Merge sort basically splits, sorts and merges the values in the array

mergeSort.js

/**
 * PSEUDOCODE
 * - Break up the array into halves until you have an empty array or array with one element
 * - Once you have the smaller sorted arrays, merge those arrays with other sorted arrays until
 * - you are back at the full length of the array
 * - Once the array has been merged back together return the merged and sorted array
*/
function mergeSort(array) {
  if (array.length <= 1) return array;
  const middlePoint = Math.floor(array.length / 2);
  const leftPortion = mergeSort(array.slice(0, middlePoint));
  const rightPortion = mergeSort(array.slice(middlePoint));
  return merge(leftPortion, rightPortion);
}

/**
 * PSEUDOCODE (merge helper function)
 * - Create an empty array called merged
 * - Look for the smallest values in each input array (ie. if sorting ASC order)
 * - While there are still values we haven't looked at
 * - if the value in the first array is smaller than the value in the second array, push
 * - the value in the first array into the merged array and move onto the next value in the first array.
 * - if the value in the first array is larger than the value in the second array, push the value 
 * - in the second array into the merged array and move on to the next value in the second array
 * - Once we exhaust one array, push in all remaining values from the other array
*/
function merge(array1, array2) {
  const merged = [];
  let i = 0;
  let j = 0;
  
  while (i < array1.length && j < array2.length) {
   if (array1[i] < array2[j]) {
      merged.push(array1[i])
      i++;
   } else {
      merged.push(array2[j])
      j++;
   }
  }
  
  while (i < array1.length) {
     merged.push(array1[i])
     i++;
  }
  
  while (j < array2.length) {
     merged.push(array2[j])
     j++;
  }
  
  return merged;
}

mergeSort([8, 32, 5, 32, 3, 100, 11])