merge sort functional style

mergesort.js

const merge = (a1,a2) => {
  if (a1.length === 0) {
    return a2;
  }
  if (a2.length === 0) {
    return a1;
  }
  if (a1[0] <= a2[0]) {
    return [
      a1[0], 
      ...merge(a1.slice(1,Infinity), a2)
    ];
  } else if (a1[0] > a2[0]) {
    return [
      a2[0],
      ...merge(a1, a2.slice(1,Infinity))
    ];
  }
}

const msort = (a) => {
  if (a.length === 1) {
    return a;
  }
  /* Calc middle */
  const m = Math.round(a.length/2);
  /* Get the first array half */
  const firstHalf = a.slice(0,m);
  /* Get the second array half */
  const secondHalf = a.slice(m, Infinity);
  /* Merge the two halves */
  return merge(
    /* Sort the first half */
    msort(firstHalf),
    /* Sort the second half */
    msort(secondHalf)
  );
}