Merge Sort in JavaScript

MergeSort.js

const mergeSort = (unsortedArray) => {
  // Check if the length is lower or equal to 1, otherwise return input.
  // This is also our break condition for the recursion
  if (unsortedArray.length <= 1) {
    return unsortedArray;
  }

  // Split input array in two equally parts when possible.
  // The half of the length could possibly be an floating number 3 / 2 = 1.5,
  // so we use Math.floor to get an whole number by remove the decimal part.
  const middleIndex = Math.floor(unsortedArray.length / 2);

  const leftArray = unsortedArray.slice(0, middleIndex);
  const rightArray = unsortedArray.slice(middleIndex);

  // Use recursion on the left and right part to break it down
  return mergeParts(mergeSort(leftArray), mergeSort(rightArray));
};

const mergeParts = (leftArray, rightArray) => {
  let mergedArray = [];
  let leftIndex = 0;
  let rightIndex = 0;

  // Compare left and right items as long as there is a counterpart (index is not out of range).
  while (leftIndex < leftArray.length && rightIndex < rightArray.length) {
    if (leftArray[leftIndex] < rightArray[rightIndex]) {
      mergedArray.push(leftArray[leftIndex]);
      leftIndex++;
    } else {
      mergedArray.push(rightArray[rightIndex]);
      rightIndex++;
    }
  }

  // Because there could be some items left on the left or right side we concat all remaining elements.
  // In theory there should only be one side affected.
  return mergedArray
    .concat(leftArray.slice(leftIndex))
    .concat(rightArray.slice(rightIndex));
};

You can find more details in this blog post