Sorting algorithms JS: Merge Sort (Recursion)

mergeSort.js

// 
// Merge Sort Implentation (Recursion)
//

function mergeSort (unsortedArray) {
  // No need to sort the array if the array only has one element or empty
  if (unsortedArray.length <= 1) {
    return unsortedArray;
  }
  // In order to divide the array in half, we need to figure out the middle
  const middle = Math.floor(unsortedArray.length / 2);

  // This is where we will be dividing the array into left and right
  const left = unsortedArray.slice(0, middle);
  const right = unsortedArray.slice(middle);

  // Using recursion to combine the left and right
  return merge(
    mergeSort(left), mergeSort(right)
  );
}

// Merge the two arrays: left and right
function merge (left, right) {
  let resultArray = [], leftIndex = 0, rightIndex = 0;

  // We will concatenate values into the resultArray in order
  while (leftIndex < left.length && rightIndex < right.length) {
    if (left[leftIndex] < right[rightIndex]) {
      resultArray.push(left[leftIndex]);
      leftIndex++; // move left array cursor
    } else {
      resultArray.push(right[rightIndex]);
			rightIndex++; // move right array cursor
    }
  }

  // We need to concat to the resultArray because there will be one element left over after the while loop
  return resultArray
          .concat(left.slice(leftIndex))
          .concat(right.slice(rightIndex));
}

/*
Merge
sort
is
a
recursive
way
to
sort
an
array.

First,
you
divide
the
array
in
half
and

recursively
sort
each
half
of
the
array.

Then,
you
combine
the
two
halves
into
a

sorted
array.


ref:
https://medium.com/javascript-in-plain-english/javascript-merge-sort-3205891ac060
*/