mergeSort.js

// divide the list into left & right
// keep dividing until only one element is left
function mergeSort(array) {
  const half = Math.floor(array.length / 2);

  if (array.length < 2) {
    return array;
  }

  let left = array.splice(0, half);
  return merge(mergeSort(left), mergeSort(array));
}

// check the first elements of the two sub-arrays
// the arrays {left & right} are already sorted, we are just combining them
function merge(left, right) {
  let arr = [];

  // end the loop if an array gets empty 
  while (left.length && right.length) {
    if (left[0] < right[0]) {
      arr.push(left.shift());
    } else {
      arr.push(right.shift());
    }
  }

  // if one array becomes empty, concatenate the remaining elements {remember the two sub-arrays are already sorted}
  return [...arr, ...left, ...right];
}

console.log(mergeSort([1, 4, 2, 8, 345, 123, 43, 32, 5643, 63, 123, 43, 2, 55, 1, 234, 92]));
// [ 1, 1, 2, 2, 4, 8, 32, 43, 43, 55, 63, 92, 123, 123, 234, 345, 5643 ]