mergesort.js

// recursively divides then merges
const mergeSort = (curSubArr) => {
  if (curSubArr.length <= 1) return curSubArr;
  const middle = Math.floor(curSubArr.length / 2);
  const left = curSubArr.slice(0, middle);
  const right = curSubArr.slice(middle);
  return merge(mergeSort(left), mergeSort(right));
};
const merge = (left, right) => {
  let pointerA = 0;
  let pointerB = 0;
  let merged = [];
  // stops when one array is used
  while (pointerA < left.length && pointerB < right.length) {
    if (left[pointerA] < right[pointerB]) {
      merged.push(left[pointerA]);
      pointerA += 1;
    } else {
      merged.push(right[pointerB]);
      pointerB += 1;
    }
  }
  // stage arrays for gc
  arr1 = [];
  arr2 = [];
  // handle left overs
  return merged.concat(left.slice(pointerA)).concat(right.slice(pointerB));
};