A merge sort alogrithm in JS

merge-sort.js

const merge = function (arr1, arr2) {
  let output = [];
  let target;
  while (true) {
    if (arr1.length == 0 && arr2.length == 0) break;
    else if (arr1.length == 0 && arr2.length != 0) target = arr2;
    else if (arr1.length != 0 && arr2.length == 0) target = arr1;
    else {
      if (arr1[0] > arr2[0]) target = arr2;
      else target = arr1;
    }
    output.push(target[0]);
    target = target.splice(0, 1);
  }
  return output;
}
const mergeSort = function (arr) {
  let output = []
  if (arr.length == 1) {
    output.push(arr[0]);
    return output;
  }
  else {
    let middle = Math.floor(arr.length/2);
    let left = (arr.slice(0, middle));
    let right = (arr.slice(middle));
    let sortedLeft = mergeSort(left);
    let sortedRight = mergeSort(right);
    output = merge(sortedLeft, sortedRight);
    return output;
  }
}

module.exports=mergeSort