Merge Sort Algorithm

mergesort.js

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

  // 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
    }
  }

  // concat here because there will be one element remaining
  // from either left OR the right
  return resultArray
    .concat(left.slice(leftIndex))
    .concat(right.slice(rightIndex));
}

// 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));
}

console.log(mergeSort([3, 43, 23, 4, 32, 65, 43, 45, 34, 5, 23]));

// Complexity of Mergesort is O(nlogn)