shift();
  }
  
  while (rightArray.length) {
    console.log('right array is: ', rightArray);
    array[index++] = rightArray.shift();
  }
  
  console.log('** end of merge function ** array is: ', array);
}