mergeSort.js

// Split the array into halves and merge them recursively
function mergeSort(array) {
  if (array.length === 1) {
    // Return once we hit an array with a single item
    return array
  }

  // Get the middle item of the array rounded down by creating a variable
  const middle = Math.floor(array.length / 2)
  // Create a variable for the items on the left side
  const left = array.slice(0, middle)
  // Create a variable for the items on the right side
  const right = array.slice(middle)

  return merge(
    mergeSort(left),
    mergeSort(right)
  )
}

// Compare the arrays item by item and return the concatenated result
function merge (left, right) {
  let result = []
  let indexLeft = 0
  let indexRight = 0

  while (indexLeft < left.length && indexRight < right.length) {
    if (left[indexLeft] < right[indexRight]) {
      result.push(left[indexLeft])
      indexLeft++
    } else {
      result.push(right[indexRight])
      indexRight++
    }
  }

  return result.concat(left.slice(indexLeft)).concat(right.slice(indexRight))
}

const arrayOfNumbers = [2, 5, 1, 3, 7, 4, 2, 3, 9, 8, 6, 3]
console.log(mergeSort(arrayOfNumbers)) // [1, 2, 2, 3, 3, 3, 4, 5, 6, 7, 8, 9]