Merge Two Sorted Arrays in Javascript

Merge two sorted arrays in Javascript - Solution with comments

const mergeArrays = (arr1, arr2) => { // function to merge two sorted arrays
  if (!arr1 || !arr2) { // if only one array is passed
    return "Invalid Array" // message is returned
  }
  if (arr1.length < 1) { // if first array is empty
    if (arr2.length < 1) { // if second array is empty
      return "Both arrays are empty" // returns message
    }
    return arr2; // else returns second array
  }
  if (arr2.length < 1) { // if second array is empty
    if (arr1.length < 1) { // if both arrays are empty
      return "Both arrays are empty" // returns message
    }
    return arr1; // else returns first array
  }
  let small = []; // initializes empty array to store the smaller array
  let large = []; // initializes empty array to store the larger array
  arr1.length < arr2.length ? [small, large] = [arr1, arr2] : [small, large] = [arr2, arr1]; // stores smaller array in small and larger array in large
  const len1 = small.length; // stores length of small in len1
  const len2 = large.length; // stores length of large in len2
  let ansArr = []; // initializes an empty array to create the merged array
  let i = 0; // initializes i to 0 to iterate through small
  let j = 0; // initializes j to 0 to iterate through large
  while (small[i]!==undefined && large[j]!==undefined) { //while element in arrays at i and j position respectively exists
    if(small[i] < large[j]) { // if element from small is smaller than element in large
      ansArr.push(small[i]); // add that element to answer
      i++; // move to the next element
    } else { // if element from large is smaller than element in small
      ansArr.push(large[j]); // add that element to answer
      j++; // move to the next element
    }
  }
  if (i < len1) { // if i has not reached the end of array
    ansArr = [...ansArr, ...small.splice(i)]; // add the rest of the elements at the end of the answer
  } else { // if j has not reached the end of array
    ansArr = [...ansArr, ...large.splice(j)]; // add the rest of the elements at the end of the answer
  }
  return ansArr; // return answer
}

console.log(mergeArrays([0,1,5], [2,3,4,6,7,8,9])); // example